The staff of a Southern California hospital system experienced a small resurgence in coronavirus infections this summer, despite more than four-fifths of its employees being fully vaccinated.
The findings join a flurry of recent reports of so-called breakthrough infections among vaccinated people. Earlier this summer, Provincetown, Mass., reported a Covid outbreak among many vaccinated residents, and the Centers for Disease Control and Prevention has confirmed that these cases are happening more often with the highly contagious Delta variant of the virus than they did with previous versions.
Breakthrough infections tend to be mild, and vaccines are still highly effective against severe disease and death from the Delta variant. Still, studies on breakthrough infections have fueled the debate over the need for a booster dose, which the Biden administration has supported, as well as masking requirements aimed at preventing the spread of Delta.
Even among its fully vaccinated workers, the University of California San Diego Health witnessed a significant increase in infections from June to July, according to a letter published Wednesday in the New England Journal of Medicine.
From March through July, a total of 227 workers tested positive, according to the letter. Of those, 130 — or 57 percent — were vaccinated.
The total number of symptomatic Covid-19 cases went up more than eightfold, from 15 in June to 125 in July, with 75 percent of the cases occurring in fully vaccinated employees.
There were no reported deaths, and one unvaccinated person was hospitalized, according to the researchers.
While the number of cases represented a tiny fraction of University of California San Diego Health’s overall work force of 19,000, the growing number of infections points to a noteworthy drop in the effectiveness of the vaccines, according to the authors.
“Our data suggest that vaccine effectiveness against any symptomatic disease is considerably lower against the delta variant and may wane over time since vaccination,” they wrote.
Given these findings, they recommended a rapid return to indoor masking and intensive testing strategies to detect the virus.