COVID’s back. Who’s most likely to get it again?

A new study is offering data to back one of the core assumptions about the spread of COVID: The intensity of exposure to the virus matters, and vaccines and prior infections can only help so much — but they do indeed help.

The research, led by scientists at the Yale School of Public Health and published in Nature Communications, also reinforces the common-sense notion that masks and air filtration can augment the protection provided by a vaccine and lower the risk of getting the virus. Anyone watching COVID cases bubble back up in their community should take that to heart: Infections aren’t unavoidable, and the tools we’ve been using to protect ourselves are not just intuitive — and certainly shouldn’t be controversial. They’re increasingly backed up by evidence.

While real-world data has given scientists a reasonable sense of COVID vaccines’ ability to protect against the worst outcomes of the virus, understanding their effectiveness at preventing transmission has been difficult.

A team of researchers struck upon a smart way of trying to show the vaccines are “leaky” — that is, they can protect against transmission, but only up to a certain level of exposure to the virus. They looked at infections in Connecticut prisons.

A correctional facility provides a unique opportunity to consider the vaccines’ effectiveness. Rarely do researchers have such complete information on who a person is directly and intensely exposed to (a cellmate), is likely in regular close contact with (others in a cell block), or is potentially exposed to in more transient situations (in the broader community). Researchers meanwhile also knew how many times everyone had been previously infected or vaccinated. Even better, testing was conducted often and according to a set of rules, rather than personal inclinations.

By tracking infections during the delta and omicron waves, the researchers found that hybrid immunity — or having been both vaccinated and previously infected — did the best job at protecting people from getting COVID, even when in close quarters with someone with the virus. Immunity acquired from vaccination or previous infections alone was leakier — they conferred reasonable protection from infection when exposure to the virus was low, but much less when exposure was high.

The paper suggests what shouldn’t be controversial by now: Avoiding a prolonged exposure to COVID, even after vaccination and prior infection, is a good way to avoid getting infected. “We’re not telling anyone anything that seems illogical based on what we’ve just gone through over the past few years,” says Margaret Lind, an epidemiologist at Yale Medical School who helped lead the study.

But it adds nuance to the conversation. The study suggests that masks and ventilation matter, not only as their own form of protection, but because lowering your exposure to the virus seems to improve the effectiveness of the immunity conferred by your vaccines and previous infections, Lind says.

And there are clear takeaways for the public. For starters, anyone still dragging their heels on getting boosted should reconsider when the updated shots arrive in a few weeks. A mere 20% of adults in the U.S. got the last booster, so there’s a lot of room for improvement. The data have long been abundantly clear that the current crop of vaccines is critical for saving lives and keeping the most vulnerable out of the hospital, and now there’s good reason to think keeping up to date on your boosters can help you also avoid some infections.

That’s a reasonable goal. After all, we still don’t have a good handle on the potential long-term health consequences of repeated infections with COVID, so using a tool to try to minimize that risk seems like a no-brainer.

As cases climb, it’s also worth revisiting tried-and-true ways of lowering your exposure to the virus. That could mean pulling out a mask in situations where the risk might be high. And it should mean continuing to invest in improving the quality of indoor air we breathe, whether that’s at home, school or work. At the very least, throw open a window while the weather is still warm.

Lisa Jarvis is a Bloomberg Opinion columnist covering biotech, health care and the pharmaceutical industry.