Human Influenza Epidemiology

Abstract

Influenza virus infections are common in people of all ages. Epidemics occur in the winter months in temperate locations and at varying times of the year in subtropical and tropical locations. Most influenza virus infections cause mild and self-limiting disease, and around one-half of all infections occur with a fever. Only a small minority of infections lead to serious disease requiring hospitalization. During epidemics, the rates of influenza virus infections are typically highest in school-age children. The clinical severity of infections tends to increase at the extremes of age and with the presence of underlying medical conditions, and impact of epidemics is greatest in these groups. Vaccination is the most effective measure to prevent infections, and in recent years influenza vaccines have become the most frequently used vaccines in the world. Nonpharmaceutical public health measures can also be effective in reducing transmission, allowing suppression or mitigation of influenza epidemics and pandemics.

Figure 1.

Age-specific infection attack rate and clinical severity profile of 2009 H1N1pdm09 infections in Hong Kong (Wu et al. 2010b). (A) Estimates of the risk of infection in each age group based on a large serologic study (Wu et al. 2010b). (B) Age-specific estimates of the clinical severity profile of H1N1pdm09 in Hong Kong based on the estimated attack rates and the numbers of severe outcomes recorded in the territory.

Figure 2.

The intended impact of nonpharmaceutical interventions, indicating how “flattening the curve” could allow a greater fraction of patients to be effectively managed by the available health system capacity and could buy time for capacity to be increased.