Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study

Abstract

Background: Assessment 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.

Methods: Using 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.

Findings: Based 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.

Interpretation: Seasonal 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.

Funding: Medical Research Council and the Wellcome Trust.

Figure 1

Rates of illness or PCR-confirmed influenza standardised by age and region Rates of acute respiratory illness, influenza-like illness, and PCR-confirmed influenza per 100 000 person-weeks. Excludes illnesses known to be due to non-influenza viruses.

Figure 2

Rates of seasonal and pandemic influenza A infection and PCR-confirmed disease Rates of infection established through seroconversion (four-fold titre rises in unvaccinated individuals) and rates of disease established through PCR-confirmation per 100 person-seasons (95% CIs).

Figure 3

Number of expected events in a surveillance practice serving a population of 10 000 people Data for a typical influenza season.

Figure 4

Immune profiles to influenza A(H1N1)pdm09 before each influenza season. Profiles before first wave (no vaccine; A), after second wave and before third wave (B), and after third wave (C). HI=haemagglutinin inhibition.