Unable to Find Enough Participants, Pfizer Stops Trial for New Covid Vaccine

Company says it couldn’t find enough healthy adults, aged 50-64, to do a large, placebo-controlled trial now required by the FDA

In late March, Pfizer and its German partner, BioNTech, notified the FDA that they were suspending a clinical trial for an updated Covid-19 vaccine because they couldn’t find enough people willing to participate – a sign that attitudes toward the vaccine have changed dramatically in the last five years.

The Pfizer/ BioNTech trial was to evaluate the efficacy and safety of an updated Covid vaccine for healthy adults aged 50 to 64 by comparing vaccine recipients with a placebo-control group.

In a March 30 letter, however, Pfizer told trial investigators that the company would stop surveillance on all study participants after April 3, Reuters reported.

The letter explained that enrollment in the trial was closed on March 6 after a review of current epidemiological trends.

The decision to halt the trial was made at a time of weakened demand for the shots. CDC data shows that only 18 percent ⁠of Americans received ​a Covid booster in the 2025–26 season.

Just 8.1 percent of children and 16 percent of adults took the Covid-19 vaccine in 2025, according to CDC figures.

Doses given in pharmacies and doctor offices declined from 27.8 million in 2024 to 20.5 million in 2025.

No Covid-19 vaccine currently has full FDA approval for healthy adults 50-64.

In 2025, the FDA withdrew emergency authorization for Covid-19 vaccines from Pfizer, BioNTech, Moderna, and Novavax. The agency did approve Pfizer-BioNTech, Moderna, and Novavax Covid-19 vaccines for people aged 65 and older.

Last August, Pfizer and BioNTech said they would implement a randomized, double-blind, placebo-controlled trial to evaluate the safety and efficacy of their Covid-19 vaccine in adults 50 to 64 who do not have any health conditions that place them at an increased risk for the virus.

The FDA approved the Pfizer-BioNTech vaccine for people 65 and older, and those 5 to 64 with a risk condition, in 2025.

Following the FDA’s updated approvals, the CDC announced that people who wanted a Covid-19 vaccine should consult with a medical professional and consider individual health factors, along with risks and benefits of the shots.

Last May, HHS stopped recommending the Covid-19 vaccination for healthy children and pregnant women.

The CDC no longer broadly recommends the Covid 19 shot. Each person should consider individual factors and consult with their doctor before getting a vaccine, the agency noted.

“Informed consent is back,” then acting director of the CDC Jim O’Neill said in a statement on Oct. 6. He continued, “CDC’s 2022 blanket recommendation for perpetual Covid-19 boosters deterred health care providers from talking about the risks and benefits of vaccination for the individual patient or parent. That changes today.”

The Pfizer/BioNTech trial intended to register between 25,000 to 30,000 healthy adults between the ages of 50 and 64. That is a population the FDA required to be studied in a large, placebo-controlled trial before the vaccine could be recommended for people in that age range.

Pfizer and BioNTech insisted that their decision was not made because of safety concerns.

“The study is not ending as a result of any safety or benefit-risk concerns,” Pfizer and BioNTech said in a statement to Reuters. “We intend to stop the study due to slow enrollment and therefore the inability to generate relevant post-marketing data.”

It was difficult to build enrollment because participants were required to have no chronic conditions, a contract research organization executive said. “This is a very difficult population to recruit.”

A similar Moderna study targeting 30,000 volunteers is also facing recruitment challenges at multiple sites, Reuters reported. The trial remains ongoing and is expected to be completed in June 2027.