Even though more than 80% of the staff at the University of California San Diego Health (UCSDH) were fully vaccinated, the system saw a significant jump in Covid-19 cases among staff—prompting concerns about waning vaccine efficacy, according to a new research letter published in the New England Journal of Medicine.

UCSDH sees rise in Covid-19 cases

The researchers found that from March 1 through July 31, 227 UCSDH health care workers tested positive for the coronavirus, including 130 workers who were fully vaccinated (or 57.3% of the workforce who tested positive). The total number of cases represented a small fraction of UCSDH's workforce of more than 19,000 employees, the New York Times reports.

However, of those who tested positive, almost 84% of the fully vaccinated workers and 88.9% of unvaccinated workers developed symptomatic Covid-19. In total, according to the Times, the number of symptomatic Covid-19 cases at UCSDH jumped eightfold between June and July, from 15 to 125—and 75% of those cases occurred among fully vaccinated workers.

However, the researchers noted that no deaths were reported among either the vaccinated or unvaccinated workers who tested positive, and just one unvaccinated worker who tested positive required hospitalization for Covid-19.

Based on those figures, researchers calculated vaccine effectiveness looking at people who tested positive for the coronavirus, had at least one symptom, and had not previously had Covid-19. They found that vaccine effectiveness declined from over 90% in March to 65.5% by July.  

Discussion

According to the researchers, the drop in vaccine effectiveness is "likely to be due to both the emergence of the delta variant and waning immunity over time, compounded by the end of masking requirements in California and the resulting greater risk of exposure in the community."

The researchers said their findings underscore the importance of reinstituting certain coronavirus preventions, such as indoor masking and improved testing, as well as continuing to increase vaccination rates.

In addition, if their findings on decreasing vaccine effectiveness are verified elsewhere, the researchers wrote that booster shots may be necessary. (Abelson, New York Times, 9/1; Keehner et al., New England Journal of Medicine, 9/1)