Volume 25, Number 10—October 2019
Research Letter
Estimated Incubation Period and Serial Interval for Human-to-Human Influenza A(H7N9) Virus Transmission
, and Timothy M. Uyeki
Table
Estimated incubation periods for avian influenza A(H7N9) virus infection in the setting of probable human-to-human transmission among 14 epidemiologically linked clusters of case-patients from 5 epidemic waves in mainland China, 2013–2017*
| All, n =14 | First, n = 2 | Second, n = 3 | Third, n = 2 | Fourth, n = 3 | Fifth, n = 4 | |
|---|---|---|---|---|---|---|
| Overall | 4 (1–12) | 6.5 (1–12); 0.297 | 3.5 (1–7); 0.295 | 4.5 (1–7); 0.857 | 6 (1–11); 0.517 | 3.5 (1–8); 0.735 |
| Minimum | 1 (1–7) | 3 (1–5); 0.830 | 1 (1–3); 0.519 | 2 (1–3); 0.914 | 1 (1–7); 1.000 | 2 (1–6); 0.581 |
| Maximum | 6.5 (3–12) | 10 (8–12); 0.072 | 4 (4–7); 0.199 | 6.5 (6–7); 0.920 | 8 (5–11); 0.304 | 4 (3–8); 0.231 |
*After the first epidemic wave of infections, defined as March–August 2013, an epidemic wave was defined as September 1–August 31 of the following year. Thirteen secondary case-patients had multiple exposure dates and 1 secondary case-patient had 1 exposure date to an index case-patient. The incubation period for secondary case-patients was defined as the time in days from the date of an unprotected exposure within 1 meter to an index case-patient for any duration beginning at the earliest before illness onset of the index case-patient to the date of illness onset of the secondary case-patient. For secondary case-patients with multiple days of exposure to an ill index case-patient, we used the earliest exposure date to define the maximum incubation period and last exposure date (such as the date of hospital isolation of the index case-patient) to define the minimum incubation period. We compared the median incubation period for each epidemic wave with the 4 other epidemic waves. We used Wilcoxon rank-sum test to compare the distribution of median incubation periods; a p value <0.05 was considered statistically significant.