There’s a new coronavirus variant topping the leaderboard in the United States: EG.5.

Nationally, EG.5 is causing about 17% of new Covid-19 cases in the country, compared with 16% for the next most common lineage, XBB.1.16, according to the latest estimates from the US Centers for Disease Control and Prevention.

EG may sound like a whole new flavor of the virus, but it’s not; it’s a spinoff of the XBB recombinant strain of the Omicron family. And it represents another incremental tweak to the virus rather than a major evolutionary leap like the original Omicron strain.

Compared with its parent XBB.1.9.2, it has one extra mutation to its spike, at position 465. This mutation has appeared in other coronavirus variants before. Scientists aren’t sure exactly what new tricks it enables the virus to do, but variant hunters are paying attention because many of the new XBB descendants have adopted it.

The 465 mutation is present in about 35% of coronavirus sequences reported worldwide, including another that’s rising in prevalence in the Northeast, FL.1.5.1, suggesting that it is conveying some kind of evolutionary advantage over previous versions.

EG.5 also now has its own offshoot, EG.5.1, that adds a second mutation to the spike. That one is also spreading rapidly.

Dr. David Ho has been testing these variants in his lab at Columbia University to see how resistant they have become to the antibodies we have to defend against them.

“Both are only slightly more resistant to neutralizing antibodies in serum of infected and vaccinated persons,” Ho, a professor of microbiology and immunology, said in an email to CNN.

Clinically, he said, these variants don’t seem to be causing different or more severe symptoms than the viruses that came before them.

“It basically has some more immune escape compared to the ones that were precedents in this XBB series,” said Dr. Eric Topol, a cardiologist at the Scripps Translational Research Institute. “It has an advantage, which is why it’s getting legs all around the world.”

Beyond the US, EG.5 is growing quickly in Ireland, France, the UK, Japan and China. The World Health Organization upgraded its status on Wednesday from a variant under monitoring to a variant of interest, a move that signals the agency thinks it should be tracked and studied further.

The variant has become the most prevalent in the US just as cases, emergency room visits and hospitalizations are going up, although there’s nothing to suggest that this specific strain is what’s driving those increases.

Instead, epidemiologists think human behavior is the engine for this uptick. They point to things like record heat driving more people indoors for air conditioning, which helps the virus spread. Summer travel is sending people outside their normal social circles, which carries viruses to new victims. School is going back into session in many parts of the country too, and as parents know, homework isn’t the only thing kids bring home from their classrooms.

Still, there are reasons to be hopeful that this wave of cases won’t be so bad, says Dr. Anne Hahn, a postdoctoral associate in the Department of Epidemiology of Microbial Diseases at the Yale School of Public Health.

“We are starting from a very low baseline in combination with high population immunity, which would speak against a big surge anytime soon. However, what these new variants will do during wintertime remains to be seen,” she said.

Levels of virus detected in wastewater in August are about where they were in March, according to data from Biobot Analytics.

“I do expect that there will be widespread infections, and I would expect that those widespread infections would generally be mild,” said Dr. Dan Barouch, an immunologist and virologist at Harvard University in Boston.

“The people who are at high risk for severe disease, obviously, have to continue to take precautions.”

Topol says this fall’s updated Covid-19 booster, which will be targeted against the XBB variant, should be a very close match to the variants circulating now, including EG.5, and he expects that it will convey strong protection — if regulatory agencies can get it to people in time.

Vaccine manufacturers have said they expect to have the updated shots ready in a few weeks, but CDC Director Dr. Mandy Cohen has projected that the updated vaccine will be available in October, since it will need sign-off from the US Food and Drug Administration and a recommendation from the CDC.

That would be full month later than boosters were available last fall.

“Why is this taking so long?” Topol said. “The whole idea of mRNA is that it would you could pivot quickly and within a couple of months, you’d have it ready.”

Topol said the vaccine is particularly important for those who are elderly and who have weakened immune systems because they no longer have effective monoclonal antibodies to help if a Covid-19 case becomes severe.

It’s not clear why the process seems to be slower than it was last year. The FDA did not immediately respond to a question about its timeline for reviewing the fall vaccines.

Topol says the US can’t afford to delay its Covid-19 vaccine rollout. Coming up behind EG.5 are a crop of variants with tweaks that enable them to bind more tightly to cells, he says, making them even more infectious.

He is worried that those might have the potential to be more consequential.

“It should be out now, because things are building up,” he said. “We need it now.”