How effective is the flu shot in preventing the flu?

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The flu shot is a common offering in doctors’ offices everywhere, even if you did not originally make an appointment to get one. This article is offers additional information to enable patients to make the most informed decision for their health this winter season.

According to the CDC,

recent studies show that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well-matched to the flu vaccines (1).

Potentially, with the flu shot, we could be half as likely to get the flu. That’s great, right? Wait, but there are a couple of major qualifiers in that statement. Let’s look at them.

First, the effectiveness of the flu vaccine depends on the age and health of the person being vaccinated. It is most effective in healthy adults with a strong immune system (2). However, healthy adults are also the population that is most likely to fight off the flu without the flu shot, and if they do get sick, are the least likely to have complications, which account for many hospitalizations each year. The populations that are most likely to have complications from the flu include the very young and the elderly. So these are the folks that should get the flu shot, right? The irony is that often their immune systems are not strong enough to get the benefits of a flu shot, or in other words, to form the anti-bodies that would then fight the actual influenza virus. So the flu shot is actually the least effective in the populations that need it the most.

The effectiveness of the flu vaccine also depends on how well matched the vaccine is to the flu virus(es) of that season. Producing a “well-matched” vaccine is more difficult than it sounds.

Influenza A and B are the types that are mostly responsible for flu epidemics in humans. However, within the A type alone, there are 16 different types of hemagglutinin (the H in H1N1, for example) and 9 different types of neuraminidases (the N in H1N1). Therefore, there are potentially 144 different subtypes of influenza A viruses. Furthermore, because the hemagglutinin gene mutates all the time, there are many variants of the same subtype virus.

Furthermore, it takes at least 6 months to produce large quantities of the influenza vaccine (3). This means that as early as January, when the current flu season is not yet over, vaccine manufacturers are already starting to grow the viruses that make up the vaccine for the following season. While the CDC and other WHO centers do their best to study up on viral trends and new, emerging strains, the predominant viral strain(s) responsible for the flu each season is usually not identified until well into the season, when it is too late to make a vaccine against it.

The predominant influenza strain of the 2017 - 2018 season was the H3N2 virus. Even if the CDC was able to predict this, not all influenza viruses lend themselves to the vaccine manufacturing process. Today the majority of influenza viruses used to make the vaccine are grown in chicken eggs (3). However, some strains, for example, the H3N2 strain, do not grow well in eggs, and it is quite difficult to produce a vaccine against these strains.

According to the CDC,

During years when the flu vaccine is not well matched to circulating influenza viruses, it is possible that little or no benefit from flu vaccination may be observed (1).

Let’s say though, you want to take your chances that there is a “vaccine match” this year. You are a healthy adult with a strong immune system, and you choose to get the vaccine because you don’t want to have to miss work or school. Or perhaps you are often around your aging parents or young children and believe that having the flu shot will help protect loved ones.

As many people who have received the flu vaccine know, the flu vaccine is not without side effects, not to mention the many ingredients that you may not want to have in your body. The most common side effects are soreness, redness, tenderness or swelling where the shot was given, as well as low-grade fever, headache and muscle aches (4). Hmm, these sounds awfully like flu symptoms. While the vaccine does not actually give you the flu, it can cause symptoms that are quite similar to those of the flu. This is because, flu viruses, while inactivated or only partially utilized, are being injected or inhaled.

If you are lucky enough to not experience any side effects, also consider that vaccination leads to several days of viral shedding, which although is less contagious, can still be spread to susceptible individuals, such as the elderly and young children. Furthermore, it takes the immune system 2 weeks following vaccination to develop the antibodies against the strains of influenza you were vaccinated against. In this 2-week post-vaccination period, you could still catch the flu despite having just received the vaccine, in addition to being somewhat contagious to those around you.

This article is NOT a blanket recommendation to avoid the flu shot. The flu can be a serious illness (9.2 - 35.6 million cases each year) and results in thousands of deaths (12,000 - 56,000) and many more hospitalizations (140,000 - 710,000) due to complications (5). In terms of percentages, deaths due to flu each year range from 0.03% to 0.61% and hospitalizations from 0.39% to 7.7%. In some people, the flu shot may be instrumental in helping reduce the severity of illness. If you are over the age of 65, a child, a pregnant woman, anyone with a compromised immune system or in close/frequent contact with these populations, we recommend speaking with one of us or your doctor to weigh the risks and benefits of a flu shot.


(1) CDC:

(2) WebMD:

(3) CDC:

(4) CDC:

(5) CDC: