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 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.”