Studies show flu vaccine reduces risk of flu-related illness and hospitalization; benefits of flu vaccine outweigh its risks

With flu season approaching in Europe, the U.S., and other parts of the Northern Hemisphere, campaigns to encourage flu vaccination in these regions are expected to roll out shortly. Public health authorities such as the World Health Organization (WHO), the European Centre for Disease Prevention and Control (ECDC), and the U.S. Centers for Disease Control and Prevention (CDC) agree that the best way to reduce the risk of flu is getting vaccinated.

Seasonal flu places a significant burden on healthcare systems. Annually, there are an estimated three to five million cases of severe flu-related illness and between 290,000 to 650,000 deaths globally, according to the WHO.

Nevertheless, misinformation about the flu vaccine’s safety regularly circulates on social media, which could affect public perception of flu vaccines and in turn lower flu vaccine uptake rates. One study of more than 260,000 Twitter posts between January 2017 and November 2022 observed that posts related to the flu vaccine were associated overall with negative attitudes^[1].

And in 2023, a survey by the U.S. National Foundation for Infectious Diseases found that roughly 40% of adults surveyed said they did not plan to get vaccinated against the flu or were unsure if they would get the flu vaccine. Some of the top reasons cited for their decision were concerns over the potential side effects of the flu vaccine and a belief that the flu vaccines don’t work very well.

These two concerns are commonly exploited in flu vaccine misinformation. One example can be seen in this Facebook reel posted on 22 September 2024, which was viewed more than 128,000 times.

In the reel, health book author Raymond Francis, who was speaking at an event organized by the Silicon Valley Health Institute in November 2017, claimed “flu shots don’t work” and instead increased the risk of getting the flu. He also claimed that the flu vaccine permanently damages the immune system and nervous system. We covered Francis’ false claims about vaccines and statins on a different occasion in an earlier review.

The Silicon Valley Health Institute previously provided a platform for individuals to propagate health misinformation, such as the false claim that vaccines cause autism (they don’t) and the baseless claim that ozone therapy is a “miracle cure” (ozone is toxic and has no known medical use).

In the same vein, Francis’ claims aren’t backed by the scientific evidence that show the benefits of flu vaccination outweigh their risks. We explain below.

Flu vaccines work, albeit imperfectly

Unlike certain vaccines, the flu vaccine’s effectiveness can vary greatly from season to season for a few reasons. One key reason is the virus’ ability to mutate quickly. As an RNA virus, the mechanism it uses to make more copies of its genetic material is more prone to introducing errors (mutations). This genomic instability is the reason why we need a new flu vaccine every year.

The Global Influenza Surveillance and Response System (GISRS), which comprises multiple laboratories and health agencies around the world, monitors flu viruses that circulate globally. Based on this data, it predicts which strains are most likely to circulate during flu season. This in turn forms the basis of recommendations for the flu strains that the upcoming flu vaccine should target.

However, making such predictions is challenging, because the virus constantly mutates, making it a moving target. This can result in a mismatch between the vaccine-targeted viruses and the predominant viruses that actually circulate when flu season hits.

When mismatch happens, the vaccine can have very little effectiveness against circulating strains. This was the case during the 2014/15 flu season, as data collected from the CDC and the ECDC showed. Therefore, flu vaccine effectiveness can vary, sometimes widely, from one season to another.

However, when the vaccine-targeted viruses do match the circulating flu viruses, the vaccine is highly effective at cutting the risk of flu-related illness and complications. For example, a review of 30 studies found that between the period of 2010 and 2015, flu vaccination reduced the risk of flu-related hospitalization in adults by approximately 40% overall^[2].

Flu vaccine also benefits children and pregnant women, both of which are groups at a greater risk of developing flu-related complications.

A study of the 2010/11 and 2011/12 flu seasons in the U.S. found that fully vaccinated children were about 80% less likely to require intensive care for flu illness compared to unvaccinated children^[3]. Another study looking at the 2019/20 season reinforced this finding, showing that the flu vaccine was 75% effective against flu-related ICU admission in children^[4].

One study of the 2010/11 and 2011/12 seasons in the U.S. found that flu vaccination cut the risk of flu illness by about one-half in pregnant women^[5]. Another study that looked at data from flu seasons 2010 through to 2016 found that the flu vaccine was overall 40% effective against flu-associated hospitalization for pregnant women^[6].

The CDC states that during the 2019/20 flu season, ”flu vaccination prevented an estimated 7 million influenza illnesses, 3 million influenza-associated medical visits, 100,000 influenza-associated hospitalizations, and 7,000 influenza-associated deaths” in the U.S.

In summary, the flu vaccine’s effectiveness from season to season may fluctuate depending on how well the vaccine targets the circulating flu viruses. But data from multiple flu seasons show that the vaccine is generally effective at reducing flu-related risks and protecting people at a greater risk of flu-related complications. Overall, the scientific evidence demonstrates that the flu vaccine indeed works.

The flu vaccine cannot cause the flu, doesn’t damage the immune system

A survey by the Annenberg Public Policy Center of the University of Pennsylvania, which polled roughly 1,600 U.S. adults in January 2023, found that nearly a third of respondents believed the flu shot could give you the flu.

That finding illustrates just how common this incorrect belief about the flu vaccine can be. As Science Feedback explained in an earlier review, the flu vaccine cannot give a person the flu, because it doesn’t contain live virus.

Francis offered no evidence to support his claim that the flu vaccine increases the risk of flu. He also didn’t provide specifics on how the vaccine allegedly increases the risk of flu, but this may be a reference to certain studies reporting that flu vaccine effectiveness was lower in people who were vaccinated in consecutive years compared to those who were vaccinated non-consecutively^[7-9].

However, this doesn’t translate to evidence of increased risk of flu or damaged immunity, as claimed. A systematic review of 83 studies found that while flu vaccination in consecutive seasons was sometimes linked to lower flu vaccine effectiveness in the current season, on the whole, vaccinated people were still better protected than unvaccinated people from the flu^[10].

Other evidence indicating that flu vaccination doesn’t damage immunity is that people who are vaccinated against the flu aren’t more likely to develop non-flu respiratory infections compared to unvaccinated people, as Science Feedback explained in an earlier review.

Flu vaccine comes with some risks, but its benefits are greater

Like any medical intervention, the flu vaccine comes with potential side effects. Concerns over these side effects have persisted for decades—in the U.S., one such concern can be traced back to the U.S. government’s flu vaccination campaign in 1976 in response to an outbreak of swine flu at Fort Dix in New Jersey. Scientists at the time feared that the virus could have pandemic potential.

However, the flu vaccine used in the campaign turned out to be linked to a slightly increased risk of Guillain-Barré syndrome (GBS), a neurological disorder that arises due to the body’s immune system attacking the nervous system. GBS produces muscle weakness, and in severe cases, paralysis, although in most cases, people recover over time.

The most common cause of GBS is infection, but vaccination is also recognized as a potential cause. The CDC states:

“The background rate for GBS in the United States is about 80 to 160 cases of GBS each week, regardless of vaccination. The data on the association between GBS and seasonal flu vaccination are variable and inconsistent across flu seasons. If there is an increased risk of GBS following flu vaccination it is small, on the order of one to two additional GBS cases per million doses of flu vaccine administered.”

Studies have found that flu vaccines post-1976 are also sometimes linked to a slightly increased risk of GBS as well. But getting the flu increases the risk of GBS much more than the flu vaccine.

A study of the 2009 flu vaccine in the U.S. found a slightly increased risk of GBS post-vaccination relative to the background risk (the risk of GBS in the absence of vaccination)^[11]. This risk translated to “about 1.6 excess cases of Guillain-Barré syndrome per million people vaccinated”, the researchers reported.

But a study of vaccinated and unvaccinated people in the U.S. at the end of the 2009 H1N1 pandemic found that overall, GBS cases were less frequent in vaccinated people compared to unvaccinated people^[12]. This finding suggests that during a flu outbreak, with all other things being equal, a vaccinated person has a lower risk of GBS than an unvaccinated person does.

This finding was also bolstered by a study in Canada. Using data obtained between 1993 and 2011, it found that the risk of GBS after vaccination was about 17 times lower than in people who sought medical attention for a flu infection^[13]. Specifically:

“The attributable risks were 1.03 Guillain-Barré syndrome admissions per million vaccinations, compared with 17.2 Guillain-Barré syndrome admissions per million influenza-coded health-care encounters.”

Narcolepsy is another neurological side effect that’s been linked to one particular flu vaccine used during the 2009 swine flu pandemic, named Pandemrix.

The hallmark feature of narcolepsy is excessive daytime sleepiness; other symptoms like sudden muscle weakness or paralysis can also occur. Narcolepsy as a side effect of Pandemix was primarily observed in Europe and was later found to be strongly linked to a specific gene variant related to the immune system. This risk wasn’t seen with other flu vaccines used during the 2009 pandemic^[14].

It’s also important to be aware that infection with the flu itself is a risk factor for narcolepsy, like GBS. Moreover, there’s no evidence showing that the seasonal flu vaccines developed each year are linked to an increased risk of narcolepsy like Pandemrix was specifically in 2009.

Taken together, the scientific evidence indicates that getting vaccinated against the flu is associated with an overall lower risk of neurological disorders like GBS and narcolepsy compared to remaining unvaccinated during a flu outbreak.

Conclusion

Flu vaccine misinformation, like the claims propagated by Francis in the Facebook reel, commonly exploit persistent misgivings about the flu vaccine, incorrectly implying that it’s safer to remain unvaccinated than to get the flu vaccine.

But studies show that remaining unvaccinated during a flu season isn’t risk-free and in fact, is associated with more risk than getting vaccinated. Like any medical intervention, the flu vaccine is associated with certain side effects, one of which is GBS. But getting the flu is associated with a greater risk of GBS than getting vaccinated. Moreover, flu can cause severe illness and death, and getting vaccinated can significantly reduce the risk of these outcomes.

REFERENCES

• 1 – Ng et al. (2023) Examining the Negative Sentiments Related to Influenza Vaccination from 2017 to 2022: An Unsupervised Deep Learning Analysis of 261,613 Twitter Posts. Vaccines.
• 2 – Rondy et al. (2017) Effectiveness of influenza vaccines in preventing severe influenza illness among adults: A systematic review and meta-analysis of test-negative design case-control studies. Journal of Infection.
• 3 – Ferdinands et al. (2014) Effectiveness of Influenza Vaccine Against Life-threatening RT-PCR-confirmed Influenza Illness in US Children, 2010–2012. Journal of Infectious Diseases.
• 4 – Olson et al. (2022) Vaccine Effectiveness Against Life-Threatening Influenza Illness in US Children. Clinical Infectious Diseases.
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• 6 – Thompson et al. (2019) Influenza Vaccine Effectiveness in Preventing Influenza-associated Hospitalizations During Pregnancy: A Multi-country Retrospective Test Negative Design Study, 2010–2016. Clinical Infectious Diseases.
• 7 – Skowronski et al. (2016) A Perfect Storm: Impact of Genomic Variation and Serial Vaccination on Low Influenza Vaccine Effectiveness During the 2014–2015 Season. Clinical Infectious Diseases.
• 8 – Valenciano et al. (2016) Vaccine effectiveness in preventing laboratory-confirmed influenza in primary care patients in a season of co-circulation of influenza A(H1N1)pdm09, B and drifted A(H3N2), I-MOVE Multicentre Case–Control Study, Europe 2014/15. Eurosurveillance.
• 9 – Lim et al. (2022) The impact of repeated vaccination on relative influenza vaccine effectiveness among vaccinated adults in the United Kingdom. Epidemiology and Infection.
• 10 – Jones-Gray et al. (2023) Does repeated influenza vaccination attenuate effectiveness? A systematic review and meta-analysis. The Lancet Respiratory Medicine.
• 11 – Salmon et al. (2013) Association between Guillain-Barré syndrome and influenza A (H1N1) 2009 monovalent inactivated vaccines in the USA: a meta-analysis. The Lancet.
• 12 – Vellozzi et al. (2014) Cumulative Risk of Guillain–Barré Syndrome Among Vaccinated and Unvaccinated Populations During the 2009 H1N1 Influenza Pandemic. American Journal of Public Health.
• 13 – Kwong et al. (2013) Risk of Guillain-Barré syndrome after seasonal influenza vaccination and influenza health-care encounters: a self-controlled study. The Lancet Infectious Diseases.
• 14 – Nellore and Randall. (2016) Narcolepsy and influenza vaccination—the inappropriate awakening of immunity. Annals of Translational Medicine.