Explaining ‘herd immunity’ may convince more people to get flu shots

The knowledge that getting a flu shot can help prevent flu from spreading in the community may help convince more people to get vaccinated, a US study suggests. 

Even though doctors recommend that nearly everyone, starting at six months of age, get a flu vaccine each year, less than half of Americans follow this advice. Each person who skips their annual flu shot diminishes what is known as “herd immunity”, or the potential for vaccinated residents in a community to help prevent the virus from spreading to the minority of residents who cannot get vaccinated for medical reasons. 

“The more people who are vaccinated in a community, the lower the risk that influenza will be able to spread even if the vaccine does not perfectly protect against the disease,” said senior study author Nicole Basta of the University of Minnesota in Minneapolis. 

“Influenza spreads by creating chains of transmission whereby one infected person infects additional people and those individuals infect others with whom they come in contact,” Basta said by email. 

At least 70 per cent of people need to get an annual flu shot to achieve herd immunity in a community, researchers note in the journal Vaccine. 

For the current study, researchers asked 554 adults at the Minnesota State Fair how often they got their annual flu shot, what proportion of people in their community got vaccinated against influenza each year, and whether they knew about herd immunity. 

Overall, 37 per cent of participants did not know about herd immunity and 76 per cent thought the influenza vaccination rate in their county was higher than it really was. Just 68 per cent of people unfamiliar with herd immunity said they planned to be vaccinated, compared to 79 per cent of participants who were knowledgeable about herd immunity. 

After researchers gave people educational materials about herd immunity, the proportion of people willing to be vaccinated rose. 

With education, 75 per cent of people who initially did not know about herd immunity said they would get vaccinated, as did 80 per cent of people who said they were familiar with herd immunity at the start of the survey. 

While the authors conclude that education on herd immunity might help improve vaccination rates, the study was not a controlled experiment designed to prove this. It is also not clear whether people who expressed willingness to get vaccinated during the study would actually follow through and get their flu shots. 

Increased willingness does not necessarily translate into vaccination, said Maimuna Majumder, a researcher at the Massachusetts Institute of Technology in Cambridge and Harvard Medical School in Boston who was not involved in the study. 

This happens “for a whole host of reasons, not the least of which include access to the vaccines themselves, how far individuals need to travel to receive it, how much the vaccine costs out of pocket once they get there, and so on”, Majumder said by e-mail. 

And even if education helps get people vaccinated one year, it might not have a lasting effect, Majumder added. 

“It’s possible that individuals may need ‘booster’ education campaigns throughout their lives to remind them of the benefits associated with vaccination,” Majumder said. 

Still, the study results suggest that education may be one way to help convince more people to get vaccinated, said Dr Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. 

“The education programme had an important impact, and we should consider adding the role of herd immunity to flu and possibly other vaccination campaigns,” Hotez, who was not involved in the study, said by e-mail.