Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to serious complications like pneumonia, or even death. There are four main types of influenza viruses: A, B, C, and D. Influenza A and B viruses are the types that cause the seasonal epidemics of disease in humans almost every winter. Influenza A viruses are further divided into subtypes based on two proteins on the surface of the virus (like H1N1 and H3N2), while influenza B viruses are broken down into specific lineages. These viruses are constantly changing, which is why new flu vaccines are needed each year. In the United States, the flu is a significant public health concern annually, with the Centers for Disease Control and Prevention (CDC) estimating it causes between 9 million and 45 million illnesses, between 140 000 and 710 000 hospitalizations, and between 12 000 and 52 000 deaths each year, depending on the season’s severity.  

While flu pandemics had occurred before, scientists didn’t actually isolate the virus responsible for human flu until 1933. A British team, including Wilson Smith, Christopher Andrewes, and Patrick Laidlaw working at the National Institute for Medical Research (NIMR) in London, were the first to identify the influenza A virus. Understanding the virus was the first step towards preventing it. The push for a vaccine gained urgency during World War II, as military leaders feared flu outbreaks could cripple the armed forces. The U.S. Army established a specific group, the Army Epidemiological Board’s Commission on Influenza, to tackle this problem. This commission funded and directed research led by Dr. Thomas Francis Jr. (who headed the commission) and his junior colleague Dr. Jonas Salk at the University of Michigan. They developed the first effective inactivated flu vaccine, using killed virus. This vaccine was first tested for safety and effectiveness on U.S. military personnel before being licensed for civilian use in 1945.  

Today, flu vaccination is recommended annually for most people aged 6 months and older in the United States. Each year, tens of millions of Americans get their flu shot, although vaccination rates vary by age group and season, often hovering around 40-60% overall coverage. While getting vaccinated isn’t a guarantee against catching the flu, it’s the best way to prevent it and its potentially serious complications. The CDC estimates that flu vaccination prevents millions of illnesses and tens of thousands of hospitalizations each year. For example, during the 2019-2020 season alone, flu vaccination was estimated to have prevented about 7.5 million influenza illnesses, 3.7 million flu-associated medical visits, 105 000 hospitalizations, and 6 300 deaths in the U.S. These numbers highlight the significant public health benefit of widespread annual flu vaccination.