Yet More Evidence That Getting Your Seasonal Flu Shot Protects You And Others Around You

Influenza vaccine programs help to considerably lower the burden of disease, even when not everyone takes up the offer of a shot. A new study has confirmed this but also provides us with a stark warning: if we allow transmission to get too high, even the best flu vaccines can’t completely stop the disease in its tracks. More evidence, if it were needed, as to why it’s so important that all those who are able get vaccinated.

The quote often attributed to Benjamin Franklin holds that nothing is certain in life but death and taxes. We’d like to add another to that list: the annual arrival of flu season. Predicting how severe a given season will be is a task even Nostradamus would have shrunk from, but it’s a vital job for scientists formulating flu vaccines.

Each season, vaccines are updated based on researchers’ best guesses as to which virus strains are likely to be circulating. Understandably, sometimes those guesses don’t quite pay off and vaccines vary in their efficacy. But until we have a universal vaccine – something many hope could be on the way fairly soon – this system is the best we have. 

But even in years when vaccine efficacy is lower, there’s no doubt that the shots save lives. That’s been made even clearer by the results of a new study out of the University of Pittsburgh.

The authors performed analytical modeling with a sample population of over 1 million “agents”, designed to closely mimic the demographics of the real-life 2010 census of Allegheny County, Pennsylvania. They ran different scenarios, altering parameters like viral transmissibility and vaccine coverage within the population.

“Even when both vaccine effectiveness and vaccine uptake were low, vaccination showed marked reductions in disease burden for transmission levels characteristic of seasonal influenza,” the paper notes.

The effects are both direct and indirect, meaning that your decision to get a flu shot not only protects you, but also those around you who are unable or unwilling to get vaccinated themselves.

However, the study can also be taken as a cautionary tale. In a scenario where transmission of the virus is very high – such as might be seen in a pandemic situation, where pre-existing immunity in the population is low – even a really effective vaccine wasn’t good enough to throw a protective blanket over the unvaccinated too.

“At the highest levels of transmissibility, such as might be found in pandemic influenza, indirect benefit decreased and eventually disappeared,” the authors write.

In any population, there will be those who are unable to get vaccinated against a particular disease. This may be due to medical factors like allergies or other conditions; it could also be an issue of access, such as lack of financial means to pay for vaccines if that is required.

At its most simplistic, the concept of herd immunity is supposed to mean that so long as enough people can get vaccinated, those who can’t will be protected because disease transmission will be considerably decreased, or halted altogether.

The margins are fine. Right now, we’re seeing what happens when measles vaccine coverage drops below the ideal thresholds, with cases of the disease in the US reportedly hitting a 33-year high – the overwhelming majority in unvaccinated individuals.

Flu is a different disease, and the seasonal flu that comes round each year is different again from pandemics like those in 1918 and 2009. Research like this new study is vital to help us prepare as best as we can for the next pandemic. The results suggest that in such a scenario, it will be more important than ever to ensure that everyone who is able to take up the offer of a vaccine.

“In our results, a substantial proportion of infections was prevented when transmissibility was similar to that encountered in typical influenza seasons, vaccine effectiveness was within the ranges estimated for those seasons, and vaccine uptake was similar to what is typically found in the US,” the authors concluded.

“However, when the level of transmission was very high, even a highly effective vaccine, while protecting vaccinated individuals, did not protect those who were unvaccinated.”

The study is published in the journal JAMA Network Open.

IFLScience's The Big Questions podcast has returned, and one of the questions we'll be tackling in season 5 is "Will We Ever Have A Universal Flu Vaccine?". Listen out for that, and catch up with the rest of the season so far wherever you get your podcasts.