Download Modes of Transmission of Seasonal Human Influenza Viruses

Modes of Transmission of Seasonal
Human Influenza Viruses
Allison McGeer, MSc, MD, FRCPC
Professor,
Professor, Dalla
Dalla Lana
Lana School
School of
of Public
Public Health
Health
University
University of
of Toronto
Toronto
Director
Director Infection
Infection Control
Control
Infectious
Infectious Disease
Disease Consultant
Consultant
Mount
Mount Sinai
Sinai Hospital
Hospital
[email protected]
[email protected]
Empiric observations about influenza
transmission
8-30% of the world’s population is infected
annually – with newly evolved clades
z
z In temperate climates, influenza is seasonal
z
z In households, secondary attack rate is ~15%
z
z In closed institutions, outbreaks can be
explosive
z
z
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Empiric observations about influenza
transmission
8-30% of the world’s population is infected
annually – with newly evolved clades
z
z In temperate climates, influenza is seasonal
z
z In households, secondary attack rate is ~15%
z
z In closed institutions, outbreaks can be
explosive
z
z
Lowen PLoS Pathogens 2007;3(10):e151
2
Empiric observations about influenza
transmission
8-30% of the world’s population is infected
annually – with newly evolved clades
z
z In temperate climates, influenza is seasonal
z
z In households, secondary attack rate is ~15%
z
z In closed institutions, outbreaks can be
explosive
z
z
Influenza A outbreak
Nursing Home, Ontario, 2005
Attack rate
(% susceptible residents)
70
60
50
40
48% in 4 days (<3 generations)
30
20
10
0
17 18 19 20 21 22 23 24 25 26 27 28 29 30
Date (Jan - Feb)
3
When are persons infectious?
Level of evidence
Systematic review, 2005
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z 2012 citations reviewed
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z 32 eligible for assessment
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z
– 8 virus survival
– 15 experimental
– 9 outbreak reports
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CONTACT
Evidence for contact spread
Humans can be infected by nasal drops
z
z Influenza virus survives on hands for 5
minutes, on cloth/paper/tissue for up to 12
hours, and on non-porous surfaces for up to
48 hours
z
z Improved hand hygiene reduces the risk of
respiratory illness and absenteeism due to
respiratory illness by ~30%
z
z
Am J Prev Med 2001;21:79-83 AJIC 2003;31:364-70
BMC Public Health 2004;4:50
5
Evidence for aerosol spread
z
z
Humans can be infected by inhalation of
aerosols
DROPLET/AEROSOL
Kowalski and Bahnfleth, 1998; Tang JW et al. J Hosp Infect 2006
6
Factors affecting viable influenza in
aerosols
Concentration of virus in respiratory
secretions
z
z Person to person variability in particle
distribution
z
z Temperature and humidity
z
z Breathing/talking/coughing/sneezing
z
z
Fabian PLoS ONE 2008;3(7)e2691
Size distribution of droplet formed by talking, coughing, and sneezing
7
Expulsion of infectious material:
Effect of Particle Size
z
z
Diameter greater
than 100 µm
(Ballistic particles)
– predominantly
affected by
gravitational forces
Kowalski and Bahnfleth, 1998; Tang JW et al. J Hosp Infect 2006
Inhalation: Inhalable size particles
account for <10% of volume of a cough
z
z
Nasopharyngeal-sized
particles
– 20-100 µm in diameter
z
z
Tracheobronchialsized particles
– 10 -20 µm in diameter
z
z
Alveolar-sized particles
(pulmonary)
– ≤10 µm in diameter
Roy CJ NEJM 2004
8
Droplet exposure
z
z
How much infection is by
– Direct deposition onto facial mucous
membranes? (eye vs. nose vs mouth)
– Deposition onto hands, with transfer to
face?
– Inhalation?
What is the evidence that influenza can be
spread by long distance aerosols?
z
z
z
z
z
z
z
z
z
z
Transmission from ferret to ferret occurs in ferrets separated
by straight, U or S shaped ducts (Andrewes, Br. J Exp Pathol
1941;22:91)
Documented in mice (Shulman, Am J Pub Health
1968;58:2092)
TB patients housed in building with ceiling UV radiation less
likely to get influenza than those in building without (Riley Am
Rev Tuberc 1957;75:420)
Increased ventilation resulted in reduced influenza (Wan
Aerosol Science Tech 2007;41:244)
Alaska Airlines outbreak (Moser, 1979)
9
Epidemic of influenza
-like illness among passengers and
influenza-like
crew exposed to an acutely ill passenger with influenza A
(Moser Am J Epidem 1979;110:1)
16
Nu m b er of cases
14
12
10
8
6
4.2 hours on ground,
2-3 without ventilation
4
2
0
Sun
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Pending studies of transmission
Booy (Australia): control vs. mask vs. respirator
z
z Leung (HK): control vs. HH vs. HH + mask
z
z Monto (US): control vs masks vs HH +mask
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z Larson (US): control vs HH vs HH + mask
z
z
Questions about H2H influenza
transmission
What is the relative contribution of contact vs.
aerosol transmission?
z
z What patients are responsible for transmission?
z
z
– Pre-symptomatic? Asymptomatic?
How important is heterogeneity in transmission
from source patients?
z
z Does long distance aerosol transmission
contribute?
z
z
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