Fully vaccinated people can still get infected with the novel coronavirus, but infections appear to be mild and very rare, according to two reports published in the New England Journal of Medicine as correspondence.

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New reports show coronavirus infections can occur among fully vaccinated people

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The first report released data—which spanned from Dec. 15, 2020 through Jan. 28, 2021—on a vaccination program launched by the University of Texas Southwestern Medical Center (UTSW) for its frontline employees on Dec. 15, 2020. The start of UTSW's vaccination campaign "coincided with a rapidly escalating number of new [coronavirus] infections in North Texas," which "strained health systems," William Daniel, a professor in clinical quality improvement at UTSW, and his colleagues wrote in the report.

According to the report, 59% of 23,234 UTSW employees received a first dose of either Pfizer/BioNTech's or Moderna's Covid-19 vaccine and 30% received a second dose within 31 days of the program's launch.

Researchers found infections occurred among partially and fully vaccinated UTSW employees but at lower rates than among those who hadn't received a vaccine. Specifically, data from the report showed infections occurred among:

• 234 of 8,969 nonvaccinated employees;
• 112 of 6,144 partially vaccinated employees; and
• 4 of 8,121 fully vaccinated employees. 

The second report released data—which spanned from Dec. 16, 2020 through Feb. 9, 2021—on vaccination campaigns launched by University of California-San Diego (UCSD) and the University of California-Los Angeles (UCLA) health systems on Dec. 16, 2020.

According to the data, between Dec. 16, 2020 and Feb. 9, 2021, a total of 36,659 health care workers received their first dose of a Covid-19 vaccine and 28,184 of them received their second dose. Among the health care workers who received one dose of the vaccine, 379 tested positive for the coronavirus at least one day after their vaccination—with 71% of positive tests occurring within the first two weeks after patients received their initial vaccine dose, the data showed.

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In comparison, only 37 health care workers who received two doses of a vaccine tested positive for the coronavirus at least one day after receiving their second dose of the vaccine—with the majority of those positive tests occurring within the first seven days after patients had their second shot, the data showed.

What the findings mean

Some experts said the latest reports should serve as a reminder that vaccinated people are still susceptible to infections.  

Francesca Torriani, an infectious disease specialist at UC San Diego Health who led the California study, said, "We felt really strongly that this data should not lead people to say, 'Let's all get vaccinated and then we can all stop wearing masks.' These measures have to continue until a larger segment of the population is vaccinated."

Other experts have noted that many questions remain unanswered about vaccines and infections among vaccinated people, which are often called "breakthrough" cases.  

For example, it remains unclear just how many vaccinated people get infected with the coronavirus. However, Pfizer and Moderna in the coming months are expected to release data that will shed light on how often vaccinated people become infected by the virus even if they don't have symptoms.

Separately, Kristen Nordlund, a CDC spokesperson, said a small team of researchers at CDC are currently examining breakthrough cases and investigating whether new variants of the novel coronavirus are playing a role in breakthrough cases. However, "[c]urrently, there is no evidence that Covid-19 after vaccination is occurring because of changes in the virus," Nordlund said.

Nordlund added that, "Based on what we know about vaccines for other diseases and early data from clinical trials, experts believe that getting a Covid-19 vaccine might also help keep you from getting seriously ill, even if you do get Covid-19."

David Wohl, the medical director of the COVID Vaccination Clinic at UNC Medical Center's UNC Hospitals Hillsborough Campus, said doctors at the University of North Carolina have found a few asymptotic cases among vaccinated patients, which suggests the Covid-19 vaccines are preventing people from getting sick even if the vaccines aren't fully stopping the virus from infecting people (Grady, New York Times, 3/23; Fernandez, Axios, 3/24; Daniel et al., NEJM, 3/23; Keehner et al., NEJM, 3/23).