Influenza vaccines, also known as flu jabs or flu shots, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their effectiveness varies from year to year, most provide modest to high protection against influenza. The United States Centers for Disease Control and Prevention (CDC) estimates that vaccination against influenza reduces sickness, medical visits, hospitalizations, and deaths. Immunized workers who do catch the flu return to work half a day sooner on average. Vaccine effectiveness in those over 65 years old remains uncertain due to a lack of high-quality research. Vaccinating children may protect those around them.
Vaccination against influenza began in the 1930s, with large-scale availability in the United States beginning in 1945. It is on the World Health Organization’s List of Essential Medicines.
The World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) recommend yearly vaccination for nearly all people over the age of six months, especially those at high risk. The European Centre for Disease Prevention and Control (ECDC) also recommends yearly vaccination of high risk groups. These groups include pregnant women, the elderly, children between six months and five years of age, and those with certain health problems.
The vaccines are generally safe; fever occurs in five to ten percent of children vaccinated, and temporary muscle pains or feelings of tiredness may occur. In certain years, the vaccine was linked to an increase in Guillain–Barré syndrome among older people at a rate of about one case per million doses. Although most influenza vaccines are produced using egg proteins, they are still recommended as safe for people who have severe egg allergies, as no increased risk of allergic reaction to the egg-based vaccines has been shown for people with egg allergies. Vaccines produced using other technologies, notably recombinant vaccines and those based on cell culture rather than egg protein, started to become available from 2012 in the US, and later in Europe and Australia. Influenza vaccines are not recommended in those who have had a severe allergy to previous versions of the vaccine itself. The vaccine comes in inactive and weakened viral forms. The live, weakened vaccine is generally not recommended in pregnant women, children less than two years old, adults older than 50, or people with a weakened immune system. Depending on the type they can be injected into a muscle, sprayed into the nose, or injected into the middle layer of the skin (intradermal). The intradermal vaccine was not available during the 2018–2019 and 2019–2020 influenza seasons.
Vaccines are used in both humans and nonhumans. Human vaccine is meant unless specifically identified as a veterinary, poultry or livestock vaccine.
In the worldwide Spanish flu pandemic of 1918, “Pharmacists tried everything they knew, everything they had ever heard of, from the ancient art of bleeding patients, to administering oxygen, to developing new vaccines and serums (chiefly against what we now call Hemophilus influenzae – a name derived from the fact that it was originally considered the etiological agent – and several types of pneumococci). Only one therapeutic measure, transfusing blood from recovered patients to new victims, showed any hint of success.”
In 1931, viral growth in embryonated hens’ eggs was reported by Ernest William Goodpasture and colleagues at Vanderbilt University. The work was extended to growth of influenza virus by several workers, including Thomas Francis, Jonas Salk, Wilson Smith and Macfarlane Burnet, leading to the first experimental influenza vaccines. In the 1940s, the US military developed the first approved inactivated vaccines for influenza, which were used in the Second World War. Hen’s eggs continued to be used to produce virus used in influenza vaccines, but manufacturers made improvements in the purity of the virus by developing improved processes to remove egg proteins and to reduce systemic reactivity of the vaccine. In 2012, the US Food and Drug Administration (FDA) approved influenza vaccines made by growing virus in cell cultures and influenza vaccines made from recombinant proteins have been approved, with plant-based influenza vaccines being tested[when?] in clinical trials.