How much protection do flu vaccinations offer?

photo of a grandfather and grandchild playing in the snow (PantherMedia / Meseritsch Herby)

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 the flu by more than half, though. This can make a big difference in years when the risk of infection is high.

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. Babies, 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 weakened or inactivated viruses (or parts of viruses) under the skin or into a muscle. There is also a flu vaccine in the form of a nasal spray. It has been approved for children between the ages of 2 and 17 years.

After it comes into contact with the vaccine, the immune system starts to produce antibodies to fight the virus. Then, if a living, active virus comes along that is exactly the same as the one introduced by the vaccine, the body recognizes it and can fight it off. The vaccine itself can't give you the flu.

Because flu viruses are constantly changing (mutating) and completely new forms of viruses also arise, one flu vaccination can't provide permanent protection. So if you want to have enough protection during the flu season, you need to have a different vaccination every year.

It's 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 be able to fight off a flu infection if you become infected. 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 viruses that are around 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 infections: Healthy adults probably only have a 2 to 5% chance of catching the flu. Children and teenagers have the highest risk of getting the flu: About 10% to 20% of them get it each season.

What does the research on flu vaccinations show?

Researchers at the Cochrane Collaboration (an international research network) analyzed the results of 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 various things, including how high the risk of infection is in a particular flu season. If larger numbers of people are already immune to the viruses that are going around during the flu season, then fewer people 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 a vaccine can never be precisely predicted for each year, or for a new virus.

Table: Effectiveness of flu vaccination in healthy adults

Flu vaccination

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:

The vaccination protects:

Season with high numbers
of infected people

10 out of 100

4 out of 100  6 out of 100

Season with moderate numbers
of infected people

5 out of 100  2 out of 100  3 out of 100

Season with low numbers
of infected people

2 out of 100  1 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.

STIKO recommends a yearly flu vaccination for:

This recommendation also applies to people who care for children, older people or sick people – like people working in nursing homes, doctor's practices, hospitals or kindergartens. The authorities recommend this for two reasons: These people are very often exposed to the viruses. And they can easily infect others who are more likely to have serious complications if they get the flu.

What are the side effects of the vaccine?

A flu vaccination may cause some temporary side effects, such as fever, tiredness, headache, muscle pain or joint pain. But these side effects usually pass within one or two days.

If the vaccine is given as an injection, there could be temporary mild pain, redness or swelling where the needle was inserted.

Vaccination using nasal spray may lead to a temporary stuffy or runny nose, cough, or sore throat. Children and teenagers who have severe asthma or a weakened immune system shouldn't be vaccinated with a nasal spray.