A large study conducted on US health workers has found that the efficacy of the Pfizer-BioNTech and Moderna COVID-19 vaccines has dropped from 91% before the Delta variant became dominant to 66% afterward, according to Medical Xpress.
The U.S. Centers for Disease Control and Prevention (CDC) has been looking at the performance of these two mRNA-based vaccines since they received FDA’s emergency authorization for use in health care personnel, first responders, and other frontline workers.
The study looked at thousands of health workers across six states and tested them weekly on the onset of COVID-19 symptoms, allowing investigators to estimate efficacy against symptomatic and asymptomatic COVID-19 infection.
The researchers looked at the rate of infections among both vaccinated and unvaccinated people along with the amount of time they were tracked. They found that the effectiveness of the vaccines was estimated at 91% in the initial study period, from December 14, 2020, to April 10, 2021.
However, during weeks in the run-up to August 14, when the highly contagious Delta variant became dominant, the efficacy dropped to 66%.
The researchers cautioned that the protection from both vaccines could be waning over time anyway, and the 66% estimate was based on a relatively short study period with few infections, per Medical Xpress.
The authors said, “Although these interim findings suggest a moderate reduction in the effectiveness of COVID-19 vaccines in preventing infection, the sustained two-thirds reduction in infection risk underscores the continued importance and benefits of COVID-19 vaccination.”
Other studies have also shown that the efficacy of these vaccines has dropped against the Delta variant, even though the level of that drop may differ between studies.
A recent CDC study conducted on New York patients found that protection against severe illness appears more stable, exceeding 90%.
Another CDC study conducted on Los Angeles patients showed unvaccinated people were 29.2 times more likely to be hospitalized with COVID-19 than the vaccinated, corresponding to an efficacy of about 97%.
In the United States, the highly transmissible Delta strain become dominant in early July and since then, the number of cases and hospitalization has increased, even in fully vaccinated people.
Another study, published in the Journal of Virological Methods, has revealed how contagious the Delta variant is, stating that the amount of virus found in the first tests of patients with Delta was 1,000 times higher compared to patients with the original strain of the virus in 2020. The article was published in Medical Xpress.
