Infections among health care workers who are fully vaccinated for COVID-19 are extremely rare, according to a new study.
Researchers examined data from employee health records of more than 36,600 health care workers in California and found that less than 1% tested positive for COVID-19 after being fully vaccinated -- meaning both doses plus two weeks for the immunity to build -- with the Pfizer or Moderna vaccines.
While anecdotal reports of individual doctors getting COVID-19 after receiving one or both doses of the vaccine have been covered in the news recently, the study offers a more comprehensive look at how often such post-vaccination infections occur in fully vaccinated people.
"This study confirms that the vaccines are highly effective at preventing COVID-19," study authors Dr. Shira Abeles and Dr. Francesca Torriani told ABC News.
"It also serves as a reminder that vaccines are not 100% effective and there are 'breakthrough' cases," the doctors added, noting that public health measures need to stay in effect to protect against stray cases.
They also pointed to a cause for optimism. "The vaccines showed strong efficacy during a surge of cases in Southern California, which is great news for all of us," said Abeles and Torriani, both of whom work at UC San Diego Health.
The study period lasted from Dec. 16, 2020, when the University of California, San Diego (UCSD) and the University of California, Los Angeles (UCLA) started their vaccination programs, until Feb. 9, 2021.
After full vaccination, the risk of testing positive for the virus was 1.19% among UCSD employees and 0.97% among UCLA employees.
The research also showed that the further along in the vaccination process health care workers were, the better the vaccine worked.
Of the 36,659 workers vaccinated, 379 tested positive after their first vaccine dose, with the majority of those individuals testing positive within two weeks of their first shot. After receiving both doses of the two-shot vaccine, 37 people tested positive, with the majority testing positive less than a week after their second dose. Only seven health workers tested positive 15 days or more after their second vaccination.
Not a single vaccinated health care worker who contracted COVID-19 was hospitalized or died and those who did get sick appeared to have milder symptoms than the unvaccinated, according to the researchers.
"We do hope this helps boost confidence in the vaccine," Abeles and Torriani said.
There were limitations to the research. Health care workers who tested positive after one dose may have been exposed to the virus prior to getting vaccinated. It's also not known which virus variants were circulating during the study period. The research was published in a letter to the New England Journal of Medicine on Tuesday.