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Breakthrough Infections

 COVID-19 Breakthrough Infections
    As of August 2021, a little over 50% of the U.S. population has been fully vaccinated against COVID-19. However, despite the increasing number of vaccinated individuals and high efficacy of the vaccines, rare breakthrough cases have occurred. With the reports of breakthrough cases, or instances when people who are fully vaccinated get infected with COVID-19, many questions have arisen. How common are breakthrough infections? What are the symptoms? How severe are the cases? These questions are addressed in a peer-reviewed study investigating breakthrough cases among healthcare workers in Israel. The study was recently published in the New England Journal of Medicine on July 28, 2021. 

About the Study

    The study setting took place in the Sheba Medical Center, which is the largest medical center in Israel staffed with 12,586 healthcare workers. By April 28, 2021, 11,453 workers (91%) had been fully vaccinated with the Pfizer vaccine. The study was conducted from January 20, 2021, 11 days after the first workers received a second vaccine dose, until April 28, 2021. Israel's pandemic surge reached its peak in January and was followed by decreasing cases after the vaccination period. The goal of the study was to identify breakthrough cases, including asymptomatic infections, that occurred among health care workers at the center during the study period.

Study Results

    Out of the 11,453 healthcare workers, 1,497 or 31% underwent COVID-19 Ag-PCR testing. Thirty-nine breakthrough cases were identified, accounting for a breakthrough infection rate of 2.6%. The average age of the 39 infected workers was 39 years with the majority (64%) being women. The B.1.1.7 variant, which was the most widespread variant in Israel at the time of the study, was identified in 85% of samples tested. Among the 39 patients, 37 had available data suggesting that an unvaccinated person was the likely source of COVID-19 infection. Of the 39 workers, 67% had mild symptoms and 33% were asymptomatic during infection. None of the patients were hospitalized. 

    The study also found that infected workers had lower neutralizing antibody levels during the peri-infection period, or the week before diagnosis, compared to the uninfected control groups. Higher peri-infection antibody levels were associated with lower infectivity.

Those who tested negative for SARS-CoV-2 tended to have lower peri-infection antibody levels

Discussion

   Among those fully vaccinated, breakthrough infections appear to be rare and are correlated with lower antibody levels during the peri-infection period. Breakthrough infection symptoms were mild or asymptomatic, suggesting that the vaccine can reduce or prevent symptomatic disease. Overall, the study found that the Pfizer vaccine is still highly effective, with only rare breakthrough cases being identified. However, these breakthrough infections could be especially risky for vulnerable populations since infections are often asymptomatic. 

Sources

1. Bergwerk, Moriah, et al. “Covid-19 Breakthrough Infections in Vaccinated Health Care Workers.” New England Journal of Medicine, 28 July 2021, doi:10.1056/nejmoa2109072.

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