In 1918, the influenza, or flu, virus infected over 20 percent of the world’s population and lead to over twenty million deaths. The United States alone had five hundred thousand deaths. This pandemic (an epidemic across continents or of worldwide proportions) was caused by a sudden shift in the antigens on the influenza virus. Usually the flu antigens change gradually in a process called antigenic drift. This allows people to retain some protection against the flu because they have seen some of the antigens before.
However, if the antigens change suddenly, in a process called antigenic shift, then the virus is effectively a new disease because people have no past immunity to the new antigens. Whenever antigenic shift occurs, more people will become ill, and the disease will spread more widely. There have been several pandemics in the past, and the World Health Organization is currently preparing for the next one.
The first flu vaccine was developed in the 1940s, with newer versions, such as the intranasal flu vaccine, developed since then. The recommendations for receiving the flu vaccine have broadened over time. The vaccine used to be reserved for people over age sixty-five or with chronic medical conditions. In the past ten years, the recommendations have expanded to include adults over age fifty, then children under age two, then children under five, and now all children between six months and eighteen years of age.
One interesting historical note involves the swine flu vaccine of 1976–77. The experts were concerned that this particular flu would be the next pandemic and strongly recommended the widespread distribution of the vaccine across the United States. Unfortunately, the vaccine had the rare but serious side effect of causing Guillain-Barré syndrome (GBS) in one out of every one hundred thousand recipients (as compared to the background population rate of one in a million people). Even though it was a rare side effect, the mass vaccination program was halted in midwinter.