Contrary to What the CDC's New COVID-19 Advice Implies, There Is Strong Evidence That Vaccination Curtails Virus Transmission

Research in Israel, the U.K., and the U.S. has found dramatic reductions in asymptomatic infections.


The Centers for Disease Control and Prevention (CDC) today issued new guidance for people who have been vaccinated against COVID-19. The agency says fully vaccinated people should feel free to forgo face masks and physical distancing while mingling indoors with each other or with people from "a single household" who are at low risk from COVID-19. But it says vaccinated people should continue wearing masks and practicing physical distancing in public places or when visiting unvaccinated people in high-risk groups.

The CDC notes "the rapidly evolving science on COVID-19 vaccines." While there is ample evidence from clinical trials that vaccination dramatically reduces the odds of serious disease, hospitalization, and death, there are still some questions about the extent to which it prevents infection and transmission. Yet several studies provide strong evidence that vaccination protects not only people who receive shots but also unvaccinated people in their vicinity.

Even without that research, there are sound reasons to expect COVID-19 vaccination to reduce the spread of the disease. "First, when the vaccines were studied in macaque monkeys (during preclinical testing), they did eliminate asymptomatic infection," a March 2 article published by the Association of American Medical Colleges notes. "Researchers swabbed the vaccinated macaques' noses and found little or no virus. Second, the types of antibodies that are stimulated by most systemic vaccines (IgG and IgA) do tend to block viral infection in the nose (and no viral load in the nose most likely translates to no transmission). Finally, when monoclonal antibodies are given to COVID-19 patients, those antibodies reduce the viral load throughout the respiratory tract, including the nose."

Several studies add to that evidence. Researchers in Israel tracked nearly 600,000 people who received the two-dose Pfizer vaccine, comparing them to an equal number of unvaccinated controls matched for age, sex, "sector" (i.e., "general Jewish, Arab, or ultra-Orthodox Jewish"), neighborhood of residence, influenza vaccination history, pregnancy, and preexisting medical conditions associated with a higher COVID-19 risk. In a study published last month by The New England Journal of Medicine, they reported sharp reductions in asymptomatic infection as well as other outcomes.

During a follow-up period beginning seven days after the second dose, vaccinated subjects were 92 percent less likely to test positive for the coronavirus, 94 percent less likely to develop COVID-19 symptoms, and 92 percent less likely to suffer serious disease. In other words, the risk of infection at this stage was much smaller than it was in the unvaccinated group. Although that risk was not completely eliminated, the CDC notes that "preliminary data from Israel suggest that persons vaccinated with Pfizer-BioNTech COVID-19 vaccine who develop COVID-19 have a four-fold lower viral load than unvaccinated persons," which implies that they are less likely to transmit the virus even when they are infected.

What about newer virus variants? "This study estimates an average effectiveness of the vaccine over multiple strains," the authors of the NEJM study say. "Although we cannot provide a specific effectiveness estimate for the B.1.1.7 variant, the plateau observed during the later periods in the cumulative incidence curve for vaccinated persons suggests that the BNT162b2 vaccine is also effective for this variant, an observation consistent with previous reports that showed preserved neutralizing antibody titers. The B.1.351 variant was estimated to be rare in Israel at the time of data extraction."

Lancet study of more than 23,000 hospital workers in Scotland and Northern Ireland who were periodically tested for the coronavirus likewise found that the Pfizer vaccine dramatically reduced asymptomatic infection. Three weeks after the first dose, infection was reduced by 70 percent. Seven days after the second dose, the reduction rose to 85 percent. "Our study demonstrates that the BNT162b2 vaccine effectively prevents both symptomatic and asymptomatic infection in working age adults," the researchers report. Since "this cohort was vaccinated when the dominant variant in circulation was B1.1.7," the study "demonstrates effectiveness against this variant."

A preprint study of about 62,000 Mayo Clinic patients in four states found that "administration of two COVID-19 vaccine doses was 88.7% effective in preventing SARS-CoV-2 infection." Complementing the data from randomized clinical trials of the Pfizer and Moderna vaccines, the researchers said, "this study demonstrates their real-world effectiveness in reducing the rates of SARS-CoV-2 infection and COVID-19 severity among individuals at highest risk for infection."

As The New York Times notes, these findings are consistent with the results of animal research and studies involving smaller samples of people who participated in the Moderna and Johnson & Johnson clinical trials. In a study of eight vaccinated monkeys exposed to the virus, for example, seven were uninfected. Among a few dozen people who received the Moderna vaccine, infection was reduced by about two-thirds after the first dose. Johnson & Johnson's single-dose vaccine reduced infection by 74 percent in a subsample of 3,000 people.

"We feel confident that there's a reduction," University of Florida biostatistician Natalie Dean told the Times. "We don't know the exact magnitude, but it's not 100 percent." While the estimates "could change with more data," said Dan Barouch, a virologist at Beth Israel Medical Center in Boston, "the effect seems quite strong."

The evidence so far indicates that people who have been vaccinated are much less likely to be infected—perhaps as much as 92 percent less likely, judging from the Israeli study. That means they are much less likely to infect other people.

Encounters among vaccinated people, the focus of the CDC's guidance, obviously pose the lowest risk. The CDC's advice also allows close encounters between vaccinated people and unvaccinated people in low-risk groups, which suggests it's OK to hug your vaccinated parents or grandparents.

But the CDC wants vaccinated people to carry on as before in public places. Here we are talking about a potential risk to strangers, so the moral calculus is different. And given the CDC's habitual risk aversion, its recommendation regarding masks and physical distancing in public is not at all surprising. After all, even an 85 percent or 92 percent reduction in the likelihood that you are carrying the virus still means the chance is greater than zero.

"They should wear masks until we actually prove that vaccines prevent transmission," says Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases. Research that tracks close contacts of vaccinated people might prove that effect to Fauci's satisfaction. In the meantime, the Times says, "when vaccinated people can ditch the masks in public spaces will depend on how quickly the rates of disease drop and what percentage of people remain unvaccinated in the surrounding community."

Many vaccinated people are apt to see things differently, especially if they think the onus should be on members of high-risk groups to avoid situations that are conducive to virus transmission. Now that vaccines are widely available to older Americans and people with preexisting medical conditions, that argument has additional force.

The minimal inconvenience of wearing a mask during a trip to the grocery store (assuming the store no longer requires it) is a pretty easy way of reassuring people who are especially vulnerable to COVID-19 but have not managed to get their shots yet, especially since they ordinarily have no way of knowing who has been vaccinated. But as time goes by and daily new cases continue to drop, vaccinated Americans are apt to lose patience with people in high-risk groups who have the opportunity to get vaccinated but fail to do so.