How much protection do flu vaccinations offer?
In most years healthy people are unlikely to get the flu (“influenza”), regardless of whether they have been vaccinated. Vaccination can reduce the risk of getting influenza by half or more, though. This can make a big difference in years when the risk of infection is high.
A flu can cause fever, chills, muscle ache and cold symptoms. People who have the flu usually feel very ill for about a week. Only rarely does the flu cause serious complications, such as pneumonia. Infants, toddlers, people who have certain medical conditions, pregnant women and older people have a higher risk of those types of complications, though, because their immune system is weaker than a typical healthy adult's is.
How flu vaccinations work
Flu vaccinations involve injecting a weakened or inactive form of the virus under the skin or into a muscle. After this happens, the immune system starts producing antibodies to fight the virus. Then, if a real virus comes along that is exactly the same as the one introduced by the vaccine, the body recognizes it and can fight it off.
Because flu viruses are constantly changing (mutating) and completely new forms of viruses also arise, one flu vaccination cannot provide permanent protection. So if you want to be sufficiently protected you have to have a different vaccination every year.
It is best to get vaccinated before flu season
After having a flu shot, it takes about 14 days for the body to produce enough antibodies to fight off a flu infection. This means that if the vaccine is to protect a person during the flu season, it needs to be given before the first flu cases arise – which is autumn in Europe. But because flu outbreaks can also occur in January, February or even later, it might still be worth getting vaccinated later.
It takes a while to develop vaccines, so the vaccine has to be produced before it is known which flu viruses will appear that season. The predictions about which viruses will emerge in the coming year are usually accurate enough, though.
If larger numbers of people are already immune to the circulating viruses during flu season, then fewer people will get the flu. In years where there is a flu epidemic, being vaccinated with the right vaccine can help protect many people from infection, also preventing serious complications. This is particularly important for young children, older people, and people with weakened immune systems.
“Real” flu is generally not as common as other respiratory tract viruses: Children have the highest risk of getting sick, and about 10% to 20% of them get the flu each season. Healthy adults have a much lower risk of catching the flu.
Research on flu vaccinations
Researchers at the Cochrane Collaboration, an international research network, evaluated studies on the effectiveness of flu vaccinations. As expected, they found that the vaccination was more effective if it matched the virus types that were going around in that particular year. Overall, the researchers estimated that the vaccination can then reduce the risk of infection by about 60% in healthy adults. But what does this number mean?
The likelihood of benefiting from a vaccination depends on a number of things, including how high the risk of infection is in a particular flu season. If a population already has a high level of immunity against the viruses going around in one particular season, then not as many people will benefit from it because only a few will get the flu anyway. The potential benefit of having a flu shot is much greater if there are a lot of new viruses and more people are infected. The figures in the table below may help make this clearer. But the benefit of vaccination can never be precisely predicted for each year, or for a new virus.
Protection offered by a flu vaccination in seasons with different numbers of infected people (figures have been rounded)
Get the flu without vaccination:
Get the flu with vaccination:
Season with high numbers of infected people
10 out of 100
|4 out of 100|
Season with moderate numbers of infected people
|5 out of 100||2 out of 100|
Season with low numbers of infected people
|2 out of 100||1 out of 100|
What the authorities recommend
In Germany, the U.S. and many other countries, health authorities recommend yearly flu vaccinations for people with a high risk of flu-related complications – before the flu season starts. The German Standing Immunization Committee (STIKO) of the Robert Koch Institute in Berlin is responsible for vaccine recommendations in Germany. It recommends a yearly flu vaccination for people over the age of 60, and for people with chronic conditions such as asthma, cardiovascular disease, diabetes, or HIV. Pregnant women are advised to have a vaccination too.
This recommendation also applies to people who care for children, older people or sick people – like people working in nursing homes, hospitals or kindergartens. The authorities recommend this for two reasons: These people are very often exposed to different viruses. And they can easily infect others who are more likely to have serious complications if they get the flu.
Demicheli V, Jefferson T, Al-Ansary LA, Ferroni E, Rivetti A, Di Pietrantonj C. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev 2014; (3): CD001269.
Jefferson T, Di Pietrantonj C, Al-Ansary LA, Ferroni E, Thorning S, Thomas RE. Vaccines for preventing influenza in the elderly. Cochrane Database Syst Rev 2010; (2): CD004876.
Jefferson T, Rivetti A, Di Pietrantonj C, Demicheli V, Ferroni E. Vaccines for preventing influenza in healthy children. Cochrane Database Syst Rev 2012; (8): CD004879.
Robert Koch-Institut (RKI). Empfehlungen der Ständigen Impfkommission (STIKO) am Robert Koch-Institut/Stand: August 2015. Epidemiologisches Bulletin 2015; 34: 327-362.
IQWiG health information is written with the aim of helping
people understand the advantages and disadvantages of the main treatment options and health
Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.
Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.