Pfizer, BioNTech to offer COVID-19 vaccine to volunteers who received placebo by March 1

The trial's Vaccine Transition Option allows all participants aged 16 or older the choice to discover whether they were given the placebo, "and for participants who learn they received the placebo, to have the option to receive the investigational vaccine while staying in the study," the companies said on their website for trial participants.

The U.S. Food and Drug Administration and a panel of its outside advisers have expressed concerns over Pfizer's "unblinding" plan, saying it could make it harder to continue collecting data on safety and effectiveness needed to win full FDA approval of the vaccine.

Trial participants who received the placebo will have two doses of the investigational vaccine reserved for them within the study, the companies said on the website.

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"The study doctor will follow the latest guidance from the U.S. Centers for Disease Control and Prevention and their local health authorities to offer the Vaccine Transition Option to participants in a prioritized manner," the companies said.