Staff at a hospital system in Southern California saw a minor resurgence of coronavirus infections this summer, despite more than four-fifths of its staff being fully vaccinated.
The results join a spate of new reports of so-called breakthrough infections in vaccinated people. Earlier this summer, Provincetown, Massachusetts reported an outbreak of Covid among many vaccinated residents, and the Centers for Disease Control and Prevention has confirmed that these cases are more common with the highly contagious Delta variant of the virus than with previous versions.
Breakthrough infections are usually mild, and vaccines are still highly effective against serious illness and death from the Delta variant. However, studies of breakthrough infections have fueled the debate over the need for a booster dose, which the government of Biden has endorsed, as well as masking requirements to prevent the spread of Delta.
Even among its fully vaccinated workers, the University of California San Diego Health saw a sharp rise in infections from June through July, according to a letter published Wednesday in the New England Journal of Medicine.
From March to July, a total of 227 workers tested positive, according to the letter. Of these, 130 – or 57 percent – were vaccinated.
The total number of symptomatic Covid-19 cases increased more than eight-fold from 15 in June to 125 in July, with 75 percent of cases occurring in fully vaccinated employees.
There were no reported deaths and an unvaccinated person was hospitalized, according to the researchers.
While the number of cases accounted for a tiny fraction of the University of California San Diego Health’s total workforce of 19,000, the growing number of infections, the authors believe, indicates a remarkable decline in vaccine effectiveness.
“Our data suggest that the vaccine’s effectiveness against any symptomatic disease is significantly less than the Delta variant and may decline over time since vaccination,” they wrote.
Given these results, they recommended a quick return to indoor masking and intensive testing strategies to detect the virus.