Influenza vaccines save millions of lives across the globe each year, but even heroes have their faults. A live attenuated flu vaccine, which contains weakened versions of the influenza virus, makes mice more prone to bacterial infections in their nasal passages. The vaccine used in the study is similar to FluMist, of which 13 million doses were distributed in the United States this year. The work helps explain why runny noses were an occasional aftereffect of FluMist in clinical trials.
“A study like this is an easy target for the antivaccine movement, but our findings shouldn’t make a concerned parent less inclined to vaccinate their kid,” says Michael Mina, a virologist at Emory University in Atlanta, who led the research.
Within the first few days of catching the flu, a person becomes susceptible to opportunistic bacteria. Microbes like Streptococcus pneumoniae and Staphylococcus aureus, which cause strep throat and staph infections, respectively, pounce on the exhausted immune system, causing secondary disease.
Mina wondered if attenuated flu vaccines could do the same. Some clinical trials of FluMist, for example, recorded an uptick in runny noses  in children between the ages of 6 months and 5 years a week or so after they got the vaccine. But no study had attempted to figure out what might be causing the side effect by studying it in an animal model.
Mina and his colleagues mutated the H3N2 influenza virus to mimic the weakened strain included in the FluMist vaccine. Mice were then given this mock vaccine or a placebo either before or after they were exposed to S. pneumoniae or S. aureus.
Both bacteria flourished after mice were vaccinated , but only in their nasal passages and upper respiratory tracts, Mina’s team reports in mBio. Although similar infections can cause runny noses in humans, they are much less dangerous than staph and strep infections in the lungs and lower respiratory tract. In fact, no infections at all transpired in the vaccinated mice’s lower respiratory tracts, which explains why zero rodents experienced severe disease or died after bacterial exposure.
Even if the findings hold true in humans, a slight increase in the risk of a runny nose shouldn’t prevent anyone from getting a flu shot, Mina says. Overall, getting vaccinated actually reduces your risk of bacterial infection by preventing influenza and, therefore, its host of possible secondary infections.
And at the end of the day, this is only a mouse model, says Vincent Racaniello, a microbiologist at Columbia University. Still, he says, it raises a mystifying question about this vaccine: “When you misdirect the immune response towards a virus, why do the bacteria take over?”
Next, Mina will search for human trends by heading to Bangladesh, where large-scale FluMist trials in children are ongoing. He will collect nasal swabs from vaccinated children to ascertain whether the patterns in his mouse model hold true. A city with the population density of Dhaka may witness an uptick in bacterial infections during mass vaccinations, he says.
According to Mina, some scientists in the field thought his study shouldn’t be published because it casts a negative light on vaccines, but this research ultimately benefits the public, says Peter Palese, chair of microbiology at the Icahn School of Medicine at Mount Sinai in New York City. “Vaccines are still the best ways of combating flu and many other diseases. Mice are not men, but it's important to learn about possible complications.”