For patients with known exposure, cluster reports suggest that the serial interval (time delay between symptom onset in primary and secondary case-patients) could be 7-8 days (online Technical Appendix Table 2).
These conclusions were robust under different assumptions about the duration of serial interval (online Technical Appendix Figures 2, 3).
We also used log-normal distributions that fit serial interval
and time-to-symptom onset ranges found in the literature (Table 1).
In the stylized example in Figure 2, the most likely time difference was 4 days (determined on the basis of the serial interval
distribution, given below the x axis), and the most likely distance is short (<1 km).
The serial interval
of an infectious disease is defined as the time between onset of symptoms in an index patient and onset of symptoms in an infected contact.
This interval was based on a serial interval
(the number of days between symptom onset in the index case-patient and household contacts) of up to 9 days to identify secondary cases, given that shedding of seasonal influenza virus rarely lasts >8 days (7,19) and a median incubation period for seasonal influenza of [approximately equal to] 1.
We calculated the serial interval
as the number of days from the onset date of illness in the index case-patient to onset date of illness in the secondary case-patient.
Changes in the epidemic curve may lag behind changes in the underlying transmission dynamics by at least 1 serial interval
, as has previously been shown for severe acute respiratory syndrome (3-5).
We used a Weibull model for the serial interval
with mean of 3.
The incubation period for influenza averages 2 days (range 1-4 days), and the serial interval
(the mean interval between onset of illness in 2 successive patients in a chain of transmission) is 2-4 days.
Early isolation of patients and quarantine of contacts successfully interrupted SARS transmission, but influenza's shorter serial interval
and earlier peak infectivity, plus the presence of mild cases and possibility of transmission without symptoms, suggest that these measures would be considerably less successful than they were for SARS (3,11,12).
4 days as the generation time of a SARS infection, as estimated from the mean serial interval
between the time from onset of symptoms in index patient to onset of symptoms in secondary case-patient in Singapore (9), we estimated the basic reproductive number of SARS infections, [R.