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Influenza: How much protection do flu vaccinations offer?

Created: ; Last Update: April 21, 2011.

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In most years healthy people are unlikely to get influenza, whether or not they are vaccinated. However, vaccination can reduce the risk of getting influenza by half or more. This can make a big difference in years when the risk of infection is high.

The flu (influenza) is caused by viruses. The infection cannot be treated with antibiotics because antibiotics only work against bacteria and not against viruses. So antibiotics only help if you have a bacterial infection of the airways as well as the viral infection. Viruses are microscopically small – even smaller than bacteria. Like many of the so-called respiratory or breathing system viruses, influenza viruses multiply very quickly once they are in our bodies. They can make us very ill and spread quickly from person to person. The body’s defense or immune system needs some time to learn how to create antibodies to fight the infection that a virus causes. The white cells in the bloodstream are an important part of the immune system. Some of these white cells develop antibodies against the virus, and then those antibodies can fight off that particular infection. There is more information about antiviral flu medications such as Tamiflu here.

Signs of influenza

Influenza typically causes fever, shivering or chills, muscle aches and pains, general discomfort, and cold-like symptoms. Some people have a high risk of complications from influenza – especially babies and small children, people with particular illnesses, and women who are pregnant as well as people over the age of 60. When the complications are severe – for example, a severe case of the lung infection pneumonia – people can die. However, most of the time the worst is over within about a week, even though people will usually still feel tired and ill for a couple of weeks afterwards. You can read more about influenza and influenza treatments in our feature.

How influenza vaccination works

There are hundreds of influenza viruses, belonging to different groups. The most serious ones for humans are groups called influenza A and influenza B. Getting infected with an influenza virus builds up a person’s immunity against that specific virus. Vaccination or immunization creates the same situation. A weakened (“live attenuated”) or inactivated form of the virus is introduced into the body to stimulate the production of antibodies. 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.

One of the difficulties with influenza vaccination, though, is that the influenza viruses keep changing. A virus has something called antigens on the outside. It is the antigen that the antibodies will recognize and start to fight. But if the virus changes its antigens, the antibodies will not recognize the virus and so they will not work. Because influenza viruses are constantly changing (mutating) and completely new forms of viruses also arise, one influenza vaccination cannot provide permanent protection. So if you want to be sufficiently protected you have to have a different vaccination every year.

An influenza vaccination must be done in time

It takes about 14 days after receiving the influenza vaccination for the body to produce enough antibodies to fight off an influenza infection. This means that if vaccination is to protect a person during the influenza season, they need to be vaccinated before the first cases of influenza arise. Because the production of vaccine does require a bit of time, it has to be manufactured before it is certain what influenza viruses will appear that season. Usually, the predictions about which viruses will emerge in the coming year are accurate enough.

If there is already quite a high level of immunity in the community to the influenza viruses circulating that season, then the viruses will not make as many people ill. But in the years when there is an influenza outbreak, and the vaccines match that year’s viruses exactly, then the influenza vaccine can prevent a lot of illness and a lot of deaths, especially among toddlers and very old people. Usually, though, true influenza is not as common as other respiratory viruses: perhaps 10 – 20% of young people get influenza during the season, for example, and they are the highest risk group for getting ill. For healthy adults, the risk of getting influenza is much lower.

What research on influenza vaccination shows

To find out how effective influenza vaccination has proven to be, researchers from Italy looked for trials that compared what has happened to vaccinated and unvaccinated people over the years. They found that when the vaccine available did not match the kinds of viruses that were circulating that year, vaccination did not make very much difference for healthy adults at low risk of getting ill. However, when the match was good, the risk of getting influenza could be reduced much more. Overall, the best estimate the research group could make was that vaccines might reduce the risk of infection by 44 to 73% in healthy adults. But what do these numbers mean?

The likelihood that someone will benefit from an influenza vaccination depends on his or her personal risk of infection. For example: If someone’s risk of getting the flu during flu season is 2%, this means that 2 out of 100 people who have this same risk will get influenza. The vaccine reduces the probability of getting influenza by half – that is, 50%. So now instead of 2 out of 100 people, only 1 out of 100 will get the flu. In other words, the vaccination will protect 1 out of 100 people from influenza.

If someone has a higher risk of infection, he or she will be more likely to benefit from the vaccination. Suppose someone has a risk of infection of 24% – that is, 24 out of 100 people who have this same risk will get influenza. In this case, a 50% reduction of risk of infection means that the vaccination will protect 12 out of 100 people from influenza. But the benefit of vaccination can never be precisely predicted for each new year, or for each new virus.

What the authorities recommend

In Germany, the USA and many other countries, health authorities recommend yearly influenza vaccination for people with a high risk of influenza complications. The German Standing Immunization Committee (STIKO) of the Robert Koch Institute in Berlin is responsible for vaccine recommendations in Germany. The German Standing Immunization Committee recommends an annual vaccination for people over the age of 60 years, people with chronic conditions such as asthma, cardiovascular disease or diabetes, and pregnant women. They also recommend that those who care for people in high-risk groups – like children, old or sick people – should be vaccinated. This applies, for example, to people who work in nursing homes, hospitals and kindergartens.

The authorities recommend this for two reasons: because these workers are exposed to so much infection, and because if they themselves become infectious, they could infect people more likely to be harmed by influenza. On top of this, the authorities recommend that people who intend to get vaccinated do so before the influenza season starts, so in autumn for those in the northern hemisphere. However, there could still be outbreaks of influenza in January, February or even a bit later, so a later vaccination might still be worthwhile.

This additional information has been provided by the U.S. National Library of Medicine

The U.S. Centers for Disease Control and Prevention (CDC) is responsible for vaccine recommendations for Americans. CDC recommends influenza vaccination for everyone 6 months and older.

Author: German Institute for Quality and Efficiency in Health Care (IQWiG)

References

  • IQWiG health information is based on research in the international literature. We identify the most scientifically reliable knowledge currently available, particularly so-called “systematic reviews”. These summarize and analyze the results of scientific research on the benefits and harms of treatments and other health care interventions. You can read more about systematic reviews and why these can provide the most trustworthy evidence about the state of knowledge here. The authors of the major systematic reviews on which our information is based are always approached to help us ensure the medical and scientific accuracy of our products.
  • Centers for Disease Control and Prevention. Prevention and control of influenza. 2007; 56: 1-54. [Full text] [PubMed: 17625497]
  • Jefferson T, Di Pietrantonj C, Rivetti A, Bawazeer GA et al. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews: Version 2010, Issue 7. CD001269 . [PubMed: 20614424]
  • STIKO. Empfehlungen der Ständigen Impfkommission (STIKO) am Robert Koch-Institut. Epidem Bulletin 2010; 30. [Full text – in German]
© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK51834
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