Transmission parameters of the A/H1N1 (2009) influenza virus pandemic: a review

Abstract

Background: The new influenza virus A/H1N1 (2009), identified in mid-2009, rapidly spread over the world. Estimating the transmissibility of this new virus was a public health priority.

Methods: We reviewed all studies presenting estimates of the serial interval or generation time and the reproduction number of the A/H1N1 (2009) virus infection.

Results: Thirteen studies documented the serial interval from household or close-contact studies, with overall mean 3 days (95% CI: 2·4, 3·6); taking into account tertiary transmission reduced this estimate to 2·6 days. Model-based estimates were more variable, from 1·9 to 6 days. Twenty-four studies reported reproduction numbers for community-based epidemics at the town or country level. The range was 1·2-3·1, with larger estimates reported at the beginning of the pandemic. Accounting for under-reporting in the early period of the pandemic and limiting variation because of the choice of the generation time interval, the reproduction number was between 1·2 and 2·3 with median 1·5.

Discussion: The serial interval of A/H1N1 (2009) flu was typically short, with mean value similar to the seasonal flu. The estimates of the reproduction number were more variable. Compared with past influenza pandemics, the median reproduction number was similar (1968) or slightly smaller (1889, 1918, 1957).

Figure 1

Mean serial intervals (red) or generation time (black) estimated for the A/H1N1 2009 pandemic, with 95% confidence interval. For serial intervals estimated in close contacts, the number of pairs infector/infectee n is coded by the size of the symbol. The dashed line is the weighted mean of mean SI in households and close‐contacts studies; diamond shows the 95% confidence interval (*median SI).

Figure 2

Reproduction number of pandemic influenza. (left) Estimates from the last five influenza pandemics (box plots show the first and third quartiles and median as thick line, see discussion for list of references). For 2009, only estimates corrected for under‐reporting and mean GT approximately 3 days were shown (right) Estimates for the A/H1N1 (2009) pandemic according to location and date of publication.