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New research: Risk of Covid-19 infection after vaccination is low, but not zero

Increased rates of infection have been strongly linked to behaviours that heighten risk of exposure, such as attending social gatherings in restaurants and bars without adequate masking and physical distancing.

By: Express News Service | New Delhi |
Updated: March 30, 2021 7:43:00 am
An elderly recipient waits to receive a shot of Covid-19 vaccine at a nursing home in Spain last week. (Source: The New York Times)

In a letter to The New England Journal of Medicine, published online on March 23, a group of researchers have reported Covid-19 infection rates for a cohort of healthcare workers previously vaccinated for the novel coronavirus.

The authors looked at pooled data from University of California—San Diego and University of California—Los Angeles healthcare workers who received either the Pfizer or Moderna vaccines between December 16 and February 9 (36,659 first doses, 28,184 second doses), a time period that coincided with a significant surge in COVID-19 infections in the region.

Within this group, 379 individuals tested positive for SARS-CoV-2 at least one day following vaccination, with the majority (71%) testing positive within the first two weeks after the first dose. Thirty-seven 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 estimated that absolute risk of testing positive for SARS-CoV-2 following vaccination was 1.19% for healthcare workers at UC San Diego 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 healthcare workers.

“There are several possible explanations for this elevated risk,” UC San Diego quoted co-author Lucy E Horton as saying. “First, the health care workers surveyed have access to regular asymptomatic and symptomatic testing. 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 strongly linked to behaviours that heighten risk of exposure, such as attending social gatherings in restaurants and bars without adequate masking and physical distancing. This connection is more strongly associated with younger age demographics.

The authors found that risk of infection 14 days after second dose, when maximum immunity is expected to be reached, was rare. “It suggests the efficacy of these vaccines is maintained outside of the trial setting,” they wrote.

Source: University of California—San Diego

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