Coronavirus transmission among vaccinated people could raise the risk of an even more dangerous variant

• The CDC says vaccinated people should mask up indoors again as cases of the Delta variant surge.
• A study suggests that transmission among vaccinated people could lead to new variants that evade vaccines.
• The virus is "just a few mutations potentially away" from evading vaccines, the CDC said.

The coronavirus could be "just a few mutations potentially away" from evolving into a variant that can evade existing COVID-19 vaccines, CDC director Rochelle Walensky said Tuesday.

According to research published Friday in the journal Scientific Reports, vaccinated people - counterintuitively - play a key role in that risk.

The best way to stop coronavirus deaths and severe illness is to roll out vaccines quickly. However, the researchers concluded that the chance a vaccine-resistant strain will emerge is highest in a scenario that combines three conditions: First, a large portion of a population is vaccinated, but not everyone. Second, there's a lot of virus circulating. And third, no measures are in place to curb potential viral transmission from vaccinated people. Sound familiar?

Before the rise of Delta, which is now responsible for more than 80% of US coronavirus cases, that situation wasn't a concern because research suggested vaccinated people weren't likely to transmit other versions of the virus. But according to a CDC study released Friday, vaccinated people may transmit the Delta variant just as easily as the unvaccinated.

That could help explain the recent surge in US cases: In the last month, the country's seven-day average of new daily cases has more than quintupled: from 12,263 on June 29 to 71,621 on Thursday.

The researchers concluded that, in an environment where Delta is spreading among all people - regardless of vaccination status - it is imperative to get more people vaccinated immediately to prevent the emergence of a new vaccine-resistant variant.

'Evolutionary arms race'

The researchers created a mathematical model that predicted which conditions are associated with the highest risk of new variants emerging that can evade vaccines.

They found if a chunk of people are vaccinated but many unvaccinated people remain, a variant that can either evade or partially evade vaccine-induced immune defenses has a competitive advantage over other versions of the virus. So over time, those less fit strains - which can't infect vaccinated hosts - die out, leaving vaccine-resistant ones to dominate the viral landscape. Then if viral transmission goes unchecked - lots of people partying maskless, say - those newly dominant variants can easily spread and further evolve.

"This means the vaccine-resistant strain spreads through the population faster at a time when most people are vaccinated," Simon Rella of the Institute of Science and Technology Austria, who worked on the study, said in an online briefing, CNN reported.

Rella and his colleagues wrote that this dynamic may give rise to "vaccine development playing catch up in the evolutionary arms race against novel strains."

Partially vaccinated people could unwittingly teach the virus to skirt our defenses

Virologists call variations of a virus that slip past vaccine- or illness-induced immune defenses "escape mutants." So far, no coronavirus variants can fully escape COVID-19 vaccines.

But the reason a future variant could do so is that the shots all target the coronavirus' spike protein - the sharp, crown-like bumps on the surface of the virus that help it invade our cells. If multiple, significant mutations alter enough characteristics of that protein, antibodies might not be able to recognize or properly fight that new variant.

Infections among people who are partially vaccinated raise the risk of a game-changing mutation because it takes time for the body to develop the antibodies, T cells, and B cells that fight the virus, and our immune response increases dramatically following the second dose. So if someone gets infected in the interim, it gives the virus a sneak peek at what it's up against. With Delta, research shows, a single shot of the Pfizer or AstraZeneca vaccines is only 33.5% effective against the variant.

"Not having all of us immunized creates a perfect circumstance for variants that are escape mutants to arise," James Hildreth, an immunologist and president of Meharry Medical College, told Insider in April. "If there are some people who have low levels of immunity, in a way, that's almost worse than having no immunity at all."

Hildreth added that partial immunity "can actually drive the formation and presence of viruses that do not bind to the antibody."

"They're going to take over, and be the ones transmitted," he said.

The new study supports the recent CDC guidance that vaccinated people should wear masks in areas of high transmission. Hildreth is fully vaccinated but said that hasn't stopped him from putting on his mask when he leaves the house.

"I don't want to become a vector and unwittingly spread the virus to others, which is another reason I wear the mask," he said.