THURSDAY, March 25, 2021 (HealthDay News) -- It's very rare, but it is possible to catch COVID-19 even if you've been vaccinated, a new study finds.
Looking at vaccinated health care workers at two University of California campuses, researchers found a tiny number tested positive for the virus. This finding highlights the need to keep wearing a mask and to keep social distancing, the researchers said.
"Because of the compulsory daily symptom screening of health care personnel, patients and visitors, and the high testing capacity at both UC San Diego Health and UCLA Health, we were able to identify symptomatic and asymptomatic infections among health care workers at our institutions," said researcher Dr. Jocelyn Keehner. She is an infectious disease fellow at University of California, San Diego (UCSD) School of Medicine.
"Moreover, we were able to describe the infection rates in a real-world scenario, where vaccine roll-out coincided with a surge of infections. We observed a low overall positivity rate among fully immunized health care workers, supporting the high protection rates of these vaccines," she said in a school news release.
For the study, Keehner's team pooled data from health care workers who received either the Pfizer or Moderna vaccines between December 16, 2020, and February 9, 2021. There were more than 36,600 first doses, and more than 28,000 were fully vaccinated (two doses).
Among those vaccinated, 379 tested positive on at least one day following vaccination, with the most (71%) testing positive within the first two weeks after the first dose of vaccine. However, 37 workers tested positive after receiving two doses, when they are supposed to have maximum protection.
The researchers estimated that the absolute risk of testing positive for COVID-19 after vaccination was about 1% for health care workers, which was higher than the risk seen in clinical trials, which were not limited to health care workers.
Researcher Dr. Lucy Horton noted that "there are several possible explanations for this elevated risk." Horton is an associate professor in the division of infectious diseases and global public health at UCSD School of Medicine.
"First, the health care workers surveyed have access to regular asymptomatic and symptomatic testing," she explained. "Second, there was a regional surge in infections overlapping with vaccination campaigns during this time period. And third, there are differences in the demographics of health care workers compared to participants in the vaccine clinical trials. Health care workers tend to be younger and have a greater overall risk of exposure to SARS-CoV-2 in the community."
Increased rates of infection have been linked to behaviors that increase the risk of exposure, such as going to restaurants and bars without masking and physical distancing.
Infection 14 days after the second dose was rare. "It suggests the efficacy of these vaccines is maintained outside of the trial setting," the researchers reported.
The risk, however, is not zero.
According to researcher Dr. Francesca Torriani, a professor of clinical medicine at UCSD, "It underscores the critical importance of continued public health mitigation measures (masking, physical distancing, daily symptom screening and regular testing), even in highly vaccinated environments, until herd immunity is reached at large."
The report was published March 23 in the New England Journal of Medicine.
For more on COVID-19, head to the U.S. Centers for Disease Control and Prevention.
SOURCE: University of California, San Diego School of Medicine, news release, March 23, 2021