High incidence of severe influenza among individuals over 50 years of age

Abstract

Age-specific epidemiological data on asymptomatic, symptomatic, and severe infections are essential for public health policies on combating influenza. In this study, we incorporated data on microbiologically confirmed infections and seroprevalence to comprehensively describe the epidemiology of pandemic H1N1 2009 influenza. Seroprevalence was determined from 1,795 random serum samples collected in our hospital in January 2007 (before the first wave of the pandemic) and March 2010 (after the second wave). Data on microbiologically confirmed infection and severe cases were obtained from the Centre for Health Protection in Hong Kong. Severe cases were most common in the 51- to 60-year-old age group. The microbiologically confirmed incidence rate was highest for children aged ≤10 years and dropped sharply for the adult population (ρ = -1.0; P < 0.01), but the incidence rate for severe disease was highest for the 51- to 60-year-old age group. For the 51- to 60-year-old age group, the seroprevalence was similar to that for the younger age groups, but the proportion of severe cases relative to seroprevalence was significantly higher than that for 11- to 50-year-old age groups. As judged from the percentage of specimens positive for other respiratory viruses compared with that for pandemic H1N1 virus, the impact of symptomatic disease due to pandemic H1N1 virus was higher than that for other respiratory viruses in people aged ≤50 years. In conclusion, the 51- to 60-year-old age group, which had the highest overall incidence and the highest rate of severe disease but is currently not considered by the World Health Organization to be an at-risk group, should be prioritized for influenza vaccination in areas where universal influenza vaccination is not practiced.

Fig. 1.

Proportions of seropositive individuals in 2007 and 2010. The y axis represents the percentage of seropositive individuals detected (those with hemagglutination inhibition antibody titers of ≥40).

Fig. 2.

Incidence of severe cases of pandemic H1N1 2009 influenza virus infection. The y axis represents the absolute number of severe cases, defined by admission to the intensive care unit, a critical condition requiring assisted ventilation, or a change of severity from critical to fatal.

Fig. 3.

Microbiologically confirmed incidence rates of pandemic H1N1 2009 influenza (per 100,000 population members). The y axis on the left represents the incidence rate of all cases with a positive virological test for pandemic H1N1 2009 influenza virus, while the y axis on the right shows the incidence rate of all severe cases.

Fig. 4.

Burden of symptomatic or severe disease. The infected population was estimated by seroprevalence. The y axis on the left represents the burden for all cases, while the y axis on the right shows the burden for severe cases.

Fig. 5.

Proportions of clinical specimens positive for pandemic H1N1 2009 influenza virus and other respiratory viruses. The y axis represents the percentage of clinical specimens that tested positive for influenza virus or other respiratory viruses.