As potential Covid-19 vaccines speed their way through development, manufacturers and U.S. regulators have largely delayed testing in children and women who are pregnant, raising the possibility that experts will lack critical safety and efficacy data in those populations when there’s a pressing need to inoculate them. Vaccines are always tested first in healthy adults, a population that is most likely to provide a clear picture of whether a vaccine triggers protection. It’s also a population deemed to be at lowest risk should there be side effects from an experimental vaccine.