Vaccinated people can transmit COVID virus, should mask up, says CDC
Faced with newly emerging data, CDC urges some vaccinated Americans to wear masks
Masks, again? Vaccines were supposed to protect us.
Faced with the threat of the highly contagious delta variant and startling new evidence that vaccinated people can spread COVID-19, the CDC is following in California’s footprints, urging us to once again to mask up.
Unaware that they are infected, vaccinated people are inadvertently infecting others, Centers for Disease Control and Prevention director Dr. Rochelle P. Walensky said at a Tuesday press briefing. Because of the virulence of the delta variant, the unvaccinated can no longer count on protection from people who have gotten the shots.
People with “breakthrough infections — rare as they are — have the potential to transmit at the same capacity as an unvaccinated person,” she said.
Masks are especially important in high-risk regions, and vaccinated people who are visiting children, the immunocompromised or the unvaccinated should wear them, she said.
Although daily case rates are rising in some highly vaccinated Bay Area counties, the overall case rate remains low here, so vaccinated people are less likely to be confronted by someone with the virus. That means they’re also less likely to become infected and transmit it. Vaccines are 79% to 95% protective against the delta variant, according to an assortment of international studies.
“The potential for this to be a problem is much much lower in areas with low amounts of disease,” said Walensky.
Vaccines are still great at their primary job: preventing serious illness, hospitalization and death. According to the latest U.S. data, 97 percent of hospitalizations and 99.5 percent of deaths in recent months occurred in unvaccinated individuals.
But the ascendance of the delta variant, which spreads much more readily than earlier strains, has changed our understanding of the risk of transmission.
In the past, vaccinated people who became infected were found to have greatly reduced levels of virus, suggesting little danger that they would pass it on.
But more recent research of delta-driven clusters of cases has revealed that levels of virus are nearly equivalent in both vaccinated and unvaccinated people, said Walensky.
“Unlike the Alpha variant that we had back in May, where we didn’t believe that if you were vaccinated you could transmit further, this is different now, with a delta variant,” Walensky said.
The problem is not that protection from the vaccine is weakening, said virologist Shane Crotty of the Vaccine Discovery Division at La Jolla Institute for Immunology. For instance, data shows that six months after inoculation, protection from the Pfizer vaccine dropped only slightly, from 94% to 91%, he said.
The challenge is that delta behaves differently. It replicates much faster.
“Delta is just incredibly more infectious than the original virus,” said Crotty. “It’s definitely a harder virus to stop.”
When multiplying, delta cranks out so many new virus particles that sometimes a few of them escape our protective troops, called antibodies, said Dr. Joel Ernst, professor of medicine and chief of the Division of Experimental Medicine at UCSF.
It’s a competition between the immune system and the virus, he said. If there are more antibodies that can bind to the virus, the antibodies win — and fend off the virus, he said. If there are fewer antibodies, the virus wins.
Once a host is infected with the virus, a different arm of the immune system is deployed, said UC San Francisco infectious disease expert Dr. Monica Gandhi. This strategy, which uses T cells, kills the virus.
“Then, instead of having serious illness, you may have some symptoms of runny nose and upper respiratory tract symptoms — before you kick start your immune system and fight it,” she said.
“It’s not that you don’t have good immunity,” she said. “But temporarily, faced with so many (viral) particles, our immune response is swamped.”
Meanwhile, you’re potentially infectious.
The big question that worries virologists now is whether the viral mutation process will spin off a new variant that can defeat the existing vaccines, worse than delta. That hasn’t happened yet.
But breakthrough cases may become more common, according to Dr. Warner Greene, director of the Michael Hulton Center for HIV Cure Research at San Francisco’s Gladstone Institute.
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