Fact box on flu vaccination for healthy young adults

The flu is a disease caused by the influenza virus. Symptoms of influenza typically include high fever (between 38°C and 40°C or higher), headache, aching muscles, and backache, as well as cough, cold, sore throat, and hoarseness [1, 2, 3].

People with flu-like symptoms are often referred to as having the flu, without being tested for the influenza virus. Flu-like illnesses primarily include influenza and other common respiratory illnesses whose symptoms (e.g. high fever, cough) are so similar to those of influenza that the illness can only be distinguished from influenza by laboratory tests of a saliva swab or blood [3].

Influenza-like illnesses such as colds cannot be prevented by the flu vaccination.

Influenza viruses are mainly transmitted by droplets containing the virus, which are expelled especially when sneezing or coughing and can enter the respiratory tract of other people over a short distance. Contact with surfaces (e.g. door handles) or hands contaminated with secretions (saliva, sputum) that contain the virus can also cause infection [2].

For the flu vaccination, inactive or weakened influenza viruses are injected into the muscle or under the skin. The immune system then forms antibodies for that particular virus. If a person subsequently comes into contact with an active influenza virus of the same type, the immune system can react more quickly to the virus and fight it [4].

The Standing Committee on Vaccination (STIKO) of the Robert Koch Institute (RKI) particularly recommends that people with an increased risk of a severe course of the disease (e.g. older people and those with chronic illness) be vaccinated annually for seasonal influenza [2].

In past years, the annual wave of influenza has usually started after the turn of the year. Since it takes about 10 to 14 days after vaccination for the body to build up sufficient antibodies, the STIKO recommends getting vaccinated in October or November. However, it can still be useful to have the vaccination at a later time (e.g. during the flu wave) [2].

Adherence to hygiene measures, e.g. regular hand washing (especially before and after contact with people at risk) and disinfection of surfaces and objects that are likely to be contaminated (e.g. door handles) can protect against infection. Keeping away from people with flu-like symptoms and taking measures to strengthen the body's own defences (e.g. balanced diet, exercise) can also reduce the risk of infection [1].

The fact box compares flu vaccination and no vaccination in terms of their benefits and side effects in healthy adolescents and adults aged 16 to 65. 

The table may be read as follows:

For every 100 healthy young adults without vaccination, about 2 suffered from confirmed flu in a flu season with a low spread of the influenza virus. In contrast, out of every 100 adults with flu vaccination, 1 suffered from confirmed flu in the same season [5]. This means that 1 out of every 100 healthy young adults can be protected from influenza by the flu vaccination - in a flu season with a low spread of the influenza virus. 

The numbers in the fact box are rounded. The figures on benefits come from randomized controlled trials with a total of about 71,000 participants. The figures for side effects come from randomized controlled trials with a total of about 35,000 (muscular pain), 24,000 (fever), and 36,000 (headache) participants.

Erklärung der Symbole: X = für diese Personen gelten die Zahlen in der Faktenbox; (X) = auf diese Personen lassen sich die Zahlen unter Vorbehalt anwenden (in solchen Fällen ist eine Rücksprache mit ärztlichem Personal empfehlenswert); - = für diese Personen gelten die Zahlen nicht; ? = es ist unbekannt, ob die Zahlen für diese Personen gelten

The number of people who become infected with the influenza virus during a flu season varies. Not every person who is infected with the influenza virus becomes ill. It is estimated that between 5 and 20 per cent of the population become infected each season (depending how widespread the influenza virus is) [5]. The actual severity of a flu session cannot be accurately estimated before a session begins.

The fluctuating benefit of influenza vaccination thus depends directly on which influenza viruses are prevalent. where and how strongly they spread, and whether the vaccine used in the respective flu season matches these viruses. Because the most prevalent viruses differ each year, it is impossible to accurately predict the benefit of vaccination in preventing the influenza virus that year or when a new virus emerges [6].

Even years where the influenza vaccine is less effective, many (severe) cases of illness can nevertheless be prevented due to the frequency of influenza. In Germany, even with the current modest vaccination rates, it is estimated that about 400,000 cases of influenza are prevented each year in people over 60 years of age [7].

The spread of the influenza virus in the 2020/2021 influenza season was particularly low due to the protective measures implemented against the coronavirus. In Germany, a total of 564 influenza cases were reported to the RKI by 21 May 2021, including severe courses of the disease and 16 deaths [8].

The state of evidence was determined by the authors of the included review. According to their assessment, the overall quality of the evidence is low. It is very likely that the findings on flu vaccination protection in relation to the seasonal spread of the influenza virus and vaccine-related side effects will be modified by further research.

• July 2021 (update of the search, no new evidence; update of the accompanying text)

• October 2020 (update of the search, no new evidence; update of the accompanying text)

• December 2019 (update of the accompanying text)

• July 2019 (update of the evidence, update of the accompanying text)

• August 2016 (development)

Information for the fact box was obtained from the following sources:

[1] Demicheli, V., Jefferson, T., Ferroni, E., Rivetti, A. & Di Pietrantonj, C. (2018). Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews. doi: 10.1002/14651858.cd001269.pub6.

[2] Robert Koch Institute (RKI) (2020a). Current data and information on infectious diseases and public health. Vaccination status of children and adolescents in Germany. STIKO: Influenza vaccinations in the COVID-19 pandemic. Epid Bull 32/33. https://www.rki.de/DE/Content/Infekt/EpidBull/Archiv/2020/Ausgaben/32-33_20.pdf (08.06.2021).  

[3] Institute for Quality and Efficiency in Health Care (IQWiG). (2019a, October 23). Flu. gesundheitsinformation.de. https://www.gesundheitsinformation.de/grippe.html (08.06.2021).

[4] Buda, S., Prahm, K., Dürrwald, R., Biere, B., Schilling, J., Buchholz, U., & Haas, W. (2018). Report on the epidemiology of influenza in Germany season 2017/18. doi: 10.17886/rkipubl-2018-003. 

[5] Robert Koch Institute (RKI). (2019, 30 January). Influenza - Frequently asked questions and answers about influenza. rki.de. https://www.rki.de/SharedDocs/FAQ/Influenza/FAQ_Liste.html;jsessionid=D138845A71D1096528DD8AD29856C00E.internet052?nn=2370434 (10.06.2021).

[6] Institute for Quality and Efficiency in Health Care (IQWiG). (2019b, 23 October). How much protection does a flu vaccination offer? gesundheitsinformation.de. https://www.gesundheitsinformation.de/wie-viel-schutz-bietet-eine-grippeimpfung.html (08.06.2021).

[7] Robert Koch Institute (RKI) (2020b). Kurz & Knapp: Faktenblätter zum Impffen. Influenza vaccination. https://www.rki.de/DE/Content/Infekt/Impfen/Materialien/Faktenblaetter/Influenza.pdf? (08.06.2021). 

[8] Buda, S., Dürrwald, R., Biere, B., Buchholz, U., Tolksdorf, K., Schilling, J., Goerlitz, L., Streib, V., Preuß, U., Prahm, K., Haas, W. & AGI Study Group (2021). Influenza weekly report week 20/2021; Arbeitsgemeinschaft Influenza - Robert Koch-Institut. https://edoc.rki.de/bitstream/handle/176904/8276/Influenza_Wochenbericht_KW20_2021.pdf (08.06.2021).

Documentation on how the numbers in the fact box were determined is available on request.