NEW YORK — Drugmaker Pfizer and German partner BioNTech have begun a nine-country study of their COVID-19 vaccine in pregnant women.

The companies said Thursday that the first volunteers have received shots in the study, which is to enroll about 4,000 healthy pregnant women aged 18 and older. Some will receive the two-dose vaccine and others dummy shots, three weeks apart and between 24 weeks and 34 weeks into their pregnancies.

The volunteers will be followed for seven months to 10 months, depending on whether they received vaccine or placebo, to see how effective and safe the vaccine is in pregnant women. Women in the U.S., Canada, Argentina, Brazil, Chile, Mozambique, South Africa, Spain and the UK will be included.

“Pregnant women have an increased risk of complications and developing severe COVID-19,” Dr. William Gruber, Pfizer’s head of vaccine clinical research and development, said in a statement. “It is critical that we develop a vaccine that is safe and effective” for them.

Women known to be pregnant were excluded from prior studies of the vaccine, which has emergency use authorization in the U.S., the European Union and other countries.

Once the babies are born, mothers who received dummy shots will be given the vaccine.

The study will assess effects on the infants for about six months, checking for safety and whether they received potentially protective antibodies from their mothers.

The companies plan later this year to begin vaccine testing in children, aged 5 to 11 and younger than five, and to test their vaccine in people with weakened immune systems. Results of their study in children aged 12 through 15 are currently being evaluated.