Although the risk is tiny, infection from the virus causing COVID-19 after receiving vaccines is possible, a report published by a team of investigators revealed Tuesday.
In a letter to The New England Journal of Medicine published Tuesday, a group of researchers from UC San Diego School of Medicine and the David Geffen School of Medicine at UCLA report COVID-19 infection rates for a cohort of health care workers previously vaccinated for the novel coronavirus.
"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 co-author Dr. Jocelyn Keehner, an infectious disease fellow at UCSD.
"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," Keehner continued.
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The authors looked at pooled data from UCSD and UCLA health care workers who received either the Pfizer or Moderna vaccines between Dec. 16, 2020 and Feb. 9 -- including 36,659 first doses, 28,184 second doses -- a period that coincided with a significant surge in COVID-19 infections in the region.
Within this group, 379 individuals tested positive for the virus at least one day following vaccination, with the majority -- 71% -- testing positive within the first two weeks after the first dose. Of the group, 37 health care workers tested positive after receiving two doses, which is when maximum immune protection is expected to be achieved with both vaccines.
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The authors estimate absolute risk of testing positive for the virus following vaccination was 1.19% for health care workers at UCSD Health and 0.97% at UCLA Health -- both higher than the risk identified in the Moderna and Pfizer clinical trials, which were not limited to health care workers.
"There are several possible explanations for this elevated risk," said co-author Dr. Lucy E. Horton, associate professor at UCSD School of Medicine and medical director of the UCSD Health Contact Tracing Unit.
"First, the health care workers surveyed have access to regular asymptomatic and symptomatic testing," Horton continued. "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."
Horton said health care workers tend to be younger and are part of a demographic which she says engage in riskier behavior such as "attending social gatherings in restaurants and bars without adequate masking and physical distancing."
The authors found that risk of infection 14 days after second dose, when maximum immunity is expected to be reached, was rare.
Nonetheless, they also noted that risk is not zero. While both Pfizer and Moderna report efficacy levels in the mid-90s, neither is 100%.
"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," said corresponding author Dr. Francesca Torriani, professor of clinical medicine at UCSD School of Medicine and program director of Infection Prevention and Clinical Epidemiology at UCSD Health.
