Aug. 24 (UPI) -- The three COVID-19 vaccines approved for use in the United States are much less effective against the Delta variant of the virus, but still protect against severe illness in most cases, according to data released Tuesday by the Centers for Disease Control and Prevention.
In 4,200 front-line workers in eight U.S. cities who received the vaccines from Moderna, Pfizer-BioNTech and Johnson & Johnson, the shots prevented serious illness from the virus in 91% before the Delta variant became the predominant one locally, the data showed.
However, after the Delta variant became the most common strain of the virus in circulation in these regions, vaccine efficacy dropped to 66%, the CDC said.
"This trend should be interpreted with caution because vaccine efficacy might also be declining as time since vaccination increases," the CDC researchers wrote.
Among those who were fully vaccinated -- meaning they had received both shots of the two-dose Moderna and Pfizer-BioNTech vaccines or the one-shot Johnson & Johnson vaccine -- protection peaked at 85% between 14 days and 119 days after inoculation.
From 120 days to 149 after vaccination, protection fell to 81% before dropping to 73% 150 days or more following receipt of the vaccine.
The findings are based on an analysis of COVID-19 cases among 4,217 first responders, healthcare workers and others working in "essential" fields between December 2020 and Aug. 14.
Of the participants, nearly 3,500, or 83% were fully vaccinated, with 65% receiving the Pfizer-BioNTech shot, 33% given the Moderna shot and 2% getting the Johnson & Johnson shot.
Before the Delta variant emerged, there were 10 cases of COVID-19 infection among 2,875 fully vaccinated participants, and 175 cases among 4,137 unvaccinated participants.
However, after the Delta variant became dominant in circulation in the local areas of study participants, there were 24 confirmed COVID-19 cases among 2,352 fully vaccinated people and 19 cases among 488 unvaccinated people, the CDC 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," the agency researchers wrote.
In addition, a separate analysis also released by the CDC on Tuesday found that of more than 43,000 confirmed cases of COVID-19 in Los Angeles County, Calif., between May 1 and July 25, 25% involved fully vaccinated people and 71% occurred in unvaccinated people.
Among these cases, hospitalization rates were 29 times higher in unvaccinated people than in, those who were fully vaccinated, the data showed.