Influenza vaccine

Influenza vaccines are essential medical tools used to protect people from infection by influenza viruses. These viruses cause the flu, a respiratory illness that can range from mild to severe. Because the virus undergoes a process called antigenic drift, it changes its structure through small mutations. This constant change means that new vaccine versions must be developed twice a year to match the circulating strains. Most vaccines provide modest to high protection against the disease. Even if a person catches a strain not covered by the vaccine, the shot can reduce the severity of the illness.

To work effectively, the vaccine triggers the body to create protective antibodies. This biological process takes about two weeks to complete following vaccination. There are several ways these vaccines are administered to patients. Some are inactivated, meaning the virus is not alive, while others use weakened viral forms. These can be given as an intramuscular injection into a muscle or an intradermal injection into the middle layer of the skin. Another method is the intranasal spray, which is applied into the nose.

The history of influenza vaccination is a journey of scientific discovery. During the 1918 Spanish flu pandemic, medical professionals tried many treatments, including oxygen and blood transfusions. A major breakthrough occurred in 1931 when Ernest William Goodpasture and his colleagues at Vanderbilt University reported that viruses could grow in embryonated hens' eggs.

This method was expanded by scientists like Thomas Francis, Jonas Salk, Wilson Smith, and Macfarlane Burnet to create the first experimental vaccines. By the 1940s, the US military had developed the first approved inactivated vaccines for use during World War II.

Manufacturing technology has evolved significantly since the 1950s when egg-based production became common. In 2012, the US Food and Drug Administration (FDA) approved vaccines grown in cell cultures. In 2013, they approved vaccines made from recombinant proteins. Many modern vaccines are now quadrivalent, meaning they target four different lineages of the virus. However, the composition of these vaccines changes over time. For example, the B/Yamagata lineage of influenza B may have become extinct due to COVID-19 pandemic measures. Because no natural cases were confirmed after March 2020, the FDA recommended removing B/Yamagata from vaccines for the 2024–2025 season.

Vaccine effectiveness is measured in two ways: efficacy and effectiveness. Efficacy is how well a vaccine works under controlled conditions, while effectiveness is how it performs in the real world. Real-world studies are difficult because influenza-like illnesses are often confused with the actual flu. On average, the efficacy against symptomatic disease is between 40% and 60%. Effectiveness can vary greatly by year. For instance, in the 2004 US season, effectiveness was 10%, but it reached 60% in 2010. In the 2019–2020 US season, effectiveness was 41% overall and 54% for people aged 65 or older.

Public health organizations provide specific recommendations for who should receive the vaccine. The World Health Organization and the US Centers for Disease Control and Prevention (CDC) recommend yearly vaccination for nearly all people over six months old. This is especially important for high-risk groups. These groups include pregnant women, the elderly, children between six months and five years, and people with certain health problems. The European Centre for Disease Prevention and Control (ECDC) also emphasizes yearly shots for these high-risk populations. Vaccination in school-age children can also provide indirect protection for other age groups in the community.

While vaccines are generally safe, they can have side effects. Common reactions include soreness at the injection site, fever, tiredness, or temporary muscle pains. In children, fever occurs in about five to ten percent of those vaccinated. In certain years, a link was found between the vaccine and an increase in Guillain–Barré syndrome among older people, occurring at a rate of about one case per million doses. However, the vaccine is considered highly beneficial for reducing mortality and hospitalization rates. One 2016 study even found that getting a flu shot was as effective or better at preventing a heart attack than quitting smoking.