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While the vaccines against SARS-CoV-2 greatly minimize risk of infection, data shows that breakthrough cases are still possible, highlighting the importance of wearing a mask and distancing even post-vaccination.

In two datasets, one by the Universities of California at San Diego (UCSD) and Los Angeles (UCLA), and the other by the University of Texas Southwestern Medical Center (UTSMC), researchers analyzed data from the health records of vaccinated healthcare workers to see how many contracted COVID-19. 

UCSD and UCLA found that only seven out of 28,184 individuals tested positive for SARS-CoV-2 after being fully vaccinated (two weeks after their second dose). In UTSMC’s case, out of 8,121 fully vaccinated employees, just four tested positive.

Based on vaccine trial results, scientists already knew that fully vaccinated individuals could still contract COVID-19. But these are the first records outside of an official trial to note this kind of data. 

[Related: What the first year of COVID tells us about the next]

In both studies, multiple employees tested positive before reaching the “fully vaccinated” stage—either during the time when they had only received one dose, or in the days immediately following the second shot. But the authors of the UCSD-UCLA study write that the fact that so few people got infected once clearing the two-week period after their second dose “is encouraging and suggests that the efficacy of these vaccines is maintained outside the trial setting.”

It’s also important to know that, so far, vaccinated people who do get infected tend to only have mild or even asymptomatic cases of COVID-19, and don’t require hospitalization. 

Dr. Francesca J. Torriani, a UCSD infectious disease specialist who led the California study, notes that, while this news is encouraging, people still need to take caution. She told The New York Times that, “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.’” 

[Related: How long will we keep wearing masks?]

Torriani’s statement echoes the CDC guidelines, which state that while vaccinations are proving effective, fully vaccinated individuals still need to continue wearing masks, distancing, and avoiding poorly ventilated spaces—especially since not everyone is vaccinated. 

Even as vaccination rollouts continue across the country, cases remain stubbornly high: The US has been averaging more than 57,000 new COVID cases per day.