A new study of more than 19,000 San Diego health care workers finds that COVID-19 vaccination may have lost some of its power to ward off “breakthrough” infections.
But that’s no reason to lose faith in the vaccines: In the study, the rate of COVID-19 infection in July among unvaccinated workers was nearly triple that of people who had gotten their shots.
Still, the study found a “dramatic change in vaccine effectiveness between June and July,” according to researchers led by Dr. Francesca Torriani, of the University of California San Diego (UCSD) Health.
Her team calculated that “vaccine effectiveness exceeded 90% from March through June, but fell to 65.5% in July.”
In June, one-third of diagnosed cases of COVID-19 occurred among fully vaccinated health care workers. But just one month later, 3 of every 4 cases occurred among the vaccinated.
Because more than 83% of all workers at UCSD Health are now vaccinated, some kind of rise in the number of breakthrough cases was expected. But the sudden, sharp uptick between June and July was still striking.
It “is likely to be due to both the emergence of the Delta variant and waning immunity over time,” Torriani and her colleagues share in a report published Sept. 1 in the New England Journal of Medicine.
Risks of infection in July were also “compounded by the end of masking requirements in California and the resulting greater risk of exposure in the community,” the authors add.
Overall, regular testing of just over 19,000 UCSD health workers uncovered 125 cases of symptomatic COVID-19 in July, up from just 13 cases in May and 15 in June. Only one case was severe enough that