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Experts say vaccinated people who contract a coronavirus infection have low viral loads and are unlikely to transmit it to another person. Tom Werner/Getty Images
  • Despite the increased spread of the delta variant, experts say it’s unlikely fully vaccinated people who have an asymptomatic infection are transmitting the virus.
  • Experts say that’s because vaccinated people who have contracted the coronavirus have much lower viral loads than unvaccinated people.
  • Experts still recommend that vaccinated people take safety precautions when they’re in large crowds or a region with a low vaccination rate.

The rapid spread of the coronavirus delta variant has led to concerns that people who have been fully vaccinated against the disease may be unwittingly spreading it because they have an asymptomatic infection and aren’t aware of it.

However, experts say that while not impossible, COVID-19 transmission by fully vaccinated people is unlikely.

The delta variant is highly infectious and potentially more deadly than previous SARS-CoV-2 variants. It currently accounts for more than half of all new COVID-19 cases in the United States.

It’s estimated that more than 99 percent of new COVID-19 cases are among unvaccinated people.

Spread of the delta variant has been particularly swift in states with low vaccination rates. But cases of the newest variant have appeared in every state, including those with high vaccination rates.

That doesn’t necessarily mean that vaccinated people are playing a major role in spreading the disease, though.

“In virology, the hallmark of transmission is viral load,” said Dr. Bruce K. Patterson, a virologist and the CEO of IncellDx, a company designing new methods of predicting, identifying, and treating long-term COVID-19 cases.

“The more virus you have, the better chance you have of spreading it,” Patterson told Healthline. “From what we know of wild COVID (the non-variant form of the novel coronavirus), people who are vaccinated are not carrying very much viral load, so the probability of them being infectious is very low.”

Not only are vaccinated people at far less risk of contracting the coronavirus, they’re also less likely to be asymptomatic carriers of the virus or transmit it to others, according to the Centers for Disease Control and Prevention (CDC).

Because the delta variant is new, it remains to be seen how much virus vaccinated people who contract an infection with this version of the coronavirus carry, said Patterson.

So far, however, the vaccines currently in use appear to remain highly protective against the delta variant as well as the other variants now in circulation.

Patterson said some “breakthrough” infections with the delta variant are bound to occur among vaccinated people, just as with other variants, because vaccination effectiveness — while exceeding 90 percent in most cases — isn’t 100 percent protective against infection and disease.

“Breakthrough cases will be infectious, but the hope is that the viral loads won’t be as high as in someone who is unvaccinated,” he said.

Overall, the spread of the delta variant and other variants is far more likely to occur among unvaccinated people than vaccinated people, probably by at least a factor of 10, Patterson said.

“Factually, it’s not wrong to say that spread by vaccinated people can occur, but the probability is” low relative to the level of concern being raised, he said.

“The risk isn’t zero, but it’s not high,” Patterson said.

Vaccination remains the best protection against all variants.

Patterson recommends continuing to wear masks in high-risk settings, such as where large crowds gather, especially in areas where spread of the delta variant is strong.

That echoes the advice offered by Dr. Anthony Fauci, head of the National Institute on Allergy and Infectious Diseases, during a recent appearance on NBC’s “Meet the Press.”

“If you put yourself in an environment in which you have a high level of viral dynamics and a very low level of vaccine, you might want to go the extra step and say, ‘When I’m in that area where there’s a considerable degree of viral circulation, I might want to go the extra mile, to be cautious enough to make sure that I get the extra added level of protection even though the vaccines themselves are highly effective,’” Fauci said.

Patterson also advised that both vaccinated and unvaccinated people continue to get tested for coronavirus infections, which are often asymptomatic but still carry the risk of transmitting the virus to others.