When toddlers and infants catch the flu, their fresh-out-of-the-uterus immune systems seldom fare well. Unlike adults, who may take a sick day or two, babies with the flu often end up in the hospital with lung infections and febrile seizures. Now, new data suggests that unvaccinated older siblings may be doubling the risk of babies ending up hospitalized due to flu infections. That’s because babies under six months are too young for flu vaccines, so their safety largely depends on their older brothers and sisters (who never wash their hands) being inoculated.
READ MORE: The Fatherly Guide to Flu Season and Kids
“We found that children, and particularly babies aged less than six months, with older siblings were more likely to be admitted to the hospital than children without older siblings,” coauthor on the study Pia Hardelid, an infectious disease epidemiologist at the University College London told Fatherly. “This is likely related to the increased risk of infection from older siblings.”
The Centers for Disease Control and Prevention estimate that up to 26,000 children in the U.S. under the age of five have been hospitalized due to flu-related complications since 2010. Complications for young children who catch the flu include pneumonia, inflammation of the brain, and severe dehydration. In rare cases, flu infections can lead to death. Because the risk is so great for young children, CDC recommends all children over the age of six months get annual flu vaccines. The problem, of course, is children under the age of six months of age.
“The younger children are, the more likely they are to be admitted to hospital,” Hardelid says.”In our study, children aged less than six months old had higher admission rates than children aged between six months and two years.” And while the best way to prevent flu infection is vaccination, “there is no influenza vaccine licensed for children aged less than 6 months old.”
So Hardelid and colleagues set out to identify risk factors and ways to prevent very young children from contracting the flu. For their study, they collected data on almost every child born in Scotland between 2007 and 2015 (roughly 400,000 total) and used hospital admissions data to track whether children contracted the flu and ended up hospitalized. They found that babies born in between July and December were at higher risk (because they were youngest during flu season) and those with rare chronic conditions or older mothers were also at increased risk.
But one, largely preventable, cause stood out—older siblings. The study suggests that almost half of all flu hospital admissions in babies under six months could be attributed to older siblings passing along the disease. “Maternal vaccinations during pregnancy have been shown to be protective against influenza, but it is not certain for how long this protection lasts,” Hardelid says. “The results from our study indicate that hospital admissions could also be reduced by reducing the risk posed by older siblings. This could be achieved by vaccinated older siblings.”
Flu vaccines often get bad press for being less effective during some seasons than others, but Hardelid says those lapses seldom apply to children, who are disproportionately protected by vaccines. “A review looking at data across several years found that live attenuated influenza vaccine, which is offered to children aged 2 years and older in the UK, is over 80 percent effective at preventing influenza in children aged less than seven years old,” she says.
And although Hardelid’s data does not reflect whether the children in her particular study were vaccinated, she is confident that vaccinating young children can protect their younger siblings. With flu season nearly upon us—and a whole lot of five-year-olds putting their sticky fingers all over their baby brothers—making sure everyone is up-to-date on their flu vaccines is crucial.
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