The infection attack rate and severity of 2009 pandemic influenza (H1N1) in Hong Kong

Background Serial cross-sectional data on antibody levels to 2009 pandemic influenza A (H1N1) virus from a population can be used to estimate the infection attack rates and immunity against future infection in the community. Methods Between April and December 2009, we obtained 12,217 serum specimens from blood donors (16–59 yo), 2,520 from hospital outpatients (5–59yo), and 917 from subjects of a community pediatric cohort study (5–14yo). We estimated infection attack rates by comparing the proportions of specimens with antibody titers ≥1:40 by viral microneutralization before and after the first wave of the pandemic. Estimates were validated using paired sera from 324 individuals that spanned the first wave. Combining these estimates with epidemiologic surveillance data, we calculated the proportion of infections that led to hospitalization, intensive care admission, and death. Results We found that 3.3% and 14% of 5–59 yo had antibody titers ≥1:40 before and after the first wave. The overall attack rate was 10.7% with the following age-stratification: 43.4% in 5–14 yo, 15.8% in 15–19 yo, 11.8% in 20–29 yo, and 4–4.6% in 30–59 yo. Case-hospitalization rates were 0.47%–0.87% among 5–59 yo. Case-ICU and case-fatality rates increased from 7.9 and 0.4 per 100,000 infections in 5–14 yo to 75 and 26.5 per 100,000 infections in 50–59 yo. Conclusions Almost half of all school-children in Hong Kong were infected during the first wave. Compared to school-children aged 5–14, older adults aged 50–59 had 9.5 and 66 times higher risk of ICU admission and death if infected.