The rapid spread of the delta variant is raising concerns among scientists that the coronavirus could mutate into more transmissible or deadlier strains.

The fresh fears come as the U.S. vaccination rate has largely plateaued, and with much of the world still unvaccinated. Such a large amount of people without even one vaccine dose gives the virus more chances to spread, replicate and potentially develop mutations.

Experts say that while mutations are not a certainty, the odds will remain high unless more vaccines are administered. And they warn that the highly transmissible delta variant, which came from mutations, could seem tame in comparison to future strains.

In announcing the new mask guidance this week, Centers for Disease Control and Prevention Director Rochelle WalenskyRochelle WalenskyCDC chief clarifies vaccine comments: ‘There will be no nationwide mandate’Overnight Health Care: CDC details Massachusetts outbreak that sparked mask update | White House says national vaccine mandate ‘not under consideration at this time’Publix will require employees to wear masksMORE addressed the anxiety among public health experts over whether potential mutations might be able to bypass existing COVID-19 vaccines.

Right now, fortunately, we are not there, Walensky said. These vaccines operate really well in protecting us from severe disease and death. 

But the big concern is that the next variant of that might emerge, just a few mutations potentially away, could potentially evade our vaccines, she added.

When COVID-19 spreads, the virus replicates its genetic material to infect more cells. In the process, that material sometimes mutates from the original strain.

Andrew Pekosz, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said the virus mutates randomly but at a steady rate.

Most changes do not help or inhibit the viruss ability to spread, but very, very infrequently its genetic material alters to give it the upper hand, he said.

Its incredibly difficult to predict when these mutations will occur, Pekosz said, while adding that the opportunities for alterations increase in areas where the virus can spread easily.

Until we make sure that the virus can’t freely replicate in the population anywhere in the world, we’re always going to have that increased likelihood of mutations occurring, he said.

The World Health Organization has classified four strains as variants of concern that give the virus an advantage: the alpha, beta, gamma and delta variants. Another four strains are labeled as variants of interest, which could possibly benefit the virus.

Officials and experts widely view immediate vaccinations in the U.S. and around the world as the best way to reduce the risk of more harmful variants from developing.

Now is the time for people to get vaccinated, Assistant Secretary of Health Rachel LevineRachel LevineBiden nominates first openly-gay woman to serve as US ambassadorBiden: ‘Pride is back at the White House’Overnight Health Care: White House acknowledges it will fall short of July 4 vaccine goal | Fauci warns of ‘localized surges’ in areas with low vaccination rates | Senate Finance leader releases principles for lowering prescription drug prices MORE told Washington Post Live on Friday. That’s the best way to protect against the development of these variants.

That message is being delivered in some hard-hit states as well.

Louisianas top medical official, Joseph Kanter, emphasized that while the COVID-19 vaccines are effective against the delta strain, that may not be the case with other variants.

There’s no guarantee that future variants will provide us that opportunity, he told reporters on Thursday.

The seven-day U.S. average in vaccinations leveled off in the past few weeks to more than 500,000 doses per day before jumping to 615,000 on Thursday. Still, the rate is a dramatic dropoff from its peak of 3.3 million doses per day in mid-April, according to Our World in Data.

Less than half of the U.S. population is fully vaccinated, including less than 58 percent of Americans 12 and older who are eligible to get the shots.

The emergence of a variant that could evade the existing vaccines would pose an ultra serious problem, said William Schaffner, a professor of infectious diseases at the Vanderbilt University Medical Center.

Then we would have to create a new vaccine to match that new variant, he said. Thats not so tough. The technology is there. That would be done within a month or two. But then we would have to give this new vaccine to everybody. Wed have to start all over again.

Only 14.2 percent of the global population is fully vaccinated against COVID-19, leaving much of the world more at risk. President BidenJoe BidenCDC chief clarifies vaccine comments: ‘There will be no nationwide mandate’Overnight Defense: First group of Afghan evacuees arrives in Virginia | Biden signs Capitol security funding bill, reimbursing Guard | Pentagon raises health protection level weeks after lowering itBiden urges local governments to stave off evictionsMORE has committed to sending millions of donated doses abroad to accelerate the worlds vaccination effort.

Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, said if vaccines were ready for the world at the same time as in the U.S., potentially we could have forestalled or prevented the delta variant.

Still, he said the kinds of mutations in current variants suggest that the viruss evolution is not selecting changes that are completely resistant to the vaccines in use now.

I’m hoping what that means is that we won’t see a doomsday COVID-19 that’s completely resistant to all of our vaccines, he said. I don’t think that’s gonna happen.

Experts agree that the CDCs updated mask guidance could help prevent the formation of harmful new variants, as masking offers an additional layer of protection.

Former Food and Drug Administration Commissioner Scott Gottlieb predicted on CNBC that the U.S. would get through the worst of the delta variant within a few weeks, adding that the CDCs new recommendations could have a negligible impact.

But some experts expressed uncertainty, pointing to how quickly the state of the pandemic could change, especially with schools across the country returning in-person in the next few weeks.

It may be that we can, that we’ll see something peaking … in the next week or two of delta, said Pekosz of Johns Hopkins. But when we move back inside in the fall, that’s another opportunity for a resurgence to occur.