By John K. Borchardt Managing modern influenza vaccines is often as much an administrative art as a science. The most devastating of the 20th-century influenza pandemics killed more than 20 million people worldwide between 1918 and 1920. Influenza activity typically peaks during December through March in temperate regions of the Northern Hemisphere and May through August in temperate regions of the Southern Hemisphere. In tropical regions, influenza typically circulates year-round.
Influenza viruses cause disease in all age groups.
Although children have the highest infection rates, rates of serious illness and death are highest among people older than 65 and people of any age with medical conditions, such as asthma, that place them at high risk for complications from influenza. The most effective way to reduce the impact of influenza is to vaccinate people at high risk for complications shortly before the influenza season each year. Influenza B viruses are not categorized into subtypes. Influenza A and B viruses are further separated into groups on the basis of antigenic characteristics. New influenza virus variants result from frequent antigenic changes that occur during viral replication.