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Covid-19 vaccines effectiveness drops from 91% to 66% after Delta variant circulation: US CDC study shows

The vaccine effectiveness against Covid-19 infection dropped from 91% to 66% once the Delta variant accounted for the majority of circulating virus, according to a study published by the US Centers for Disease Control and Prevention.

“While we did see a reduction in the protection of the Covid-19 vaccine against the Delta variant, it’s still two-thirds reduction of risk,” lead author Ashley Fowlkes, an epidemiologist for CDC Covid-19 Emergency Response said.

The study is in line with others from the US and around the world showing Delta’s increased tendency to cause largely minor infections among fully vaccinated people.




Still the effectiveness of vaccines against severe disease, including hospitalization and death has remained high against all known variants.

The study does not cover disease severity. The paper has been following “health care personnel, first responders, and other essential and frontline workers” who receive weekly PCR tests in eight locations across six US states. The vast majority are vaccinated.

The study’s reliance on regular, weekly testing makes it possible to capture a more complete picture of Covid-19 infections in a group, since people with mild or no symptoms may be less likely to get tested overall. In the official vaccine trials, efficacy was calculated against symptomatic Covid-19 not all infections.



The study’s authors are planning future analyses comparing the different Covid-19 vaccines as well as the types of symptoms experienced by vaccinated and unvaccinated people who become infected.

“It’s still a very powerful vaccine,” Fowlkes added, especially when it comes to more severe outcomes. “But we are also looking towards continuing to use masks for a little bit longer.”

Separately, a new study out of Los Angeles County reaffirms that fully vaccinated people are far less likely than unvaccinated people to be hospitalized, admitted to an intensive care unit, require mechanical ventilation or die from Covid-19.

“On July 25, infection and hospitalization rates among unvaccinated persons were 4.9 and 29.2 times, respectively, those in fully vaccinated persons,” researchers from the Los Angeles County Department of Public Health wrote in the study.