The flu is a contagious viral respiratory infection. And while the illness caused by the flu is often self-limiting, it can have a considerable impact on our daily life ⁽¹⁾.

Vaccination remains the most effective way of preventing the flu ⁽¹⁾. Flu vaccines are estimated to be around 30% to 60% effective ⁽²⁾, where their effectiveness is an estimate of their ability to prevent the flu ⁽²⁾.

In Europe, the European Centre for Disease Control (ECDC) set up the I-MOVE (Influenza – Monitoring Vaccine Effectiveness) network to measure flu vaccine effectiveness ⁽²⁾, which it has since the 2008/2009 flu season. The team works with 29 partners, including the European arm of the World Health Organization (WHO-EURO), regional and national public health institutes, hospitals, small and medium enterprises, and universities from across Europe ⁽³⁾.

While vaccine effectiveness can vary from season to season, flu vaccination reduces the overall risk of the flu – both for those vaccinated and the people in contact with them ⁽⁴⁾. I-MOVE studies showed, for example, that flu vaccination during the 2019/20 flu season protected between one-third and two-thirds of people vaccinated ⁽⁵⁾. And while some people who get vaccinated still get sick, vaccination has been shown in several studies to reduce the severity of their illness ⁽⁶⁾.

How well the flu vaccine works depends on a variety of factors. Let’s take a closer look.

A good match?

When there is a good match between the viruses circulating and the flu vaccine, the vaccine provides substantial benefits in preventing flu illness and complications ⁽⁷⁾.

But frequent mutations in circulating flu viruses can sometimes result in a mismatch between the flu viruses in the vaccine and the flu viruses spreading in the community ⁽⁴⁾⁽⁷⁾. In seasons when the flu vaccine is not well matched to circulating flu viruses, the vaccine’s effectiveness may be lower than expected ⁽⁴⁾.

Your age and your health

How effectively a vaccine season protects you against the flu also depends on your age and health ⁽⁴⁾⁽⁸⁾. Flu vaccine effectiveness can also differ depending on whether you have previously been infected or vaccinated, and whether you are ‘naïve’ to the viruses circulating ⁽⁴⁾.

In general, a flu vaccine works best among healthy younger adults and older children ⁽⁸⁾. Some older people and people with certain chronic illnesses may develop less immunity after vaccination ⁽⁸⁾. And while flu vaccination is not a perfect tool, it is the best way to protect against the flu ⁽⁸⁾.

When you get your flu shot

There may also be slight differences in flu vaccine effectiveness depending on when you get it. The annual flu season rarely starts before mid-November and often runs to the end of May the following year ⁽⁹⁾. The vaccination season usually starts in October and lasts until January.

The sooner you get your flu shot, the better. Why? Because it takes 10 to 14 days following your flu vaccination before your immune response kicks in and your body develops protection ⁽⁹⁾. So, if you’re exposed to the flu within that time, you might still get sick.

The protection provided by the vaccine is expected to last for at least one flu season, but it does wane with time ⁽⁹⁾. So, don’t forget to get your flu shot every year.

References
(1) https://www.ecdc.europa.eu/sites/portal/files/documents/seasonal-influenza-antiviral-use-2018.pdf
(2) https://www.ecdc.europa.eu/en/seasonal-influenza/prevention-and-control/vaccine-effectiveness
(3) https://www.imoveflu.org/
(4) https://www.euro.who.int/.../vaccination/influenza-vaccination-coverage-and-effectiveness
(5) https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.10.2000153#html_fulltext
(6) https://www.cdc.gov/flu/prevent/misconceptions.htm
(7) https://www.cdc.gov/flu/vaccines-work/vaccineeffect.htm
(8) https://www.cdc.gov/flu/prevent/keyfacts.htm
(9) https://www.ecdc.europa.eu/en/seasonal-influenza/prevention-and-control/vaccines/timing