Each infection — whether in an adult in Yemen or a kid in Kentucky — gives the virus another opportunity to mutate. Protecting a new, large chunk of the population anywhere in the world limits those opportunities. 

That effort got a lift with 28 million U.S. kids 5 to 11 years old now eligible for child-sized doses of the Pfizer-BioNTech vaccine. Moves elsewhere, like Austria’s recent decision to require all adults to be vaccinated and even the U.S. authorizing booster shots for all adults on Friday, help by further reducing the chances of new infection.

Vaccinating kids also means reducing silent spread, since most have no or mild symptoms when they contract the virus. When the virus spreads unseen, scientists say, it also goes unabated. And as more people contract it, the odds of new variants rise.

According to AP, David O’Connor, a virology expert at the University of Wisconsin-Madison, likens infections to “lottery tickets that we’re giving the virus.” The jackpot? A variant even more dangerous than the contagious delta currently circulating.

“The fewer people who are infected, the less lottery tickets it has and the better off we’re all going to be in terms of generating the variants,” he said, adding that variants are even more likely to emerge in people with weakened immune systems who harbor the virus for a long time.

Researchers disagree on how much kids have influenced the course of the pandemic. Early research suggested they didn’t contribute much to viral spread. But some experts say children played a significant role this year spreading contagious variants such as alpha and delta.

Getting kids vaccinated could make a real difference going forward, according to estimates by the COVID-19 Scenario Modeling Hub, a collection of university and medical research organizations that consolidates models of how the pandemic may unfold. The hub’s latest estimates show that for this November through March 12, 2022, vaccinating 5- to 11-year-olds would avert about 430,000 COVID cases in the overall U.S. population if no new variant arose. If a variant 50% more transmissible than delta showed up in late fall, 860,000 cases would be averted, “a big impact,” said project co-leader Katriona Shea, of Pennsylvania State University.

Delta remains dominant for now, accounting for more than 99% of analyzed coronavirus specimens in the United States. Scientists aren’t sure exactly why. Dr. Stuart Campbell Ray, an infectious disease expert at Johns Hopkins University, said it may be intrinsically more infectious, or it may be evading at least in part the protection people get from vaccines or having been infected before. 

Another big unknown: Dangerous variants may still arise in largely-unvaccinated parts of the world and make their way to America even as U.S. children join the ranks of the vaccinated.