Initial human transmission dynamics of the pandemic (H1N1) 2009 virus in North America

Background Between 5 and 25 April 2009, pandemic (H1N1) 2009 caused a substantial, severe outbreak in Mexico, and subsequently developed into the first global pandemic in 41 years. We determined the reproduction number of pandemic (H1N1) 2009 by analyzing the dynamics of the complete case series in Mexico City during this early period. Methods We analyzed three mutually exclusive datasets from Mexico City Distrito Federal which constituted all suspect cases from 15 March to 25 April: confirmed pandemic (H1N1) 2009 infections, non-pandemic influenza A infections and patients who tested negative for influenza. We estimated the initial reproduction number from 497 suspect cases identified prior to 20 April, using a novel contact network methodology incorporating dates of symptom onset and hospitalization, variation in contact rates, extrinsic sociological factors, and uncertainties in underreporting and disease progression. We tested the robustness of this estimate using both the subset of laboratory-confirmed pandemic (H1N1) 2009 infections and an extended case series through 25 April, adjusted for suspected ascertainment bias. Results The initial reproduction number (95% confidence interval range) for this novel virus is 1 center dot 51 (1 center dot 32-1 center dot 71) based on suspected cases and 1 center dot 43 (1 center dot 29-1 center dot 57) based on confirmed cases before 20 April. The longer time series (through 25 April) yielded a higher estimate of 2 center dot 04 (1 center dot 84-2 center dot 25), which reduced to 1 center dot 44 (1 center dot 38-1 center dot 51) after correction for ascertainment bias. Conclusions The estimated transmission characteristics of pandemic (H1N1) 2009 suggest that pharmaceutical and non-pharmaceutical mitigation measures may appreciably limit its spread prior the development of an effective vaccine.