CHICAGO (Reuters) — Pfizer Inc and BioNTech SE have started an international study with 4,000 volunteers to evaluate the safety and effectiveness of their COVID-19 vaccine in healthy pregnant women, the companies said on Thursday.

Pregnant women are at higher risk of developing severe COVID-19, and many public health officials have recommended some women in high-risk professions take coronavirus vaccines even without proof they are safe for them.

Last week, the U.S. National Institutes of Health called for greater inclusion of pregnant and lactating women in COVID-19 vaccine research.

Bioethicists, vaccine and maternal health experts have argued for years that pregnant women should be included early in trials of pandemic vaccines so they would not need to wait until long after a successful one emerges.

Nevertheless, pregnant women were excluded from the large U.S. trials used to obtain emergency use authorization of COVID-19 vaccines.

Drugmakers have said they first need to make sure the vaccines are safe and effective more generally. In the United States, regulators require drugmakers to conduct safety studies in pregnant animals before vaccines are tested in pregnant women to ensure they do not harm the fetus or lead to a miscarriage. The companies said those studies revealed no new risks.

Pregnant women in the United States have already received their first doses, the companies said.

The new study will test pregnant women aged 18 and older in the United States, Canada, Argentina, Brazil, Chile, Mozambique, South Africa, the UK and Spain.

Women will receive the vaccine during weeks 24-34 of gestation, getting two shots 21 days apart – the same regimen used in the larger clinical trial.

Shortly after giving birth, participants who got a placebo in the trial will be given an opportunity to get the actual vaccine, while remaining part of the study, the companies said.