(CNN) — Nearly all the staff at Johns Hopkins Bloomberg School of Public Health are vaccinated against Covid-19. Yet they are all still wearing masks to work.

These researchers, who are among the most well-versed in the tricks of the coronavirus, aren’t taking any chances. They’re advising the rest of the country and the world to be similarly careful as strains like the Delta variant arise and spread.

“We still have a masking policy here, particularly in group situations,” Andrew Pekosz, a professor of immunology at Johns Hopkins who is studying the coronavirus, told CNN. “This pandemic isn’t over yet.”

Dr. Mark Mulligan, director of the New York University Langone Vaccine Center, is equally cautious. “I would just say maybe keep your mask in your pocket and if you feel you are in a situation that warrants it, it certainly is good to have it available,” he told a briefing sponsored by the International Antiviral Society Tuesday.

“Very clearly vaccination and the variants are in a footrace.”

On Monday, the Los Angeles County Department of Public Health urged even fully vaccinated people to start wearing masks again in some circumstances.

“With increased circulation of the highly transmissible Delta variant, the Los Angeles County Department of Public Health strongly recommends everyone, regardless of vaccination status, wear masks indoors in public places as a precautionary measure,” it said in a statement. “In the week ending June 12, Delta variants comprised of nearly half of all variants sequenced in Los Angeles County.”

US Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said Wednesday that the coronavirus vaccines available offer protection against the variant, telling ABC’s Good Morning America, “the vaccinated, we believe, still are safe.” The unvaccinated, however, remain at risk.

“We are still seeing uptick in cases in areas of low vaccination, and in that situation we are suggesting that policies be made at the local level,” Walensky said. “Those masking policies are really intended to protect the unvaccinated.”

The Delta variant of coronavirus accounted for 26% of Covid-19 cases in the United States as of June 19, the US Centers for Disease Control and Prevention said in updated estimates Tuesday. Mulligan predicted that would rise to 40% soon, and the genetic sequencing company Helix said its testing indicates Delta already accounts for 40% of cases.

The B.1.1.7 or Alpha lineage of the virus, first spotted in the UK, was still the dominant variant in the US as of June 19, representing 47.8% of cases, the CDC estimated.

Delta does look to be more transmissible, said Dr. Philip Landrigan, director of the Program in Global Public Health & Common Good at Boston College.

“What we do know about the Delta variant is that it is very contagious — more contagious than some of the other variants, which is why it spreads more rapidly than some of the other variants,” Landrigan told CNN.

Rolling the dice

All viruses mutate constantly as they replicate in the body and while the novel coronavirus is not as mutation-prone as influenza, it changes at a good clip. Each identified variant has a particular pattern of mutations that make it stand out and give it new properties.

It makes sense, Pekosz said, that variants would gradually evolve to become more and more transmissible. Delta, the latest iteration, is about half again as transmissible as B.1.1.7 or Alpha.

“From a virus perspective, it has a few unique mutations, particularly in the spike protein, that would suggest it is able to bind to human cells better and perhaps evade antibody responses that target the spike protein,” Pekosz said. The spike protein is the structure the virus uses to grapple cells, and it’s the part all the coronavirus vaccines are designed to train the body to recognize.

“From a population perspective, reports coming out of England and perhaps some other areas suggest it is spreading in the population faster than the Alpha variant. That spread really suggests that this virus is probably better at infecting humans.”

But there’s little evidence Delta can bypass the protection offered by full vaccination. Numerous studies have shown the vaccines — especially the mRNA vaccines made by Pfizer/BioNTech and Moderna — provide strong, broad protection that gives a cushion of extra immunity above and beyond what’s provided by natural infection.

“For people who are vaccinated, the likelihood of their picking up the Delta variant is very low,” Landrigan said.

This is why the US Centers for Disease Control and Prevention says fully vaccinated people can stop wearing masks in most circumstances. “You can resume activities without wearing a mask or staying 6 feet apart, except where required by federal, state, local, tribal, or territorial laws, rules, and regulations, including local business and workplace guidance,” it says in online guidance.

There’s no indication that guidance will change. “For the most part, you don’t need a mask,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN’s Don Lemon on Tuesday.

“I think there is a degree and there should be a degree of flexibility and discretion distributed throughout the country,” Fauci added. “If you are in an area where viral dynamics are really high, you’ve really got to be careful about pulling back from mask mandates.”

And Landrigan notes the risk is not zero for fully vaccinated people.

“These vaccines give 95% or so protection, but that said, the protection is not 100%. There are always a few people in the population that don’t get a take from a vaccine,” he said.

“We certainly are having breakthrough infections everywhere. The vaccines are not 100%,” Mulligan said in his presentation. “There will be breakthrough infections, even for vaccines that are 95, 90, 80% protective.”

Plus, kids under 12 cannot get vaccinated in the US — which means they are still vulnerable to infection.

‘Better safe than sorry’

So Mulligan, Landrigan, Pekosz and others recommend wearing a mask, even if fully vaccinated, when indoors and around people who may not be vaccinated and who may be carrying the virus.

“If you are gathering with friends, family whom you know well, in small groups, you’re outdoors in the open air and everybody has been vaccinated, you don’t need to wear a mask,” Landrigan said.

“On the other hand, if you go to a baseball game or some other setting where there are hundreds or thousands of people, some of whom might not be vaccinated, some of whom might be carrying the virus, I would recommend wearing a mask. Better safe than sorry.”

This is also what World Health Organization officials recommend — although their guidance is aimed at a largely unvaccinated world, as opposed to the US, where close to half the population is vaccinated.

“Vaccines alone won’t stop the community transmission and we need to ensure that people follow the public health measures. People need to continue to use masks consistently,” Dr. Mariangela Simao, WHO’s assistant director general for access to medicines, vaccines and pharmaceuticals, said last week.

“Be in ventilated spaces. Practice hand hygiene and respiratory etiquette, everything, the physical distancing. Avoid crowding. This still continues to be an extremely important, even if you are vaccinated, when you have a community transmission ongoing,” Simao added.

This is in no small part because of evolution. When a virus changes, it does so randomly, but the changes will shift as the pressure on the virus shifts — including pressure from vaccination.

If the virus is circulating, people who have been vaccinated will be exposed to the virus. Their vaccine-boosted immune systems are likely to fight off the virus, but it could start to mutate little by little, and the versions of the virus that successfully evade the vaccinated body’s immune response will have an advantage that will allow them to take off.

The more vaccinated and unvaccinated people are swapping virus among themselves, the more chances it will have to evolve.

That’s why even vaccinated people should still be taking care, says Pekosz.

“As mutations come up in viruses, the ones that persist are the ones that make it easier for the virus to spread in the population,” he said.

That is an argument for using masks, even when vaccinated, he said. “Particularly when it comes to situations where vaccinated people are going indoors and are under conditions there where the virus may be a little easier to spread,” he said.

“Certainly, we are not out of the woods yet because we haven’t had a vaccination rate where we get those herd immunity effects.”

The spread of the Delta variant changes the herd immunity equation, Pekosz added.

“Every time a virus gets better at transmitting, the number of people that have to be vaccinated to reach herd immunity increases,” he said.

“The thing about mutations is that they occur randomly. The more virus replication you have, the more likely that you’ll get a certain mutation. If a mutation has a one in a million chance of making that virus better, if you let the virus roll that dice 900,000 times, you are more likely to get that more successful mutation.”

That’s why unvaccinated people anywhere are a danger to everyone.

“So we really need to pay attention to this and increase our vaccination rates as soon as possible,” Pekosz said.

For the same reason, WHO is urging its worldwide audience to keep up with the precautions.

“While a COVID-19 vaccine will prevent serious illness and death, we still don’t know the extent to which it keeps you from being infected and passing the virus on to others. The more we allow the virus to spread, the more opportunity the virus has to change,” WHO said in a statement.

People should still keep their distance from others, use masks when around others, “especially in crowded, closed and poorly ventilated settings,” keep hands clean and ventilate rooms when inside with others.

“Doing it all protects us all,” WHO said.

“This is not a political issue. This is a matter of life and death,” concluded Landrigan.

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