Any new coronavirus vaccine will have to work at least 50% better than a placebo in preventing infection or serious disease in people, the US Food and Drug Administration said in new guidance released Tuesday to vaccine makers.

New vaccines should also be at least as safe as other vaccines against infectious diseases, and testing should include three to four weeks’ follow-up in volunteers to make sure there are no adverse events, the FDA said. 

“To ensure that a widely deployed Covid-19 vaccine is effective, the primary efficacy endpoint point estimate for a placebo-controlled efficacy trial should be at least 50%,” the agency said in its guidance.

While that’s not considered desirable for a vaccine, it’s within the range of current influenza vaccines, for instance. In some years flu vaccines can have 40% efficacy or even less in preventing infection. That compares to measles vaccines, which have about 97% efficacy after two doses, according to the US Centers for Disease Control and Prevention.

The FDA also says that vaccine makers should specifically ask about adverse events in volunteers for at least seven days after they get the vaccine, and then wait for any unsolicited reports of side-effects for 21 to 28 days after each vaccination. And any study should include watching for serious events requiring a medical intervention for at least six months after the last vaccination.

“All pregnancies in study participants for which the date of conception is prior to vaccination or within 30 days after vaccination should be followed for pregnancy outcomes, including pregnancy loss, stillbirth, and congenital anomalies,” the FDA adds in its guidance.

But it doesn’t say pregnant women should be excluded.

And companies making vaccines using new and untested designs will have to test in animals first, the FDA said.

The guidelines also consider reports that show Blacks and Hispanics are especially hard hit by the virus.

“FDA strongly encourages the enrollment of populations most affected by COVID-19, specifically racial and ethnic minorities,” the guidelines say.