Flu vaccine reduces risk of flu-related illness and hospitalization, even though it hasn’t eradicated flu

Key takeaway

Vaccines have contributed to historical successes in eradicating certain infectious diseases, such as smallpox and polio. But success at disease eradication depends on more than the availability of a vaccine; another important factor is the characteristics of the infectious agent itself. Eradicating the flu is challenging because it mutates readily to produce new strains, and because it can infect animals as well as humans. Nevertheless, the flu vaccine is effective at protecting people against flu-related illness and complications.

Reviewed content

Verdict:

Lacks context

Claim:

“We’ve had the flu vaccine for 78 years. We still have the flu.”

Verdict detail

Lacks context: Disease eradication depends on several factors, including the way a disease-causing microorganism behaves. Unlike diseases that have been eradicated, like smallpox and polio, the flu virus is more challenging to eradicate due to its tendency to mutate rapidly and its ability to infect animals in addition to humans.

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Full Claim

“We’ve had the flu vaccine for 78 years. We still have the flu.”

A Threads post from early February 2025 stated that “We’ve had the flu vaccine for 78 years. We still have the flu”, implying that the flu vaccine doesn’t work. Earlier iterations of this statement were documented by AFP and USA Today.

The claim that the flu vaccine doesn’t work is popularly based on its imperfect nature. This is an example of the nirvana fallacy, in which a solution to a problem is rejected because it doesn’t work perfectly.

Vaccines have indeed led to historical successes in disease eradication. As Science Feedback discussed in a previous review, vaccines were key to eradicating smallpox globally and polio in most of the world, two diseases that have existed for thousands of years.

But this has also resulted in vaccines becoming a victim of their own success. Some, like the Threads poster, have come to expect that a vaccine should eradicate a disease, and if they don’t do this, then the vaccine must be ineffective. This is incorrect, however. The main purpose of a vaccine is to protect people from disease, and published studies have shown that the flu vaccine does so by reducing the risk of serious illness and flu-related complications in adults and children^[1-5].

This article in The Conversation explains how even imperfect vaccines provide significant public health benefits—like reducing the burden of disease in a population—even if they cannot eradicate the disease.

But why is it that smallpox and polio could be eradicated with the help of a vaccine, while the same cannot be said of the flu? We explain below.

Flu virus mutates readily, making it difficult to target with a vaccine

In fact, successfully eradicating diseases depends on various factors, of which the availability of a vaccine is just one. Another factor is the characteristics of the infectious agent itself. The flu virus behaves differently from the smallpox and polio viruses, explaining why we’ve been successful with eradicating smallpox and polio, but not the flu.

One reason is because the flu virus mutates readily. This is in part due to its imperfect genome replication mechanism, increasing the chances of mutations in important viral genes that facilitate infection. This allows new strains of flu viruses to arise all the time.

Another contributing factor to this genetic variation is reassortment, a process in which two different flu viruses that infect the same cell can swap gene segments with each other.

Reassortment is one of the reasons why experts are on high alert for avian influenza (bird flu) infections in humans. Unlike the seasonal flu, the H5N1 avian influenza virus isn’t as well-adapted for infecting humans, although rare cases of human infections still occur. But if the two viruses co-infect the same individual, there’s a possibility that a genetic exchange produces a novel flu virus that infects humans with ease. In fact, the last three flu pandemics in human history have arisen from strains that underwent this form of genetic exchange.

The flu virus’ high degree of mutability essentially makes it a moving target that’s difficult to tackle with a vaccine. Scientists around the world have to monitor flu viruses that circulate globally to predict which strains are most likely to circulate during flu season, so that they can recommend which flu viruses the vaccine should target.

But these predictions can sometimes be wrong since the virus is constantly changing. A mismatch between vaccine-targeted viruses and the viruses that actually circulate during flu season can occur, which results in lower flu vaccine effectiveness.

The flu virus’ mutability is also why the flu vaccine needs to be updated every year, unlike other vaccines, like those against smallpox and polio.

Flu virus can infect other hosts apart from humans, which complicates eradication

Another obstacle to eradicating the flu is the flu virus’ ability to infect animals like birds and pigs. This sets it apart from the viruses that cause smallpox and polio, which don’t naturally infect other animals apart from humans. This means that there’s no naturally occurring reservoir for smallpox and polio to hide in. This also means that efforts at eradicating these diseases only need to focus on the human population.

Conversely, to eradicate the flu virus, we’d need to vaccinate the majority of animal carriers at the same time, in addition to people—and this is assuming that we have a flu vaccine that’s both more effective than the existing one and that targets all existing flu virus strains. The fact that there are migratory species of birds which can carry the virus to another location further complicates matters.

Since vaccination on such a scale isn’t feasible, the flu virus will continue to have the opportunity to survive and multiply in an animal reservoir, even if we had a highly effective vaccine that is received by the majority of humans. In the animal reservoir, the flu virus can continue to mutate, potentially into a form that is able to overcome protection from the vaccine. Through contact between humans and animals, these newly generated viruses have the chance to once again infect humans (zoonotic infection).

Therefore, the nature of the flu virus itself means that we’re unlikely to eradicate the virus through concerted vaccination campaigns like those against smallpox and polio. But while disease eradication represents the ideal outcome, it’s not the only goal or the main goal of vaccination in general. It’s important to keep in mind that the flu causes between 290,000 to 650,000 deaths every year worldwide, and the flu vaccine still provides important public health benefits by mitigating the risks of flu-related illness.

Conclusion

While some diseases like smallpox have been eradicated, in large part due to vaccination, other diseases are much more challenging to eradicate due to the way the disease-causing microorganism behaves. In the case of the flu virus, some of the key obstacles are its propensity to mutate and produce new strains, as well as its ability to infect animals apart from humans, making it much more difficult to target with vaccination.

However, vaccines don’t need to eradicate a disease to be considered effective. The main purpose of a vaccine is to reduce the risk of disease. In this regard, the flu vaccine has been shown to work, particularly when vaccine-targeted strains match well with the flu virus strains that circulate during flu season.

REFERENCES

• 1 – 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.
• 2 – 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.
• 3 – Olson et al. (2022) Vaccine Effectiveness Against Life-Threatening Influenza Illness in US Children. Clinical Infectious Diseases.
• 4 – Thompson et al. (2013) Effectiveness of Seasonal Trivalent Influenza Vaccine for Preventing Influenza Virus Illness Among Pregnant Women: A Population-Based Case-Control Study During the 2010–2011 and 2011–2012 Influenza Seasons. Clinical Infectious Diseases.
• 5 – 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.