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Colds, flu and other respiratory infections in the
home
Respiratory infections are the most common illnesses in people of all ages. Good
respiratory hygiene can help to reduce the spread of these diseases. This briefing
material has been produced for those who work in healthcare professions, the media
and others who are looking for background understanding of hygiene and hygiene
issues and/or those who are responsible for providing guidance to the public on how
to reduce the risks of spread of respiratory infection in their homes.
Worldwide, respiratory tract (RT) infections are the most common illnesses in people
of all ages. Data from the United States, suggest that the mean number of respiratory
illnesses experienced per year in adults is around 2-5, and in children under 5 years
around 4-8. About 80% of upper respiratory tract infections are caused by viruses.
Winter months see increased viral respiratory infections, which are caused by many
different viruses. The common cold, influenza and other respiratory infections may
occur concurrently and, as the symptoms are similar, it is often difficult to assess the
relative impact of each virus during an outbreak situation.
Authorities world-wide remain concerned about the possibility of a global influenza
pandemic. Pandemics arise when a new virus emerges which is capable of
spreading in the world-wide population. Across the world, health agencies have been
focusing on improving preparedness for a future influenza pandemic. This
information and advice sheet however, deals mainly with colds and “seasonal
influenza”.
How do I recognise a viral respiratory infection?
Common colds
Colds tend to begin slowly, with the first symptom usually a sore throat, followed by
sneezing, a runny nose and nasal congestion. Children may also develop a slight
fever (raised temperature). Symptoms usually last around 7 days, but may last longer
in some people. Viral shedding in nasal secretions can continue for up to 3 weeks.
The common cold, a viral infection of the upper respiratory tract, can affect all age
groups and can be caused by any of up to 200 different viruses. Rhinoviruses cause
up to 40% of common colds. Coronaviruses are responsible for up to one-third of
common colds. Other causative viruses include parainfluenza virus, respiratory
syncytial virus (RSV) and adenovirus. Rhinoviruses and coronaviruses have been
found to cause a greater disease burden in elderly people living at home, compared
to influenza virus or RSV. Rhinoviruses cause infections all year round, with one
peak in the autumn, usually as a result of children returning to school.
Although colds are generally mild and self-limiting, they represent a significant
economic burden due to loss in productivity and medical costs. Furthermore,
secondary infections produce complications, such as otitis media, sinusitis, or lower
respiratory infections including pneumonia, with its risk of mortality, particularly in the
elderly. Studies have demonstrated that colds are also a trigger for asthma. RSV is
the major cause of viral RT infection in young children worldwide. Child daycare
attendance in North America carries with it a very high risk of RSV infection within the
first 2 years of life, and may account for 0.5-1.0% of hospitalised infants in the USA.
Seasonal Influenza
Influenza, is a more serious RT illness, although there is a wide spectrum of severity
of illness ranging from minor symptoms through to pneumonia and death. Common
symptoms of flu include sudden onset of fever, headache, chills, fatigue, muscle
aches and pains, runny nose, sore throat and dry cough. The symptoms quickly
become more severe than those of a common cold.
Influenza occurs mostly in the winter months, normally peaking between December
and March in the Northern hemisphere. Risks are highest among persons >65 years,
children <2 years, and persons who have medical conditions (e.g. diabetes, chronic
lung disease). The elderly and persons with underlying health problems are at
increased risk from complications.
There are three types of influenza virus: A, B and C. Type A constantly changes, with
new strains appearing regularly, and is usually responsible for the large epidemics.
Influenza A is usually a more severe infection than influenza B, which causes
smaller, more localised outbreaks. Type C is less common. Influenza virus can be
shed before symptoms appear and up to 7 days after onset of illness, therefore
people are potentially infectious before symptoms develop as well as afterwards.
In the northern hemisphere, annual influenza epidemics occur during autumn and
winter affecting approximately 5-15% of the population.
SARS
Severe Acute Respiratory Syndrome (SARS) generally begins with a fever greater
than 100.4°F [>38.0°C]. Other symptoms include headache, an overall feeling of
discomfort and body aches. After 2-7 days, patients may have a dry cough and have
trouble breathing. The incubation period for SARS is typically 2 to 7 days, but may be
as long as 10 days.
SARS is caused by SARS coronavirus (SARS CoV). It was first recognised in
Guangdong Province in China in November 2002, and spread worldwide before
being contained by July 2003. Between July 2003 and May 2004, four small and
rapidly contained outbreaks of SARS were reported; three of which appear to have
been linked to laboratory releases of SARS. The source of the fourth outbreak
remains unclear. The possibility of SARS re-emergence remains, and there is a need
for continuing vigilance.
Other viral infections of the respiratory tract
A variety of other respiratory viruses can cause ‘flu-like’ symptoms, sometimes with
infection of the lower respiratory tract. Respiratory Syncytial Virus (RSV) can infect
the same person several times during a lifetime. It causes more severe illnesses (e.g.
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bronchiolitis, pneumonia) in children, but only a ‘common cold-like infection’ in adults.
It can also produce a flu-like illness indistinguishable from influenza. RSV affects
about 90% of children by the age of 2 years. It is often carried home by school
children and passed onto their siblings in the home. Infections occur mainly in winter
to early spring and are associated with high incidence of secondary pneumonia and
death in the elderly. Human metapneumovirus (hMPV) is a closely related to RSV. It
was only recently identified in 2001. It is associated with mild respiratory infections as
well as severe bronchiolitis and pneumonia. hMPV infections are thought to occur
mostly during winter. The number of people that suffer from hMPV each year is still to
be determined. Infection occurs in infants and young children but hMPV has been
found in older children and adults suggesting re-infection may occur later on in life.
Parainfluenza viruses (PIV) are a major cause of acute respiratory tract infections.
They may cause lower respiratory illnesses (LRTI) (bronchitis, pneumonia) in young
children. In older children and adults, parainfluenza virus causes upper respiratory
illnesses (URTI) (e.g. common colds) which are usually only mild. Parainfluenza
peaks in the late autumn to early winter. Most LRTI due to adenoviruses are mild and
indistinguishable from other viral respiratory infections. Adenoviruses are a less
frequent cause of LRTI in children than RSV or PIV, but can cause epidemics of
severe LRTI in young children. Adenoviruses are implicated in 5-11% of URTI, and
are also implicated in pharyngitis, pneumonia, bronchiolitis and croup in children.1
How are respiratory viruses spread?
The mode of transmission of colds and influenza remains highly contentious. There
are 3 possible routes of transmission:
Droplet transmission occurs when the infected individual directly sprays large
droplets (droplets of size > 5µm) of infected mucous by coughing or sneezing, which
propels the droplets onto conjunctiva of the eye (the virus enters the tear fluid and
drains down a duct into the nose) or the lining of the nose of a susceptible host,
where the virus then infects the mucous membranes.
Airborne transmission involves droplet nuclei (droplets of size < 5µm) and does not
require face-to-face contact with the infected individual. Droplet nuclei settle from the
air slowly and are respirable i.e. the virus can be drawn down directly into the
alveolar region of the lungs where they infect the tissue of the lungs.
Contact transmission involves hands and surfaces. Infected droplets of mucous are
deposited on surfaces either by settling of airborne droplets or being touched with
contaminated fingers. An individual can pick up the virus if they touch a contaminated
surface or shake hands with an infected individual with contaminated hands. They
can then become infected if they rub their eyes or nose with contaminated hands
when the virus infects the nasal mucosa.
It is probable that colds and flu can be transmitted by all 3 pathways, but there is
considerable disagreement as to the relative importance of each pathway.
Spread of cold viruses
A significant amount of investigation was carried out during the 1970s and 80s to
better understand the mode of transmission of cold viruses. The findings of these
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investigations are comprehensively reviewed by Goldmann2 as summarised in
Appendix 1.
The commonly held belief is that colds are spread by particles of infected mucous
generated by coughs and sneezes. However, increasingly, there is evidence that
colds are transmitted via hands and surfaces. Infection can spread when fingers
become contaminated by contact with the infected nose, or when surfaces such as
handkerchiefs and tissues, tap and door handles or telephones become
contaminated by droplets of infected mucous shed from the nose. The virus is
passed onto another person either by handshaking or when contaminated surfaces
are touched by that person. Individuals then infect themselves by touching their own
nose or eyes with contaminated hands. Cold viruses deposited on surfaces can
remain viable, in large numbers, for several hours and the ‘infectious dose’ (the
number of viral particles required to cause infection) may be very small. For
rhinovirus the infective dose may be less than ten particles.
Some investigators maintain that inoculation of the eyes or nose by contaminated
hands is of paramount importance. Others maintain that the evidence favours droplet
and droplet nuclei transmission as the most important mode of spread. Goldman
maintains that, since coughs and sneezes tend to spray saliva from the pool at the
front of the mouth (as opposed to mucous droplets from the nose), and saliva
contains little or no cold virus, coughs and sneezes are unlikely to spread infection.
Since saliva contains very little cold virus, it is also unlikely that colds are caught by
kissing.
For RSV, there is general agreement that the hands are the primary route for the
spread of infection.
Spread of influenza
Prompted by concerns about the possibility of a flu pandemic, a considerable amount
or work has been carried out to try and establish the relative importance of the
different routes of transmission. The findings of these investigations are summarised
in Appendix 1.
Influenza virus is shed in large numbers from an infected person. Survival times for
aerosolized influenza vary between 1 and 24 h. The virus can also survive on
surfaces such as stainless steel and plastic for 24-48 hours, and for up to 12 hours
on soft surfaces such as cloth, paper and tissues. By contrast the virus survives only
short periods of time on the hands; investigations suggest that, after transfer to
hands from surfaces, viable virus falls to a low level within 5 min.
It is probable that flu can be transmitted by all 3 pathways, but there is considerable
disagreement as to the relative importance of each pathway. Whereas some
investigators believe that droplet transmission is the major pathway for spread of flu
and airborne transmission is of minor importance, others maintain that the role of
droplet transmission has been overrated and that airborne transmission is a
potentially important transmission pathway in indoor environments. Like colds, flu can
also be spread via the hands by contact with objects that an infected person has
contaminated with infectious nose and throat secretions, although there is less
supporting evidence for this mode of spread than for colds.
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Current WHO and ECDC recommendations on measures to reduce the spread of
influenza are based on the supposition that influenza mainly spreads from person-toperson by large droplets from an infected person coughing and sneezing, and by
contact transmission.
Understanding the relative importance of the different routes of transmission is key
for development of strategies for containing a flu pandemic in the early stages, when
a vaccine is not yet available. Recommending the use of face masks only makes
sense if airborne or droplet transmission are important. Handwashing promotion is
only worthwhile if contact transmission is significant. A 2007 analysis of measures
such as isolation, quarantine, social distancing, barriers, personal protection and
hygiene to prevent the spread of viruses such as influenza and SARS3, indicated that
handwashing and wearing masks, gloves and gowns were effective individually in
preventing the spread of respiratory viruses, and were even more effective when
combined. Evidence that measures such as hand hygiene, can reduce spread of
influenza comes from the SARS outbreaks in Hong Kong, which coincided with the
latter part of influenza season, when it was observed that, as extensive personal and
community public health measures took place, influenza case numbers fell
significantly, more so than usual for the time of year.4
Spread of SARS virus
Based on current evidence, close contact with an infected person poses the highest
risk of cross infection from one person to another. Infectious mucous droplets are
produced by sneezing, coughing etc. These droplets may contaminate the hands or
can settle on nearby objects or surfaces. It is thought the virus may remain infective
for up to 24 hours on dry surfaces. Other coronaviruses studied to date have not
remained viable beyond 3-4 hours.5 As with the common cold, it is thought that the
virus can be spread by inhalation of infected droplets, or by people touching other
people, or objects and surfaces that are contaminated with infectious droplets.
Infection then occurs by transferring the virus from the hands to the eye(s), nose, or
mouth. It is also possible that SARS is spread more broadly through the air or by
other ways that are currently not known. SARS, like colds, appears to be less
infectious than influenza.
Indications are that spread from faecal matter infected with the virus was the cause
of the majority of the 300+ SARS cases in the apartment block outbreak in Hong
Kong. An official investigation concluded that leaking sewage pipes and inadequate
seals on U-bends were major contributors to the outbreak and that airborne particles
carried the virus throughout the complex.
Nine out of 10 studies carried out to evaluate the effectiveness of hand hygiene
showed that handwashing was protective against SARS transmission, but only in 3
studies was this result statistically significant. The authors concluded that the
evidence was “suggestive, but not conclusive”, although the small size of the studies
may have been an important factor. 6
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If I get a cold or the flu, will it help to build my immunity against further colds
or flu?
Unfortunately, one of the main problems with cold and flu viruses is the way in which
new strains are continuously developing which means that immunity built against one
strain will not protect us against the next one that appears.
Spread of common cold infections is dependent on viruses circulating around the
community and a supply of susceptible noses to infect. In isolated communities that
do not have regular contact with the rest of the world, the community can be free of
colds until visitors introduce new viruses. There are many reports of epidemics of
colds occurring in isolated island communities after the landing of a ship that brought
in visitors with colds, but the colds die out as the population develops resistance.
Practical advice to prevent the risk of infection within the home
There has been a tendency to assume that, since nothing can be done to cure colds
or flu, that catching these viruses is inevitable. We can seek vaccination that provides
effective protection against the most likely strains of flu, but otherwise, colds and flu
advice tends to be focused on how to cope once infected.
Avoidance of cold and flu viruses is worthwhile, but trying to prevent the spread of
cold and flu germs through good respiratory hygiene also reduces the risks of
infection. Good respiratory hygiene is particularly recommended in families where
there are people more vulnerable to infection, something that is becoming more and
more common. For these people the consequences of catching a cold may
potentially be much more serious. Adopting good hygiene measures will reduce the
chances of spreading infection in the home, particularly when someone in the family
is already infected.
If you are already infected
“Good respiratory hygiene” reduces the risks or spread of infection to others. The key
to good respiratory hygiene (as promoted in the 2007 winter communications
campaign7) is “catch it, bin it, kill it” which means:
 avoid touching your nose as much as possible. Block coughs or sneezes
preferably with a tissue, or with your hands - but remember they are now
contaminated and could spread infection.
 use disposable tissues rather than a cotton handkerchief to blow your nose.
Dispose of tissues immediately and ‘safely’. DO NOT leave them lying around for
other people to pick up and become infected.
 wash your hands thoroughly using soap and water. Make sure you use good
mechanical action and rinse your hands under clean running water to remove any
infected mucus. Make sure your hands are dry, as wet or moist hands are more
likely to spread germs.
 alternatively if a washbasin is not available, use a alcohol ‘wet wipe’ tissue or a
suitable waterless hand sanitiser* to clean your hands.
Also:
 think before you shake hands with anyone.
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 remember that you can transmit the virus to other people via computer keyboards,
TV remotes, telephones, door handles tap handles etc. – so wash your hands
frequently
 cleaning cloths and sponges can readily spread germs from one surface to
another. Make sure that cloths are disinfected immediately after use using a
bleach disinfectant or other disinfectant that is capable of killing cold and flu
viruses**, and thoroughly dried until next use. Alternatively use a disposable cloth
or wipe to clean surfaces.
 do not share your towels, facecloths, toothbrushes, eating utensils, etc. with other
family members.
 wash your laundry (especially handkerchiefs, towels, face-cloths) used by ill
people separately from other laundry, and at a higher temperature (at least 60ºC)
to ensure viruses are inactivated.
 if you are infected with influenza, stay indoors and keep contact with other people
to a minimum until seven days after your symptoms have improved and you have
had no fever for at least 48 hours.
Advice for carers and the family
 Treat your hands as potentially contaminated and avoid touching your nose and
eyes.
 If someone leaves a tissue lying around on surfaces be aware that your hands will
be contaminated after touching it – so wash them immediately whenever possible.
 Wash your hands thoroughly using soap and water after touching any suspect
item. Rinse them under running water to remove any infected mucous. Make sure
your hands are dry, as wet or moist hands are more likely to spread germs.
 Alternatively if a washbasin is not available, use an alcohol ‘wet wipe’ tissue or a
suitable waterless hand sanitiser* to clean the hands.
 When looking after a baby with a continually runny nose, keep a plastic bag with
you to collect tissues until you have the opportunity to dispose of them.
 Be aware that you and other family members could pick up infection from hand
contact surfaces around the home. Ensure that surfaces frequently touched by
different people are regularly cleaned and disinfected, e.g. door and cupboard
handles, toilet flush handles, wash basin taps, telephones, computer keyboards
and TV remotes. Where possible, use a bleach-based cleaner or other
disinfectant/cleaner that is capable of killing viruses* but take with e.g. co,puter
keyboards which could become damaged.
 Cleaning cloths and sponges can readily spread germs from one surface to
another. Make sure that cloths are disinfected immediately after use using a
bleach disinfectant** and thoroughly dried until next use. Alternatively use a
disposable cloth or wipe to clean surfaces.
 Do not share towels, facecloths, toothbrushes, eating utensils etc. with an infected
person.
 Wash any laundry (especially handkerchiefs, towels, face-cloths) used by ill
people separately from other laundry, and at a higher temperature (at least 60ºC)
to ensure viruses are inactivated.
 Make sure that as far as possible the home is kept well ventilated, and avoid
spending time in rooms which are poorly ventilated.
*Waterless hand sanitisers (also called hand rubs) which are effective in
inactivating cold and flu viruses on the hands are alcohol-based gels (or other
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products) containing not less than 62% v/v alcohol. For inactivation of cold and flu
viruses on hands. An alternative formulation is also available which is a
pyroglutamate/succinate buffering system (pH 3.) in a topical foam formulation. The
formulation also contains a polymer that physically traps viral particles, reducing
transfer from hands to the nasal mucosa or conjunctiva of the eye. The effect is thus
achieved through a combination of virus entrapment and low pH inactivation of the
virus. The action is sustained on hands for a few hours after application which helps
protect against ongoing exposure to respiratory viruses.
**Disinfectants and disinfectant cleaners - Make sure you use a disinfectant or
disinfectant/cleaner such as a bleach-based product, which is active against
respiratory viruses. For more details on choosing the appropriate disinfectant, see
the IFH information sheet “Cleaning and disinfection: Chemical Disinfectants
Explained”. Consult the manufacturers’ instructions for information on the “spectrum
of action”, and method of use (dilution, contact time etc). For bleach (hypochlorite)
products, use a solution of bleach, diluted to 0.5% w/v or 5000ppm available chlorine.
Household bleach (both thick and thin bleach) for domestic use typically contains 4.5
to 5.0% w/v (45,000-50,000 ppm) available chlorine. Bleach/cleaner formulations
(e.g. sprays) are formulated to be used “neat” (i.e. without dilution). It is always
advisable, however, to check the label as concentrations and directions for use can
vary from one formulation to another.
You should also be aware that:
 although mucous in the nose and throat can be a source of infection for others,
the production of mucous helps the patient because it traps bacteria and viruses
and limits their spread in the body.
 hot tasty drinks and spicy foods increase mucous production and this can help
relieve symptoms of sore throat and cough. In contrast, a hot dry atmosphere in a
room can dry up mucous in the nose and throat and make us more vulnerable to
infection.
 a flu injection can greatly reduce your chances of catching flu. The best time to get
a flu injection is during October and November. Flu injections are most important
for people of 65 years or older, people with health problems such as asthma or
heart disease, or people who care for the elderly or sick
 antibiotics do not work against colds and flu.
In response to the need for education on respiratory hygiene, ECDC has produced
an “Influenza Communication Toolkit”. The aim is to assist health communicators
in devising communication campaigns to tackle seasonal influenza. The toolkit can
be found at:
http://www.ecdc.europa.eu/en/healthtopics/seasonal_influenza/communication_to
olkit/Pages/communication_toolkit.aspx
Other Resources



CDC. Seasonal influenza basics.
http://www.cdc.gov/flu/about/disease/index.htm
WebMD How flu spreads http://www.webmd.com/cold-and-flu/how-fluspreads
ECDC. Seasonal influenza.
http://www.ecdc.europa.eu/en/healthtopics/seasonal_influenza/
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

WHO Europe. Influenza. http://www.euro.who.int/en/healthtopics/communicable-diseases/influenza
WHO. Influenza. http://www.who.int/topics/influenza/en/
IFH teaching/self-learning resources on home hygiene
 Home hygiene - prevention of infection at home: a training resource for carers and
their trainers. (2003) International Scientific Forum on Home Hygiene. Available
from:
http://www.ifh-homehygiene.com/best-practice-training/home-hygiene%E2%80%93-prevention-infection-home-training-resource-carers-and-their
 Home Hygiene in Developing Countries: Prevention of Infection in the Home and
Peridomestic Setting. A training resource for teachers and community health
professionals in developing countries. International Scientific Forum on Home
Hygiene. Available from: www.ifh-homehygiene.org/best-practice-training/homehygiene-developing-countries-prevention-infection-home-and-peri-domestic. (Also
available in Russian, Urdu and Bengali)
Appendix 1 How are colds and flu spread? – supplementary
information
Spread of cold viruses
A significant amount of work was carried out during the 1970s and 80s to find out
how cold viruses are spread. These studies were reviewed by Goldmann in 20003:
 studies showed that colds can be induced by inoculating secretions from infected
patients into the nose or eyes of healthy volunteers.
 studies showed that it is difficult to transmit the infection orally or by kissing. When
volunteers with colds kissed ‘cold free’ volunteers for up to 1.5 minutes, only one
case of cross infection occurred in 16 trials.
 studies show that the virus can be recovered from objects in the surroundings of
people with rhinovirus colds, and that clean hands can pick up the virus by
touching such objects. A study showed that most subjects with colds had
rhinovirus on their hands, and virus could be recovered from 43% of surfaces they
touched. For people with rhinovirus colds, virus was found on 39% of hands and
6% of objects in their immediate environment.
 in a study where asthmatic children were trained not to touch their nose and eyes
so frequently, a 47% reduction in cold infections and 45% reduction in coldassociated asthma attacks was noted.
 after handling contaminated coffee cup handles, 50% of subjects developed an
infection.
 mothers who regularly disinfected their hands with dilute iodine solution had a
slightly lower infection rate than mothers using an inactive hand wash.
 Volunteers with colds played cards with healthy volunteers for 12 hours whilst
prevented from touching their nose or eyes by a neck collar and arm brace. Over
half of healthy volunteers developed colds, indicating that the infection could only
have occurred via aerosol transmission.
 volunteers suffering from colds were recruited to stay overnight in hotel rooms.
After checkout, virus was found on 35% of objects, including door handles, light
switches, pens, faucet and toilet handles, and television remote controls. In a
second study where the same subjects stayed overnight in a hotel room where
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hand contact surfaces (light switch phone button and handset) had been
contaminated with rhinovirus-contaminated mucus, 60% of subjects became
contaminated with rhinovirus.8
 Homes of subjects with rhinovirus colds were tested. Sixty-seven (42%) of 160
surfaces were positive for rhinovirus. Surfaces most frequently contaminated were
doorknobs (6/18), refrigerator door handles (8/14), TV remote controls (5/10), and
bathroom faucets (8/10). Infected nasal mucus from 6 subjects was deposited on
household objects. Infectious rhinovirus was detected on 23.5%, 4% and 0% of
fingertips following contact with these objects which had been contaminated 1, 24
and 48 hours previously.9
Some investigators maintain that contamination of the hands followed by innoculation
of the eyes or nose is of paramount importance and that there is little evidence that
rhinovirus colds can be transmitted orally or by kissing, and little evidence of droplet
or droplet nuclei transmission. Others maintain that the evidence favours droplet and
droplet nuclei transmission as the most important mode of spread.10
Spread of influenza
The mode of transmission of influenza remains highly contentious. The relative ability
of the virus to cause infection by airborne, droplet or contact transmission is
determined by a number of factors including the number of virus particles shed from
the infected person, the distance which infected airborne particles can travel, the
extent to which the virus can survive in the environment and the infective dose. It
also depends upon how people behave. The available data is reviewed in a 2008
report by Weber et al, commissioned by the European Commission11:
Shedding of virus particles from the infected person,
 People infected with influenza viruses shed large quantities of virus-laden
mucous. Up to 107 infectious influenza particles per ml has been detected in nasal
secretions.12
Transmission of infected airborne particles
 Coughing and sneezing produces droplets in size range from <1 to up to 2000 µm.
Particles larger than 10µm probably contain more than 99.9% of the pathogen
load of a cough or a sneeze.
 Particles smaller than 488 µm (cough) or 232 µm (sneeze) will not travel further
than 60 cm and settle quickly on surfaces.
 Fine particles (droplet nuclei) can remain airborne for a considerable time and
travel over a distance of up to >3m to contaminate surrounding surfaces.
 The ability of particles to penetrate into the lungs decreases with particle size.
Weber concludes that most particles expelled by coughing or sneezing are not
inhaled into the lungs, because they are too large or because they have settled
quickly after expulsion.
Virus survival in the environment
Pathogen-loaded mucous can only cause disease if the pathogens survive in the
airborne state or on surfaces. Inactivation rates vary by several orders of magnitude.
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On inanimate surfaces and in aerosols, daily inactivation rates are in the order of 1102. On hands the daily inactivation rate is of the order of 103. This means that the
influenza virus can survive in aerosols for several hours, on hands for only a few
minutes. Nasal mucous has a survival-enhancing effect, which may explain why the
virus can be found on surfaces in homes and daycare centers. Investigations have
shown that:
 maximum survival times for aerosolised influenza vary between 1 h (80% RH) and
24 h (20% RH). Several investigators found that aerosolised influenza virus
survives well at low RH and is inactivated quickly at medium and high RH, whilst
others identified a survival minimum at middle RH and an increase in survival at
high RH. Low temperatures increase survival.
 at 35-40% RH, typical for indoor environments, influenza A virus survived for more
than 24-48 h on stainless steel and plastic surfaces. Transfer from stainless steel
to hands occurred for up to 24 h
 at 35-40% RH influenza A virus dropped to undetectable levels after 8-12h on
porous surfaces such as paper tissue, pajamas or paper. Transfer of viable
influenza A virus from paper tissue to hands was only possible for 15 min,
 after the transfer to hands from both surface types, viable virus fell to a low level
within 5 min.
 on Swiss banknotes influenza A viruses of the subtypes H1N1 and H3N2 have
very low inactivation rates. H1N1 shows a low inactivation rate of approximately
0.05 day_1.
 in a study of US daycare centres and domestic homes, influenza A virus was
detected on 23% of daycare centre surfaces sampled during the fall and 53% of
surfaces sampled during the spring. Whilst no influenza was detected on home
surfaces during the summer, influenza was detected on 59% of surfaces sampled
during March in 5 homes where there was an influenza-infected child. No virus
was recovered from 3 other homes where household members were healthy.
Influenza virus was recovered most frequently from telephone receivers (80%)
and least frequently from computer keyboards (40%). Other surfaces found to be
contaminated included refrigerators, kitchen taps, light switches, microwaves, TV
remote controls, door knobs, baths and taps and toilet handles. Influenza virus
was recovered from 69% of the daycare centre nappy changing areas indicating
presence of virus in infant faeces.
The infective dose
 It is assumed that infection can occur via both upper and lower respiratory tract
down to the alveoli, although the nasal infectious dose of influenza A is several
orders of magnitude larger than the airborne infectious dose. The nasal infectious
dose ID50 (the dose that infects 50% of the exposed persons) is in the range of
100-1000 TCID50, whereas the airborne ID50 is in the range of 0.6-3.0 TCID50. A
rough calculation based on volumes of droplets and virions suggests that between
103 and 107 virions fit into droplets with diameters between 1 and 10µm. i.e. an
infective dose could easily fit into one droplet.
 There are no estimates for the ocular infectious dose of influenza A; it remains to
be seen whether viruses inoculated in the eye could find their way to the nasal
mucosa through the lacrymal ducts in sufficient amounts to initiate an infection.
 The ability of particles to penetrate into the lungs decreases with particle size.
Weber concludes that most pathogens expelled by coughing or sneezing are not
inhaled into the lungs, either because they are too large or because they have
settled quickly after expulsion.
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The evidence suggests that flu can transmit by all 3 pathways, but there is
considerable disagreement as to the relative importance of each. Whereas some
investigators believe that droplet transmission is the major route of spread of flu and
airborne transmission is of minor importance, others maintain that the role of droplet
transmission has been overrated and that airborne transmission is an important
transmission pathway especially in indoor environments. The data and arguments for
and against are summarised in a number of publications.13,14,15,16,17,18, 19,20
WHO,21 and ECDC22 recommendations are based on the supposition that viral
respiratory infections mainly spread by large droplets from an infected person
coughing and sneezing and by contact transmission. ECDC currently consider that
fine droplet and aerosol spread may occur, but is a possibility only in a minority of
infections e.g. in healthcare settings where medical procedures can generate
aerosols containing virus. From a 2007 systematic literature review, Brankston et al19
also conclude that, in reality, transmission occurs at close range rather than over
long distances, suggesting that airborne transmission, as traditionally defined, is
unlikely to be of significance, but this conclusion is contested by others.18,20
By contrast, Weber et al.11 conclude the following:
 with regard to airborne transmission of droplet nuclei, the biology of virus
inactivation and the physics of aerosols make it likely that this is an important
transmission pathway in indoor environments, especially in unventilated
conditions. Under these conditions influenza A virus may remain infectious for a
considerable time in the airborne state. Whereas large droplets expelled by
coughing or sneezing are not inhaled into the lungs, because they are too large or
have settled too quickly, it is estimated that 103 to 107 virions could easily fit into
the smaller droplet nuclei which can be inhaled into the lung. Since the airborne
infectious dose is around 0.6-3.0 (compared with 100-1000 for the nasal infectious
dose) transmission through fine droplets becomes more plausible.
 Since large droplets travel only short distances and infection depends on direct
deposition on the mucosa of a susceptible person, droplet transmission will only
occur if infected and susceptible persons are in close contact (several tens of cm
apart), of comparable height and the sneeze or cough is directed in the ‘‘right’’
direction. Weber argues that the aerodynamics of large droplets show that even a
close cough is unlikely to cause infection, and only a close, unprotected,
horizontally directed sneeze could be enough to cause droplet transmission. It
remains unknown how commonly this occurs, but such sneezes are probably quite
rare. Droplet transmission is also constrained by the fact that the nasal infectious
dose is around 100-1000 TCID50. Weber et al. thus conclude that “transmission
through this pathway is most likely a rare event”.
Assuming that contact transmission involves hand transfer to nasal mucosa or
conjunctiva of the eye, inactivation on environmental surfaces and human skin are
the major limiting factors. Since inactivation occurs very rapidly on hands, this
appears to be the most important. If, however, virus on surfaces is being constantly
renewed and there is frequent hand contact with these surfaces, then even short
survival times on human skin make contact transmission probable. The data suggest
that if people are not being watched, nose-picking and eye-rubbing occur at a rate of
approximately 0.4 h-1, although if people are facing each other the rate is 10 times
less. The fact that influenza virus shows high stability on non-porous surfaces such
as door handles, light switches or telephone buttons increases the possibility of
contact transmission in highly frequented public settings such as hotel rooms, public
transport, public toilets etc. Transmission via hands has been demonstrated by hand
hygiene intervention studies; a systematic review showed that hand cleansing can
Page 12/14
cut the risk of respiratory infection in home and community settings by up to 23%.23
Weber concludes that contact transmission could be a key transmission mode.
A further analysis published in 200824, involving a detailed mathematical model of
influenza transmission infection risk in a four-person household, also concluded that
“droplet transmission might also be possible but the results strongly indicate a
dominance of airborne transmission for influenza”.
Overall, the question as to which transmission mode/s is responsible for most
influenza infections remains controversial. For influenza, none of the three possible
transmission modes has unambiguously been demonstrated to be responsible for
most infections. There appears to be agreement that airborne transmission is at least
possible, but strong disagreement about its importance relative to droplet and contact
transmission. Given the need to formulate robust and efficient non-pharmaceutical
control measures in case of a new pandemic, more quantitative estimates for the
importance of the different transmission modes are called for.
This fact/advice sheet was last updated in 2015
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