The first two Covid-19 vaccines to enter large-scale US trials will not be tested in pregnant women this year, raising questions about how this vulnerable population will be protected from the coronavirus, researchers told Reuters.

Moderna and Pfizer, which has partnered with Germany’s BioNTech, this week separately launched clinical trials that use a new and unproven gene-based technology. Both companies are requiring proof of a negative pregnancy test and a commitment to using birth control from women of childbearing age who enrol.

Drug makers say they first need to make sure the vaccines are safe and effective more generally. In addition, US regulators require that drug makers conduct safety studies in pregnant animals before the vaccines are tested in pregnant women to ensure they don’t harm the fetus or lead to miscarriage.