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Pfizer and BioNTech 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, according to Reuters.

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.

Dr. William Gruber, senior vice president of vaccine clinical research and development for Pfizer, said in an interview the company could have results by the fourth quarter of 2021.

He added that data so far suggest that pregnant women with COVID-19 have higher rates of severe disease. They also have higher rates of pregnancy complications, such as premature birth, compared with pregnant women not infected by the coronavirus.

In late January, the World Health Organization updated its guidelines on the coronavirus vaccines produced by Pfizer and Moderna, removing its recommendation that pregnant women not receive these vaccines.

Previously, the WHO recommended that pregnant women get vaccinated only if they were at risk of exposure to coronavirus, adding that the Moderna vaccine is not recommended for them.

Meanwhile, Prof. Osnat Wolfish, director of the obstetrics and gynecology department at Hadassah Mount Scopus Hospital, has recommended that pregnant women receive the coronavirus vaccination after the completion of the first trimester of pregnancy.

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

The trial will also assess whether vaccinated pregnant women transfer protective antibodies to their babies.