The CDC released new information Thursday on the transmissibility of COVID-19 in vaccinated people.


In controlled clinical trials, the Pfizer-BioNTech and Moderna vaccines have already proven that they can protect against SARS-CoV-2 infection, along with symptomatic, severe disease, and hospitalization from COVID-19. Now, the Centers for Disease Control and Prevention (CDC) have provided more information that suggests those vaccinated by mRNA vaccines are also less likely to have asymptomatic infection or to spread COVID-19.

The information comes from an update to a science brief on the CDC website regarding COVID-19 vaccines, vaccination, and background on recommendations for fully-vaccinated people. According to the newly-updated brief, multiple studies from different countries "suggest that people vaccinated with Pfizer-BioNTech COVID-19 vaccine who develop COVID-19 have a lower viral load than unvaccinated people." Because viral load—or how much virus an infected person has in their body—is a "key driver of transmission," a lower viral load may indicate "reduced transmissibility."

The updated brief also referenced two studies from the UK, which found that those who were exposed and infected with SARS-CoV-2 after vaccination were far less likely to spread the virus to household contacts.

This news comes after CDC Director Rochelle Walensky, MD, made an appearance on The Rachel Maddow Show on March 29, where she made the comment that "vaccinated people do not carry the virus, don't get sick." Dr. Walensky was referencing data from a then-new CDC study showing that the vaccines' effectiveness—which were already proven effective in clinical trials—also held up in real-word settings.

The study Dr. Walensky referenced, published in the March 29 issue of the CDC's Morbidity and Mortality Weekly Report, included 3,950 health care personnel, first responders, and other essential and frontline workers, whose jobs put them at greater risk than the general population of being exposed to the virus. Of these workers, 2,479 (62.8%) received both recommended doses of either the Pfizer or Moderna vaccine and 477 (12.1%) received only one of the doses.

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For 13 weeks, between December 14, 2020, and March 13, 2021, the CDC routinely tested the workers for COVID-19. The tests were done each week and also as soon as anybody started to feel potential symptoms of the virus. The study participants were from six states: Arizona, Florida, Minnesota, Oregon, Texas, and Utah.

The risk of infection was reduced by 90% among those who were fully vaccinated (meaning at least two weeks had passed since their second dose). Partial vaccination also provided protective benefits, reducing the risk of infection by 80% two weeks after receiving the first dose. And these risk reductions go for not only symptomatic infections, but also asymptomatic and pre-symptomatic infections.

"This study shows that our national vaccination efforts are working. The authorized mRNA COVID-19 vaccines provided early, substantial real-world protection against infection for our nation's health care personnel, first responders, and other frontline essential workers," Dr. Walensky said in a statement. "These findings should offer hope to the millions of Americans receiving COVID-19 vaccines each day and to those who will have the opportunity to roll up their sleeves and get vaccinated in the weeks ahead. The authorized vaccines are the key tool that will help bring an end to this devastating pandemic."

So is it less likely that vaccinated people would pass the virus to others, vaccinated or not? Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, says that that is what the study and Dr. Walensky's comments seem to suggest. "As the CDC director has stated, the new study provides real-world evidence that shows that if you are fully vaccinated, you are virtually unable to be infected with the virus or serve as a vector of spread," he tells Health.

But a spokesperson for the CDC has provided some clarity about what Dr. Walensky said on Maddow's show, telling The New York Times that "Dr. Walensky spoke broadly during this interview. It's possible that some people who are fully vaccinated could get COVID-19. The evidence isn't clear whether they can spread the virus to others. We are continuing to evaluate the evidence."

And so, the CDC is still recommending that fully vaccinated people follow its COVID-19 safety guidelines, which include avoiding medium- or large-sized gatherings and wearing a mask in public. Especially because, while public health experts believe that the latest study suggests that vaccinated people can't pass on the virus since they do not contract it in the first place, it's still something that the CDC needs to confirm. That's because the CDC is still learning, through studies and real-word situations, just how well COVID-19 vaccines keep people from spreading the disease. "Early data show that the vaccines may help keep people from spreading COVID-19, but we are learning more as more people get vaccinated," the agency says.

The CDC is also still learning how long COVID-19 vaccines can protect people, as well as how effective the vaccines are against variants of the virus that causes COVID-19.

According to the CDC's COVID Data Tracker, more than 166 million COVID-19 vaccine doses have been administered in the US so far. Of the entire US population, 50.1% have received at least one dose, and 40.2% are fully vaccinated.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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