THURSDAY, Sept. 2, 2021 (HealthDay News) -- 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 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 in 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 said 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 added.
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 the individual required hospital care, and that case occurred in an unvaccinated worker, the researchers said. No one died.
And although 94 of the 125 cases reported in July occurred in vaccinated workers, the key number to look at is what the researchers call the "attack rate."
According to the study, in July there were about 5.7 cases of COVID-19 per every 1,000 vaccinated workers at UCSD Health.
Compare that to the attack rate among the unvaccinated: 16.4 cases of COVID-19 per every 1,000 workers.
That means that an unvaccinated person was nearly three times more likely to contract COVID-19 compared to a fully vaccinated person.
As for symptoms, in a tally of cases occurring among UCSD health workers from March through July, "symptoms were present in 109 of the 130 fully vaccinated workers [83.8%] and in 80 of the 90 unvaccinated workers [88.9%]," the study found.
The new study represents a detailed look at COVID infection rates, because UCSD Health "has a low threshold for SARS-CoV-2 testing, which is triggered by the presence of at least one symptom during daily screening or by an identified exposure, regardless of vaccination status," the authors noted.
Dr. Amesh Adalja is an expert in infectious disease and senior scholar at the Johns Hopkins Center for Health Security in Baltimore. Reading over the findings, he said that they support the effectiveness and necessity of getting vaccinated against the new coronavirus.
"Vaccines aren't force fields -- breakthrough infections will occur, especially as people get back to their activities in the midst of the more contagious Delta variant," Adalja said.
"The breakthroughs were all mild, with no one hospitalized," he added. "To me, that shows the vaccines are doing what they were designed to: taming the virus."
For more on COVID-19 vaccination, see the U.S. Centers for Disease Control and Prevention.
SOURCES: Amesh Adalja, MD, senior scholar, Johns Hopkins Center for Health Security, New England Journal of Medicine, Sept.1, 2021