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

Abstract

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 1·51 (1·32–1·71) based on suspected cases and 1·43 (1·29–1·57) based on confirmed cases before 20 April. The longer time series (through 25 April) yielded a higher estimate of 2·04 (1·84–2·25), which reduced to 1·44 (1·38–1·51) 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.

Prevalence of seroprotection against the pandemic (H1N1) virus after the 2009 pandemic

“…Various estimates of R o have been derived for the pandemic (H1N1) virus, ranging up to 3.0 in the setting of a school outbreak, but are generally between 1.5 and 2.0 on average. [16][17][18][19][20] On that basis, community-level protection against the pandemic (H1N1) virus in the range of 33%-50% overall would be enough to prevent an epidemic, assuming random mixing of people of different ages and homogeneous immunity. School-aged children, however, are thought to contribute disproportionately to influenza transmission.…”

“…About 100 samples were assembled for each of the following age groups: < 5, 5-9, 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80-89 and ≥ 90 years. To enable finer stratification in pediatric age groups, additional samples were collected between June 15 and 17, 2010, from children less than two years (7) and two to four years of age (20), which gave us totals of 26 and 100 serum samples in these two youngest age groups.…”

Simulating school closure policies for cost effective pandemic decision making

“…On the other hand, estimates for the basic reproduction number (R 0 ) of 2009 H1N1 pandemic influenza (and other strains of pandemic flu) range between 1.1 and 2.1 [33]. Accordingly, we considered two transmission rate scenarios: R 0 values are sampled from age-specific triangular distributions with values ranging either from 1.1 to 1.5 (low transmission scenario) with mean 1.3 or from 1.5 to 2.1 (high transmission scenario) with mean 1.8 (Table 1).…”

Modelling mitigation strategies for pandemic (H1N1) 2009

“…The basic reproductive number (R 0 ) is the average number of secondary infections produced by an infected person in an otherwise susceptible population. 4,[22][23][24] This important index determines the epidemic potential of an infectious disease. Simulations using the calibrated parameters gave an R 0 value of 1.8 (Appendix 1).…”