Pandemic Potential of a Strain of Influenza A (H1N1): Early Findings

Abstract

A novel influenza A (H1N1) virus has spread rapidly across the globe. Judging its pandemic potential is difficult with limited data, but nevertheless essential to inform appropriate health responses. By analyzing the outbreak in Mexico, early data on international spread, and viral genetic diversity, we make an early assessment of transmissibility and severity. Our estimates suggest that 23,000 (range 6000 to 32,000) individuals had been infected in Mexico by late April, giving an estimated case fatality ratio (CFR) of 0.4% (range: 0.3 to 1.8%) based on confirmed and suspected deaths reported to that time. In a community outbreak in the small community of La Gloria, Veracruz, no deaths were attributed to infection, giving an upper 95% bound on CFR of 0.6%. Thus, although substantial uncertainty remains, clinical severity appears less than that seen in the 1918 influenza pandemic but comparable with that seen in the 1957 pandemic. Clinical attack rates in children in La Gloria were twice that in adults (<15 years of age: 61%; ≥15 years: 29%). Three different epidemiological analyses gave basic reproduction number (R0) estimates in the range of 1.4 to 1.6, whereas a genetic analysis gave a central estimate of 1.2. This range of values is consistent with 14 to 73 generations of human-to-human transmission having occurred in Mexico to late April. Transmissibility is therefore substantially higher than that of seasonal flu, and comparable with lower estimates of R0 obtained from previous influenza pandemics.

Absence of cross-reactive antibodies to influenza A (H1N1) 2009 before and after vaccination with 2009 Southern Hemisphere seasonal trivalent influenza vaccine in children aged 6 months-9 years: a prospective study

“…In Melbourne, as elsewhere, children played a key role in fuelling the initial phase of the 2009 winter outbreak 3 , 4 , 5 , 7 . These data support Australian Government recommendations for the immunisation of children with monovalent A (H1N1) 2009 vaccines both for direct protection and to limit infection transmission beyond the 2009 Southern Hemisphere Influenza season (http://www.healthemergency.gov.au).…”

“…Early epidemiologic reports indicated higher symptomatic attack rates in children than adults, with variable experience of disease severity 2 , 3 , 4 . Established transmission of the new strain within Australia, one of the earliest countries in which the infection was introduced during the Southern Hemisphere winter, was first described among school children in the Northern suburbs of Melbourne 5 .…”

Estimating age‐specific cumulative incidence for the 2009 influenza pandemic: a meta‐analysis of A(H1N1)pdm09 serological studies from 19 countries

“…Together with early investigations elucidating age‐specific clinical attack rates13, 14 and transmission characteristics15 of the new virus, these studies provided critical input into and reduced uncertainty around national and global policy decisions. Numerous seroepidemiological studies have subsequently been published, but the comparison and direct interpretation of the results of serological studies is difficult due to the varied epidemiological methods used to collect sera, the heterogeneity in the populations under study, variation in laboratory assays used, and criteria for seropositivity 12, 16…”

New Influenza A/H1N1 (“Swine Flu”): information needs of airport passengers and staff

“…Frankfurt airport is one of the world’s major air travel hubs. While the main peaks of that pandemic are now over, the results may have implications for how we react to future outbreaks and evolving pandemics 8 . In addition to contrasting the potential information needs of passengers and staff, we also assessed whether their level of fear or concern about the swine flu outbreak, and whether passengers were arriving from or going to a region of the world that was heavily affected by swine flu, had any effect on the desire for more information or the type of information that was requested.…”