No, the Flu Shot Is Not 100 Percent Effective. Yes, You Still Need It
I study infectious diseases for a living. I’m also a mom of three kids. So every fall, I schedule an appointment with the pediatrician for my youngest and drag the older kids with me to a pharmacy, and we all get our flu shots. I do this even though I know that, most years, the vaccine is generally only about 40 to 60 percent effective at preventing the development of illness from the influenza virus.
These stats make some people wonder, so why bother?
For anyone skeptical about getting a flu shot due to its shortcomings, there are two things you should think about: influenza vaccination is not just about protecting yourself against acute infection, but also from ongoing complications if you do fall ill. Secondly, it’s not just about you.
Let’s take a step back and first discuss why the flu vaccine isn’t 100 percent effective.
Because the circulating influenza viruses change from year to year, a new vaccine is needed every fall to maintain protection. To choose the composition of each year’s vaccine, more than 100 centers around the globe track influenza viruses. Based on these data, and with input from the World Health Organization (WHO), specific vaccine strains are selected by February of each year for the following season. The strains chosen are the ones that the data suggest are the most likely to spread and cause illness.
Each new flu vaccine contains two strains of influenza A, which is typically the flu virus we associate with severe disease as well as causing influenza pandemics. Depending on which vaccine you get, it will also contain one or two strains of influenza B. (Traditional vaccines are “trivalent,” meaning they are made to protect against three viruses, but there are also “quadrivalent” vaccines that are designed to protect against four.) Influenza B is generally considered more mild, but it can also lead to serious infections and even death.
In the injected vaccine, all of the viruses included are “killed” or “inactivated," so they cannot replicate in your body. In the nasal spray that is once again being offered this year (FluMist), the viruses are live but attenuated, meaning they do not cause disease.
So, do these strains always match perfectly with the illness that ends up circulating in the fall and winter? No, not always. In some years, a new virus starts to dominate the population after strains have already been chosen for the vaccines in February, resulting in a bad match by the time influenza season is in full swing. This happened during the 2004 to 2005 season with one of the influenza A strains, and over 2005 to 2006 with an influenza B strain.
The vaccine viruses themselves can also be difficult to develop. As the Centers for Disease Control and Prevention (CDC) explains, flu vaccine viruses are often grown in chicken eggs. But some viruses, like H3N2 viruses, don’t grow well in eggs, making it tricky to get a viable vaccine virus with no mutations.
In fact, the H3N2 portion of the vaccine was modified for the 2018 to 2019 influenza season. Litjen (L.J) Tan, M.S., Ph.D., chief strategy officer with the Immunization Action Coalition, tells SELF that “the H3N2 strain changed from last year partly in response to the concerns that there was some adaptation happening in the vaccine virus strain that could have potentially made the vaccine less effective.” This mutation is believed to be in part due to how difficult it is to grow the H3N2 virus in eggs without adaptations developing.
Figuring out how effective the vaccines are each year is a complicated process.
As mentioned, there is constant surveillance taking place for influenza infection. At the sites that carry out this surveillance work, researchers type the viruses that are making people sick to see how well they match current vaccine strains. They also try to find out the medical history of sick individuals, including whether they received the vaccine. However, this gathering of data takes months to do correctly, so while we get some preliminary numbers during influenza season, we don’t know the final results of a vaccine’s effectiveness until the following fall.
Case in point: We’re still waiting on final numbers from the 2017 to 2018 influenza season, but preliminary data shows that last year’s vaccine effectiveness was about 36 percent overall—much higher than the 10 percent figure reported by many news agencies based on Australian data. It was even more effective in kids: about 59 percent. For the H1N1 strain of the virus, it was about 67 percent effective, while for the H3N2 strain it was lower (25 percent). Effectiveness against the 2017 to 2018 influenza B viruses was 42 percent.
But getting the flu vaccine, even if it’s imperfect, helps keep both the individual and the community healthy and safe.
A report published in April in the journal PNAS (Proceedings of the National Academy of Sciences of the United States) showed that even if a vaccine is only 20 percent effective, it could still prevent 20 million infections or illnesses, 129,000 hospitalizations, and 61,000 deaths compared to no vaccine—even if only 43 percent of the population gets it (which is roughly the amount of people we see get vaccinated each year).
The flu vaccine also reduces a person’s risk of needing to go to the doctor by 30 to 60 percent, even if you do get sick. And it reduces the risk of hospitalization and entering intensive care due to the flu. In children, the vaccine lowers risk of death and ICU admission for serious influenza-associated complications. And if you’re vaccinated, that means you’re less likely to spread the virus to others around you, including those you might not realize are vulnerable to a serious influenza infection, like infants, elderly persons, and anyone with a compromised immune system.
There are a number of people who are particularly vulnerable to the effects and complications associated with influenza. “Pregnant women really are impacted negatively by flu, so you don’t want to catch it while you’re pregnant,” Tan says. Pregnant women who develop influenza have an eight-fold increased risk of complications, including death, as well as complications for the fetus, such as stillbirth. The WHO and CDC both have pregnant women as the top risk group on their vaccine priority list. Getting the vaccine during pregnancy can also help to protect newborns after birth, before they can get their first influenza vaccine at 6 months of age.
Many adults also think that because they’re healthy and, perhaps, have never experienced a serious bout of influenza infection, they don’t need the influenza vaccine. But Tan cautions against relying on this false sense of security: “A lot of adults have chronic health conditions that they are not aware of, especially if they are over 50. You might have an underlying heart condition, you might have an underlying respiratory disease, and you don’t know it because you haven’t been diagnosed. And for those folks with underlying chronic conditions, influenza is certainly nasty, and potentially deadly.”
For those over 65, it gets worse. “There’s a term a lot of gerontologists use called ‘inflammaging,’ which is a low-level chronic inflammation that happens as you get older,” Tan explains. “And we know that inflammaging is one of those factors that lead to severe influenza responses.” This can include an increased risk of cardiovascular events such as heart attack and stroke in the time period after an influenza infection.
So even though it’s not perfect, the flu vaccine is one more precaution we can all take to protect ourselves and those around us, just like washing your hands regularly and keeping your distance from sick people.
Ultimately, the flu vaccine is a safe, simple way to protect yourself and your family during influenza season. And, as previously mentioned, there's also a nasal spray vaccine available if needles really aren't your thing. That nasal spray option, FluMist, was taken off the market for the past two seasons due to an unexpected lack of protection in the 2014 to 2015 influenza season; but with the new formulation, preliminary tests have suggested it will work again this year. As SELF reported previously, there isn't as much data to support the nasal spray compared to the shot. So, the shot is still the primary recommendation. But if it’s a question of getting the spray or not getting vaccinated at all, the spray is a good option.
As Tan notes, “We have a safe vaccine. We have an effective vaccine in multiple outcomes. So why are you gambling with potentially your independence, potentially your vacation, potentially your ability to live a high quality of life in this upcoming flu season? It doesn’t make sense to me.” I have to agree—and it’s not too early to get yours now.
Tara C. Smith, Ph.D., is an infectious disease epidemiologist and professor at the Kent State University College of Public Health.
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