Moderna and Pfizer COVID-19 vaccines may block infection as well as disease

Studies suggest fully vaccinated people pose a low risk for transmitting the coronavirus

a woman receives her first dose of the COVID-19 vaccine at a hospital

Diana Carolina, a pharmacist at Memorial Healthcare System, gets her first dose of a COVID-19 vaccine. Studies in health care workers indicate the shots reduce the risk of catching coronavirus.

Joe Raedle/Staff/Getty Images News

Vaccines against COVID-19 are about 90 percent effective at blocking coronavirus infections, real-world studies of health care workers, firefighters, police, teachers and other essential workers suggest.

Even after just one dose of the mRNA vaccines made by Pfizer and Moderna, the vaccines  reduced the chance of getting infected with SARS-CoV-2, researchers report March 29 in Morbidity and Mortality Weekly Report. “We clearly showed in our study that if you were at least 14 days out from your first shot, you had 80 percent protection” from infection, says Jeff Burgess, associate dean for research at the Mel and Enid Zuckerman College of Public Health at the University of Arizona in Tucson. The study is part of a growing body of evidence suggesting that the vaccines not only reduce the risk of getting seriously ill with COVID-19, but can prevent catching the virus in the first place.

“If you can’t get infected, you can’t infect anyone else, which means the vaccines can reduce transmission as well as the disease,” says Marm Kilpatrick, an infectious diseases researcher at the University of California, Santa Cruz, who was not involved in the study.

That is welcome news coming on the heels of data indicating that cases, hospitalizations and deaths are on the rise again in the United States as states lift mask mandates and open businesses at full capacity.

“Right now I’m scared,” Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention said during a White House briefing on March 29, noting “the recurring feeling I have of impending doom.” She urged people to “hang on a little longer” and continue to wear masks, social distance and get vaccinated to head off a potential fourth surge of the disease. “We have so much to look forward to. So much promise and potential of where we are and so much reason for hope,” she said.

Part of the reason for hope comes from the MMWR study. The study “is tremendously encouraging and complements other recent studies,” Walensky said.

Nearly 4,000 health care workers, first responders and other essential workers in six states took part in the study led by CDC researchers. From December 14 through March 13, the workers submitted weekly nasal swabs for coronavirus testing. Both symptomatic and asymptomatic infection rates fell after vaccination. A small number of vaccinated people in the study still got infected.

Other real-world data collected from health care workers in California and Texas also seem to back up those findings, researchers say in separate reports published March 23 in the New England Journal of Medicine. In the Texas report, 234 of 8,969 nonvaccinated employees at the University of Texas Southwestern Medical Center in Dallas tested positive for the coronavirus from December 15 to January 28. That’s 2.61 percent, compared with 1.82 percent (112 of 6,144) of employees that had gotten one shot and 0.05 percent (four of 8,121) of fully vaccinated employees.

In the California report, infections among health care workers also fell with increasing vaccination levels. Only seven infections occurred among 4,167 people who were at least 15 days out from getting their second dose of vaccine. The vaccines prevented health care workers in the study from becoming seriously ill, says study coauthor Francesca Torriani, an infectious diseases physician and hospital epidemiologist at the University of California, San Diego.

Reducing the severity of illnesses will help relieve the burden on hospital systems if there should be a fourth wave, she says. But to really stop transmission of the virus, it’s important to vaccinate 18- to 30-year-olds. “Those are the ones who are right now transmitting the disease.” Motivating healthy young people who are less likely to become severely ill in the first place to take the shots could be difficult, Torriani says. “There’s not much in it for them, but there is a lot in it for their families, so I’m hoping that realization” will push young people toward getting vaccinated.

Because some vaccinated people can still get infected, the CDC and other public health agencies have recommended that people who have gotten their shots continue to wear masks in public and take other precautions to avoid spreading the virus.

Data from Israel does suggest that the Pfizer vaccine might block transmission of the virus (SN: 2/12/21). Unvaccinated people produce 2.58 to 4.5 times more virus than vaccinated people do, researchers report March 29 in Nature Medicine. Those data show vaccinated people have a lower “viral load” and are less likely to pass the coronavirus to others if they do become infected, but the effect is not as strong as might be hoped to truly limit transmission, Kilpatrick says. That reduction in viral load amounts to about an 11 percent decrease in infectiousness, he says. “That’s good … but you’d like to be half as infectious or three-quarters lower infectiousness.”

There is not yet enough data to say for sure that vaccines prevent transmission, Torriani says. “There is definitely heavy suggestion,” but further studies on viral load are needed.

So are data from everyday citizens. Aside from the MMWR study, almost all of the real-world vaccine data collected so far in the United States has been among health care workers. Those workers may not give a true picture of transmission risks because they have better personal protective equipment and ventilation than the average person does, Torriani says. Far more indicative would be studies to determine whether vaccinated people are less likely to infect household members. “If [household transmission] goes away with vaccination, that would be the proof.”

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