Vaccines Prevented Fewer Infections as Delta Emerged, Researchers Find

Coronavirus vaccines provided strong protection against infection for essential workers earlier this year, but became less effective as the highly contagious Delta variant became the dominant form of the virus, according to a study published on Tuesday by federal health officials.

It was not clear whether the decline in protection was caused by the emergence of the Delta variant or the lengthening period of time since the inoculations were begun. Vaccine effectiveness showed possible signs of decline starting four months after vaccinations were first rolled out.

“What we were trying to figure out is: Is this Delta, or is this waning effectiveness?” said Ashley Fowlkes, an epidemiologist on the Covid-19 response team at the Centers for Disease Control and Prevention, and the study’s lead author. “Our conclusion is that we can’t really tell.”

Researchers followed thousands of first responders, health care workers and others who could not work remotely in eight locations in Arizona, Florida, Oregon, Texas, Utah and Minnesota. The participants were tested for coronavirus infection every week for 35 weeks, as well as any time they developed Covid-like symptoms.

Most of the vaccinated workers received the Pfizer-BioNTech vaccine; one-third received the Moderna vaccine and 2 percent the Johnson & Johnson vaccine.

Overall, the vaccines reduced infections among vaccinated workers by 80 percent from Dec. 14, when the U.S. vaccination campaign began, to Aug. 14, compared with unvaccinated workers. (The results were adjusted for factors including occupation, demographic characteristics, frequency of close social contact and mask use.)

But while the shots reduced infections by 91 percent before the emergence of the Delta variant, their protectiveness dropped to 66 percent as the variant became dominant in each region.

“We really wanted to let people know that we were seeing a decline in the effectiveness of the vaccine in protection against any infection, symptomatic or asymptomatic, since the Delta variant became dominant,” Dr. Fowlkes said.

“But we also want to reinforce that 66 percent effectiveness is a really high number,” she added. “It’s not 91 percent, but it is still a two-thirds reduction in the risk of infection among vaccinated participants.”

The drop-off in effectiveness “should be interpreted with caution,” however, because the observation period during which Delta was dominant was short, Dr. Fowlkes said, and the overall number of infections was small.

Another C.D.C. study released on Tuesday analyzed infections and hospitalizations in Los Angeles County from May 1 to July 25 of this year. While vaccinated people did become infected, the researchers concluded that among the unvaccinated, infection rates were 4.9 times as high, and the hospitalization rate was 29 times as high.

Of 43,127 known infections in Los Angeles County among residents 16 and older, 25 percent were in fully vaccinated people, 3.3 percent in partly vaccinated people and 71.4 percent in unvaccinated people. (The proportion of fully vaccinated Los Angeles County residents increased to 51 percent on July 25, from 27 percent on May 1.)

Three percent of vaccinated individuals were hospitalized, 0.5 percent were admitted to intensive care units, and 0.2 percent required mechanical ventilation. The comparable rates for unvaccinated individuals were 7.6 percent, 1.5 percent and 0.5 percent, the study reported.

Those who were hospitalized despite vaccination were also older, on average, than the unvaccinated who were hospitalized. The death rate among the vaccinated was lower: 0.2 percent, compared with 0.6 percent among the unvaccinated. The median age at death was also higher among the vaccinated, at 78, compared with a median age of 63 among the unvaccinated.