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

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).