Carlos SolisA new study from the University of Pittsburgh finds that widespread flu vaccination protects both vaccinated and unvaccinated people — but vaccinated people are still best protected.
Researchers ran computer models simulating 1.2 million people in a single U.S. county. The model tracked the daily spread of influenza within families, schools, workplaces, and neighborhoods.
The modeling assessed a vaccine with 40% efficacy across different flu seasons, varying transmission rates, and different levels of vaccination coverage. Each scenario was simulated 100 times, and the researchers reported average outcomes.
What Did the Modeling Reveal?
When vaccination coverage reached 51% of the population, the overall number of flu cases fell by 32.9% to 41.5%, depending on the severity of the season. The benefit showed up in both vaccinated and unvaccinated people — though it was smaller for the unvaccinated.
“Vaccination against strains similar to those circulating during seasonal flu provided indirect benefits to unvaccinated people, but the direct benefits for the vaccinated were always greater,” the report said.
In other words, by getting vaccinated you not only greatly increase your chances of avoiding infection but also help protect others who haven’t been vaccinated.
This isn’t just about people who refuse vaccines. The finding is especially important for people who can’t be vaccinated for medical reasons.
So the study confirmed: the more people who are vaccinated, the fewer total infections there will be. Both vaccinated and unvaccinated people have less exposure to sick people, ScienceAlert reported.
However, the authors warned that this effect shouldn’t be an excuse for people who are eligible for vaccination to skip their shots.
In the simulations, unvaccinated people were still 43% to 73% more likely to contract influenza, depending on the vaccine’s effectiveness. That indirect protection was almost wiped out when the virus spread very rapidly, as it did in the early stages of the pandemic. So directly vaccinating as many people as possible remains crucial.
“When vaccine efficacy and coverage levels were low, vaccination showed a noticeable reduction in illness at transmission levels typical of seasonal flu. However, when transmission rates were very high, even an effective vaccine did not protect unvaccinated individuals,” the team said.
The data also showed limits to herd immunity. When a population builds enough resistance to infection through natural immunity or vaccination, the spread of disease slows and that helps protect unvaccinated people. But that indirect protection doesn’t hold up against pathogens that spread very rapidly.
The study’s results were published in JAMA Network Open.
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