The flu vaccine isn’t foolproof, but that shouldn’t dissuade you from getting vaccinated. 

So you had a flu shot last year and you still got the flu — should you get vaccinated this year or take your chances? This is a common question, and there’s a reason that the flu vaccine may not always prevent the illness. On average, the annual flu vaccine is around 40 percent effective. By comparison, most other vaccines that protect against infectious diseases have to be at least 90 percent effective to be considered successful.

Why the discrepancy? There are numerous strains of the influenza virus, and the ones that circulate vary from year to year. The CDC and the World Health Organization (WHO) track the flu diligently and work with the Food and Drug Administration (FDA) to update the vaccine so that it protects against strains that are circulating in the current year. 

One of the reasons we have to vaccinate each year is that the flu virus changes each year.

The flu vaccine for the 2019–2020 seasons has been formulated to protect against both H1N1 and H3N2, the two strains that dominated last year’s flu season, as well as other strains that are currently circulating. But decisions about which flu strains will be included in the vaccine each year have to be made well before flu researchers know which strains will be dominant during the upcoming season.

In many ways, making recommendations for flu vaccinations each year is like vaccinating against a moving target — as we saw with last year’s flu season, the dominant strain that’s spreading can change at a moment’s notice. That said, the vaccine still offers the greatest chance at protection.   

Your best defense against the flu is to get your flu shot every year to help protect against the strains of flu that are most likely to be circulating that season. 

You can also take steps to help stop germs from spreading by avoiding people who are sick — and stay home when you’re sick; cover your mouth and nose when you sneeze — preferably with a tissue; wash your hands regularly with soap and water — or use hand sanitizer; clean areas around you — disinfect surfaces regularly to help remove germs; keep germs out — avoid touching your eyes, nose, or mouth and focus on overall health — eat nutritious meals, keep hydrated, stay physically active, manage stress, and get good sleep.