Rise of Delta at California Hospital Tells a Cautionary Tale

Over half of healthcare workers in a California hospital who tested positive for COVID-19 from March to July were fully vaccinated and symptomatic, though other factors may have been at play, researchers found.

University of California San Diego Health reported 227 workers testing positive from March 1 to July 31, and 57.3% of those were fully vaccinated. Also, 84% of those fully vaccinated workers had symptoms, reported Francesca Torriani, MD, of University of California San Diego Health, and colleagues.

However, the attack rate in July was much higher in unvaccinated individuals (16.4 per 1,000 persons) compared to fully vaccinated individuals (6.7 per 1,000 for those vaccinated in January and February; 3.7 per 1,000 for those vaccinated from March to May), they wrote in a correspondence in the New England Journal of Medicine.

They cited two potential reasons for this surge in cases: the Delta variant, and the end of California’s mask mandate on June 15.

“Our findings underline the importance of rapidly reinstating nonpharmaceutical interventions, such as indoor masking and intensive testing strategies, in addition to continued efforts to increase vaccinations, as strategies to prevent avoidable illness and deaths and to avoid mass disruptions to society during the spread of this formidable variant,” the authors wrote.

However, they also noted waning vaccine effectiveness, which fell from over 90% (March to June) to 65.5% (95% CI 48.9-78.9) in July. The authors added that their data suggest “vaccine effectiveness against any symptomatic disease is considerably lower against the Delta variant.”

They said that the Delta variant first emerged in mid-April, and accounted for almost all (95%) of isolates from the health system by the end of July. As of July, 83% of healthcare workers at University of California San Diego Health were vaccinated, allowing for a bigger denominator where 75% of the 94 cases in July were among vaccinated workers.

The authors also said if these findings on waning immunity are verified by others, “booster doses may be indicated.” CDC staff recently proposed that healthcare workers could receive booster doses based on a “risk-based approach” to preserve the public health infrastructure.