UC San Diego Health reported a significant rise in staff members testing positive for COVID-19 earlier this summer, even with 87 percent of its workforce being fully vaccinated as of July, according to a research letter published Sept. 1 in The New England Journal of Medicine.
The health system saw fewer than 30 cases involving healthcare workers between March and June. However, after California's mask mandate ended June 15 and the delta variant quickly gained dominance in the U.S., infections increased rapidly, the health system said.
In March, just three fully vaccinated employees had symptomatic COVID-19 cases, compared to 94 in July.
Overall, 227 members of the health system's 19,000-person team tested positive between March and July, and 57 percent of cases involved people who were fully vaccinated. Nearly 84 percent of vaccinated workers and 89 percent of unvaccinated workers had symptoms. One staff member required hospitalization, but no one died.
"The dramatic change in vaccine effectiveness from June to July 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," researchers said.
The findings underscore the importance of reinstating public health precautions such as indoor masking and intensive testing to help prevent more infections and deaths, they added.
To view the full research letter, click here.