Vaccines Prevented Fewer Infections as Delta Emerged, Researchers Find

Coronavirus vaccines offered strong protection against infection for key workers earlier this year, but became less effective when the highly contagious Delta variant became the predominant form of the virus, according to a study published Tuesday by federal health officials.

It was not clear whether the decrease in protection was caused by the advent of the Delta variant or the increase in the time since vaccinations began. The effectiveness of the vaccine showed possible signs of decline as early as four months after the vaccinations were introduced.

“What we were trying to find out is, is the delta or is the effectiveness dwindling?” Said Dr. Fowlkes. “Our conclusion is that we can’t really tell.”

The researchers followed thousands of first responders, health care workers, and others unable to work remotely in eight locations in Arizona, Florida, Oregon, Texas, Utah, and Minnesota. Participants were tested for coronavirus infection every week for 35 weeks, as well as every time they developed Covid-like symptoms.

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

Overall, the vaccines reduced infections in vaccinated workers by 80 percent compared to unvaccinated workers from December 14, when the U.S. vaccination campaign began, to August 14. (Results were adjusted for factors such as occupation, demographics, frequency of close social contacts, and mask use.)

But while the vaccinations reduced infections by 91 percent prior to the advent of the Delta variant, their protective effects dropped to 66 percent as the variant became dominant in each region.

“We really wanted people to know that the vaccine’s effectiveness in protecting against any infection, whether symptomatic or asymptomatic, has been decreasing since the Delta variant became dominant,” said Ashley Fowlkes, an epidemiologist on the Covid-19 Response team Centers for Disease Control and Prevention and the lead author of the study.

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

The decrease in efficacy “should, however, be interpreted with caution” because the observation period while Delta was dominant was short, said Dr. Fowlkes, and the total number of infections was low.

Understand US vaccination and mask requirements

    • Vaccination rules. On August 23, the Food and Drug Administration fully approved Pfizer-BioNTech’s coronavirus vaccine for people aged 16 and over, paving the way for increased mandates in both the public and private sectors. Private companies are increasingly demanding vaccines for employees. Such mandates are legally permissible and have been confirmed in legal challenges.
    • Mask rules. The Centers for Disease Control and Prevention in July recommended that all Americans, regardless of vaccination status, wear masks in public places indoors in areas with outbreaks, reversing the guidelines offered in May. See where the CDC guidelines would apply and where states have implemented their own mask guidelines. The battle over masks is controversial in some states, with some local leaders defying state bans.
    • College and Universities. More than 400 colleges and universities require a vaccination against Covid-19. Almost all of them are in states that voted for President Biden.
    • schools. Both California and New York City have introduced vaccination mandates for educational staff. A survey published in August found that many American parents of school-age children are opposed to mandatory vaccines for students but are more likely to support masking requirements for students, teachers and staff who are not vaccinated.
    • Hospitals and medical centers. Many hospitals and large health systems require their employees to have a Covid-19 vaccine, due to increasing case numbers due to the Delta variant and persistently low vaccination rates in their communities, even within their workforce.
    • New York City. Proof of vaccination is required by workers and customers for indoor dining, gyms, performances, and other indoor situations, though enforcement doesn’t begin until September 13. Teachers and other educational workers in the city’s vast school system are required to have at least one vaccine dose by September 27, with no weekly testing option. City hospital staff must also be vaccinated or have weekly tests. Similar rules apply to employees in New York State.
    • At the federal level. The Pentagon announced that it would make coronavirus vaccinations compulsory for the country’s 1.3 million active soldiers “by mid-September at the latest. President Biden announced that all civil federal employees would need to be vaccinated against the coronavirus or undergo regular tests, social distancing, mask requirements and travel restrictions.

Another CDC study, published Tuesday, analyzed infections and hospital stays in Los Angeles County between May 1 and July 25 of this year. The researchers concluded that although the infection rates were infected in the unvaccinated individuals, the infection rates were 4.9 times higher in the unvaccinated and the hospitalization rate in the unvaccinated 29 times higher.

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

Three percent of the vaccinated had to be hospitalized, 0.5 percent were admitted to the intensive care unit and 0.2 percent had to be ventilated. The comparable rates for unvaccinated individuals were 7.6 percent, 1.5 percent, and 0.5 percent, the study reported.

Those admitted to hospital despite being vaccinated were on average older than those who had not been vaccinated. The mortality rate for the vaccinated was lower: 0.2 percent versus 0.6 percent for the unvaccinated. The mean age at death of the vaccinated was also higher, at 78 years, compared with a mean age of 63 for the unvaccinated.

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