Science

Covid-19 infection after vaccination possible but rare

Prashasti Awasthi Mumbai | Updated on March 24, 2021

A new study says the chance is around 1 per cent or less

A new study carried out by researchers from the University of California San Diego School of Medicine and the David Geffen School of Medicine has revealed that people can contract Covid-19 even after vaccination though the chance is around one per cent or less.

For the study published in The New England Journal of Medicine researchers pooled data from UC San Diego and UCLA health care workers who received either the Pfizer or Moderna vaccines between December 16, 2020 and February 9, 2021 — 36,659 first doses, 28,184 second doses.

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The researchers found that within this group, 379 individuals tested positive for coronavirus at least one day following vaccination while the majority (71 per cent) of those individuals tested positive within the first two weeks after the first dose.

Notably, 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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Possible reasons

The authors stated that the absolute risk of testing positive for the virus following vaccination was 1.19 per cent for health care workers at UC San Diego Health while it was 0.97 per cent 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 Lucy E. Horton, MD, MPH, associate professor in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine and medical director of the UC San Diego Health Contact Tracing Unit.

“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.”

Inappropriate behaviour

The researchers noted that increased rates of infection have been strongly linked to behaviours that heighten the risk of exposure. These include 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 also found that the risk of infection 14 days after the 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.

Published on March 24, 2021

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