Moreover, estimates of the serial interval
(i.e., time between index case and symptom onset in secondary infection) are limited to pandemic influenza in nonhousehold settings (6,12-15).
According to the case-ascertained study by Gordon and colleagues, which of the following statements about overall risk for influenza infection in household contacts, mean serial interval
for within-household transmission, and transmissibility of different strains in low-income urban households in Managua, Nicaragua, is correct?
The probability of observing additional cases was derived by using the serial interval
; that is, the time from illness onset in the primary case-patient to illness onset in a secondary case-patient, and the transmissibility of MERS (online Technical Appendix, http://wwwnc.cdc.gov/EID/article/22/1/15-1383-Techapp1.pdf).
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).
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.
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 3.6 days and standard deviation of 1.6 days, based on data from a recent community study (10).
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).