Pfizer Confirms It Ended COVID-19 Vaccine Pregnancy Trial Early

A nurse prepares a Pfizer-BioNTech COVID-19 vaccine in Hartford, Conn., on Jan. 6, 2022. Joseph Prezioso/AFP via Getty Images

Pfizer has confirmed it stopped its clinical trial analyzing COVID-19 vaccine safety and efficacy in pregnant women early.

The company said that enrollment in the study stopped in the fourth quarter of 2021 after health experts, including the U.S. Advisory Committee on Immunization Practices, began recommending the Pfizer-BioNTech vaccine for pregnant women.

“With the declining enrollment, the study had insufficient sample size to assess the primary immunogenicity objective and continuation of this placebo controlled study could no longer be justified due to global recommendations,” Pfizer told journalist Maryanne Demasi.

The U.S. Food and Drug Administration and European Medicines Agency both agreed to the proposal to halt the study, according to Pfizer. The regulatory agencies did not respond to requests for comment.

An internal email, disclosed in a court case, previously indicated Pfizer had stopped enrollment early.

“The study enrolment was stopped with incomplete numbers because recruitment was slow and it became unreasonable/inappropriate to randomise pregnant women to placebo given the amount of observational evidence that the vaccine is safe and effective, coupled with increasing number of technical committees supporting immunization of pregnant women,” Jelena Vojicic, vaccines medical lead at Pfizer Canada, wrote in the 2022 email.

The randomized, placebo-controlled study in question was launched in early 2021 after pregnant women were excluded from the phase three trial that led to the authorization of the vaccine in the United States and a number of other countries.

Pfizer and BioNTech said they expected to enroll some 4,000 pregnant women but actually enrolled just 349, according to the trial record. Some of the participants were to receive a vaccine while others would receive a placebo, according to the original protocol. Women were expected to be studied for seven to 10 months.

The trial was listed as completed on Aug. 23, 2022, but no results have been released yet.

Pfizer told Demasi that it still does not have “a complete data set” from the study and did not say when it expected to receive it. Pfizer said it and BioNTech plan to complete an analysis of the trial “and seek publication or presentation as is our standard practice.”

Pfizer officials told CDC advisers in September 2022 that the study was completed.

“We will be generating data from that study despite the difficulties in enrollment ... due to the wide recommendations for pregnant women to be vaccinated, but we will generate those data for sure,” Nicholas Kitchen, a senior Pfizer vaccine official, told the panel.

Pfizer did not respond to requests for more information.

‘Shocking’

Dr. Robert Malone, a vaccine researcher, said that the disclosure was surprising.

“This is shocking. Another example of a placebo-controlled randomized clinical trial being terminated early. There must be hearings,” Malone wrote on Twitter.

The U.S. Centers for Disease Control and Prevention (CDC), based in part on advice from its advisory committee, on Aug. 11, 2021, recommended pregnant women get vaccinated.

The agency relied on data from its v-safe system, which sends surveys out to some vaccine recipients, and observational data from other CDC-run systems.

Some of the research has since been corrected, undermining the conclusions, critics have said.

Malone said that the “premature” recommendations from bodies such as the CDC “basically killed” Pfizer’s trial. “The FDA and CDC basically presumed that this was safe without having the data to support that decision,” he said.

The CDC did not respond to a request for comment.

The FDA only required Pfizer to conduct one study on vaccination in pregnant women when it approved the company’s vaccine two weeks after the CDC’s recommendation. Pfizer was scheduled to complete the study by June 30, 2025, and submit a final report on the results by the end of 2025, according to the FDA, which has allowed the company to delay at least one critical postmarketing study.

Some Pregnancies Occurred

While women known to be pregnant were excluded from the original Pfizer trial, some pregnancies were reported before or after receipt of one or more doses.

Dozens of women reported pregnancies and some suffered miscarriages, according to documents released under court order by the FDA.

Miscarriages among vaccinated women have also been reported following authorization, including 23 miscarriages among 270 pregnancies reported to Pfizer through Feb. 28, 2021, another document showed.

Thousands of miscarriages following COVID-19 vaccination have been reported since authorization to authorities in the United States and elsewhere through February 2023.

The CDC says COVID-19 vaccination in pregnant women is safe and effective, pointing to data from its monitoring systems. Several CDC studies, though, have drawn criticism. Researchers reanalyzed one and found that there was an increased risk of miscarriage. Other data have caused concern, including the finding that some rat fetuses, in Moderna testing, ended up with malformed ribs.

Dr. Tom Shimabukuro, a CDC vaccine safety official, said in September 2022 that to date there had been no “concerning findings for pregnancy” from reviews of vaccine safety monitoring systems. He said that the CDC would present data on vaccine safety during pregnancy at a future meeting, but it has not done so to date.

Moderna Trial

Moderna is the second most-used COVID-19 vaccine in the United States. Moderna also excluded pregnant women from its trial but launched a trial in 2021 to analyze safety and effectiveness in the population.

The company said it expected to enroll 1,000 women in the observational study.

The trial is still recruiting, according to an update posted on Jan. 9.

The estimated primary completion date is listed as Dec. 9, 2023.

Moderna did not respond to a request for comment.

The trial was listed among those required following approval of the company’s vaccine. Moderna told the FDA the study would be completed by Sept. 30, 2023, and a report on the results would be finalized by June 30, 2024.

Moderna officials told CDC advisers in September 2022 that the study was still being conducted and was examining approximately 800 pregnancies.

A second observational study, examining vaccine safety among pregnant women in five European countries, was also listed, with a completion date of March 31, 2023. The FDA was to receive a final report on the results by the end of 2023.