Moderna Inc. is preparing to offer its COVID-19 shot to trial participants who received a placebo in the final-stage study once the vaccine is authorized for emergency use in the U.S.

The participants will first be given the opportunity to learn whether they got the vaccine. If not, they’ll be able to request the shot, according to a letter from a study investigator to participants that was obtained by Bloomberg.

The drugmaker hopes to start offering the vaccine to volunteers within one to two weeks after it gains emergency-use authorization from the U.S. Food and Drug Administration. Ray Jordan, a Moderna spokesman, confirmed the letter was being prepared.

FDA staff released a favorable report on the shot Tuesday, saying that it is 94.1 per cent effective at preventing COVID-19 symptoms. A panel of advisers will meet later this week to vote on their recommendation, which the agency isn’t obligated to follow, but often does. If the adviser vote to support the vaccine, it could be authorized by the FDA soon afterward.

Pfizer Inc. and BioNTech SE, whose COVID shot was authorized by the FDA last week, have also said they aim to give trial participants who received a placebo the opportunity to get actual shots.

“This option will be voluntary and implemented in alignment with the regulatory authorities where the trial is being conducted,” Pfizer said Sunday.