Babies may get a lot of diseases, but that’s mostly because they’re only encountering infections for the first time, says Donna Farber, professor of microbiology & immunology at Columbia University Vagelos College of Physicians and Surgeons. By the time we reach adulthood, our bodies have encountered all sorts of pathogens and developed resilience to many of them. Meanwhile, infants are faced with these pathogens for the first time, and they do a pretty good job of handling them.

“Adults don’t get sick as often because we’ve recorded memories of these viruses that protect us,” Farber says, “whereas everything the baby encounters is new to them.”

In a new study, Farber and colleagues wanted to see the immune system of infants and adults go toe to toe without the previous experience of adult immune systems. To level the playing field, they only tested the immune system’s ability to respond to a new pathogen. Infants have less developed immune systems than adults but can still respond to a plethora of infections and in some cases better than adults, the researchers write.

To conduct a relevant comparison, they collected naïve T cells (cells that have never encountered a pathogen before) from both adult and infant mice, as well as adult and infant humans. The cells were then placed into an adult mouse that had been infected with a virus.

When it came to eradicating the virus, the T cells from infant mice won handily, proliferating faster and traveling in greater numbers to the site of infection, mounting a strong defense against the virus. The researchers found the same thing when they looked at human cells.

“We were looking at naïve T cells that have never been activated, so it was a surprise that they behaved differently based on age,” Farber says. “What this is saying is that the infant’s immune system is robust, it’s efficient, and it can get rid of pathogens in early life. In some ways, it may be even better than the adult immune system, since it’s designed to respond to a multitude of new pathogens.”

The findings are particularly important in the case of COVID-19 — a disease that’s new to everyone, and seems to affect adults much more than infants. Although there could be multiple explanations for this, researchers suspect that children’s surprisingly robust immune systems have something to do with that.

“SARS-CoV-2 is new to absolutely everybody, so we’re now seeing a natural, side-by-side comparison of the adult and infant immune system,” Farber says. “And the kids are doing much better. Adults faced with a novel pathogen are slower to react. That gives the virus a chance to replicate more, and that’s when you get sick.”

The finding could also explain why many vaccines seem to be more effective when administered in childhood — because the T cells are more robust. Studies such as this one could not only help develop better vaccines for children but also help doctors pinpoint what the ideal period for vaccination is. According to Farber, that period is during childhood, and doctors shouldn’t be afraid to recommend multiple vaccines during that time window.

“That is the time to get vaccines and you shouldn’t worry about getting multiple vaccines in that window,” Farber says. “Any child living in the world, particularly before we started wearing masks, is exposed to a huge number of new antigens every day. They’re already handling multiple exposures.”

“Most vaccine formulations and doses are the same for all ages, but understanding the distinct immune responses in childhood suggests we can use lower doses for children and could help us design vaccines that are more effective for this age group,” Farber concludes.

The study was published in Science Immunology.