Download Infectious period - Open Study at LSHTM

The logic behind isolation:
incubation, latent and infectious periods
Judith Glynn
In October 2014 Nurse Kaci Hickox was placed under
quarantine for 21 days when she returned from West Africa to
the United States.
She protested and eventually got the order overturned.
Who was right? And why 21 days?
Key time periods for an infection:
• incubation period
• latent period
• infectious period
INFECTION
ACQUIRED
ONSET OF
SYMPTOMS
RESOLUTION/
DEATH
INCUBATION PERIOD
DISEASE
Incubation period:
time from infection to first clinical symptoms
INFECTION
INFECTIOUS
PERIOD
LATENT
PERIOD
INFECTION
ACQUIRED
ONSET OF
INFECTIOUSNESS
END OF
INFECTIOUSNESS
RESOLUTION/
DEATH
Latent period:
from infection to onset of infectiousness
Infectious period:
time during which infected person can transmit to others
INFECTION
ACQUIRED
ONSET OF
SYMPTOMS
RESOLUTION/
DEATH
INCUBATION PERIOD
DISEASE
INFECTION
INFECTION
ACQUIRED
LATENT PERIOD
ONSET OF
INFECTIOUSNESS
INFECTIOUS
PERIOD
END OF
INFECTIOUSNESS
Incubation period:
time from infection to first clinical symptoms
Latent period:
from infection to onset of infectiousness
Infectious period:
time during which infected person can transmit to others
ONSET OF
SYMPTOMS
INFECTION
ACQUIRED
RESOLUTION/
DEATH
INCUBATION PERIOD
DISEASE
Incubation period
Incubation period
Characteristic for each disease
Variation
Distribution usually lognormal (single peak and long right tail)
May be influenced by infecting dose
Important for:
when (and therefore where) infection occurred
how long to quarantine for after known exposure
Salmonella outbreak, UK
80
70
No. of individuals
60
50
40
30
20
10
0
6
12
18
24
30
36
42
48
54
60
66
72
78
Hours between dinner and onset
84
90
96
102
108
114
120
Latent period
INFECTION
INFECTIOUS
PERIOD
LATENT PERIOD
INFECTION
ACQUIRED
ONSET OF
INFECTIOUSNESS
END OF
INFECTIOUSNESS
RESOLUTION/
DEATH
Latent period
Earliest that infection can
be transmitted to others
INFECTION
INFECTIOUS
PERIOD
LATENT PERIOD
INFECTION
ACQUIRED
ONSET OF
INFECTIOUSNESS
END OF
INFECTIOUSNESS
Latent period: from infection to onset of infectiousness
RESOLUTION/
DEATH
Infectious period
Transmission
INFECTION
INFECTIOUS
PERIOD
LATENT PERIOD
INFECTION
ACQUIRED
ONSET OF
INFECTIOUSNESS
END OF
INFECTIOUSNESS
Determines:
- how infection will be transmitted in the population
- how long patients should be quarantined for
RESOLUTION/
DEATH
Disease A (eg measles)
INCUBATION PERIOD
DISEASE
DISEASE
INFECTION
LATENT PERIOD
INFECTIOUS PERIOD
Disease B (eg SARS)
INCUBATION PERIOD
DISEASE
DISEASE
INFECTION
LATENT PERIOD
INFECTIOUS PERIOD
Ebola
Incubation periods from person-person transmission in the first Ebola outbreak
Zaire (DRC) 1976
10
9
8
Number of cases
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Days
Incubation period: median 8-9 days, minimum 2 days, maximum 21 days
19
20
21
Ebola
Symptoms (including fever) start before infectious period
INCUBATION PERIOD
INCUBATION PERIOD
DISEASE
INFECTION
LATENT PERIOD
INFECTIOUS PERIOD
Ebola
Symptoms (including fever) start before infectious period
INCUBATION PERIOD
INCUBATION PERIOD
DISEASE
INFECTION
LATENT PERIOD
INFECTIOUS PERIOD
Infectiousness increases as disease gets more severe
Infectious period continues up to (and including) death
For survivors, infection in semen up to about 3 months