Vaccines prevented fewer infections as Delta emerged, federal 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?” Dr. Fowlkes said. “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 workers who were vaccinated received the Pfizer-BioNTech vaccine; one-third received the Moderna vaccine, and 2 percent received 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,” 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.

“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 while Delta was dominant was short, Dr. Fowlkes said, and the overall number of infections was small.

Understand Vaccine and Mask Mandates in the U.S.

    • Vaccine rules. On Aug. 23, the Food and Drug Administration granted full approval to Pfizer-BioNTech’s coronavirus vaccine for people 16 and up, paving the way for an increase in mandates in both the public and private sectors. Private companies have been increasingly mandating vaccines for employees. Such mandates are legally allowed and have been upheld in court challenges.
    • Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in indoor public places within areas experiencing outbreaks, a reversal of the guidance it offered in May. See where the C.D.C. guidance would apply, and where states have instituted their own mask policies. The battle over masks has become contentious in some states, with some local leaders defying state bans.
    • College and universities. More than 400 colleges and universities are requiring students to be vaccinated against Covid-19. Almost all are in states that voted for President Biden.
    • Schools. Both California and New York City have introduced vaccine mandates for education staff. A survey released in August found that many American parents of school-age children are opposed to mandated vaccines for students, but were more supportive of mask mandates for students, teachers and staff members who do not have their shots.  
    • Hospitals and medical centers. Many hospitals and major health systems are requiring employees to get a Covid-19 vaccine, citing rising caseloads fueled by the Delta variant and stubbornly low vaccination rates in their communities, even within their work force.
    • New York City. Proof of vaccination is required of workers and customers for indoor dining, gyms, performances and other indoor situations, although enforcement does not begin until Sept. 13. Teachers and other education workers in the city’s vast school system will need to have at least one vaccine dose by Sept. 27, without the option of weekly testing. City hospital workers must also get a vaccine or be subjected to weekly testing. Similar rules are in place for New York State employees.
    • At the federal level. The Pentagon announced that it would seek to make coronavirus vaccinations mandatory for the country’s 1.3 million active-duty troops “no later” than the middle of September. President Biden announced that all civilian federal employees would have to be vaccinated against the coronavirus or submit to regular testing, social distancing, mask requirements and restrictions on most travel.

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