In addition to incest risks, experts say it can be psychologically damaging for children of donors to find out they have hundreds of genetic half-siblings.
Current rules mean a single sperm donor can only create ten families in this country. But there is no regulation limiting the number of families that donors can create overseas.
British researchers reviewed the findings of more than 20 studies involving 120,000 pregnant women given two doses of either the Moderna or Pfizer vaccine.