Abstract
SummaryBackgroundAssessment of the effect of influenza on populations, including risk of infection, illness if infected, illness severity, and consultation rates, is essential to inform future control and prevention. We aimed to compare the community burden and severity of seasonal and pandemic influenza across different age groups and study years and gain insight into the extent to which traditional surveillance underestimates this burden.MethodsUsing preseason and postseason serology, weekly illness reporting, and RT-PCR identification of influenza from nasal swabs, we tracked the course of seasonal and pandemic influenza over five successive cohorts (England 2006–11; 5448 person-seasons' follow-up). We compared burden and severity of seasonal and pandemic strains. We weighted analyses to the age and regional structure of England to give nationally representative estimates. We compared symptom profiles over the first week of illness for different strains of PCR-confirmed influenza and non-influenza viruses using ordinal logistic regression with symptom severity grade as the outcome variable.FindingsBased on four-fold titre rises in strain-specific serology, on average influenza infected 18% (95% CI 16–22) of unvaccinated people each winter. Of those infected there were 69 respiratory illnesses per 100 person-influenza-seasons compared with 44 per 100 in those not infected with influenza. The age-adjusted attributable rate of illness if infected was 23 illnesses per 100 person-seasons (13–34), suggesting most influenza infections are asymptomatic. 25% (18–35) of all people with serologically confirmed infections had PCR-confirmed disease. 17% (10–26) of people with PCR-confirmed influenza had medically attended illness. These figures did not differ significantly when comparing pandemic with seasonal influenza. Of PCR-confirmed cases, people infected with the 2009 pandemic strain had markedly less severe symptoms than those infected with seasonal H3N2.InterpretationSeasonal influenza and the 2009 pandemic strain were characterised by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation. In the community the 2009 pandemic strain caused milder symptoms than seasonal H3N2.FundingMedical Research Council and the Wellcome Trust.
Highlights
Influenza causes roughly 250 000–500 000 deaths worldwide each year.[1]
WHO declared an end to the pandemic on Aug 10, 2010.8 a further pandemic wave occurred in some European and other countries outside North America[9] in 2010–11 with reports of excess deaths in, for example, England.[10]
There have been some cohort studies reporting on the 2009 pandemic from Hong Kong, southeast Asia, and Mali[21,22,23,24,25] as well as Lancet Respir Med 2014; 2: 445–54
Summary
Influenza causes roughly 250 000–500 000 deaths worldwide each year.[1]. In the 20th century there were three influenza pandemics for which there are varying mortality estimates: 1918 A/H1N1 at least 20–40 million excess deaths, 1957 A/H2N2 about 4 million excess deaths, and 1968 A/H3N2 about 2 million excess deaths.[2,3,4] In 2009 a new pandemic virus,[5] influenza A(H1N1)pdm[09], emerged in Mexico[6] and spread globally over 2009–10, causing an estimated 200 000 respiratory deaths and 83 000 cardiovascular deaths during the first 12 months of circulation.[7]. Underestimation of the number of community cases leads to overestimates of severity.[12,13] Heightened concern during a pandemic can change patient consultation thresholds and clinician recording and investigation behaviour, distorting surveillance information.[14] Information on the community burden of influenza is key to informing control,[15] but is not routinely collected. The available data for periods of seasonal influenza are largely derived historically from household cohort studies of families with children in communities in the USA between 1948 and 1981,16–19 and a more recent study from rural Vietnam.[20] There have been some cohort studies reporting on the 2009 pandemic from Hong Kong, southeast Asia, and Mali[21,22,23,24,25] as well as Lancet Respir Med 2014; 2: 445–54
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