This actually has been the subject of a recent debate/article in Nature. The answer is that the approach varies, depending on the perceived risk for the subjects... One method previously used is to create a 3rd arm for participants on the placebo that receive[d] the actual vaccine later.
Once a vaccine is granted emergency approval, there is pressure on developers to offer the immunization to trial participants who received a placebo. But if too many people cross over to the vaccine group, the companies might not have enough data to establish long-term outcomes, such as safety, how long vaccine protection lasts and whether the jab prevents infection or just the disease.
On 10 November, Pfizer sent a letter to participants, seen by Nature, which states that the company is exploring ways to allow interested participants in the placebo group who meet eligibility criteria for emergency access to cross over into the trial’s vaccine arm. A spokesperson told Nature that the company would have “an ethical responsibility to inform all study participants about the availability of an Emergency Authorized Vaccine.”
The Pfizer spokesperson says that the company will discuss with the FDA how it will gather data to comprehensively measure safety and efficacy if participants cross over. The company’s clinical-trial plan says it intends to monitor participants for two years after their final vaccine dose.
There are ways of managing such disruptions without jeopardizing the trial outcome, says Kathleen Neuzil, director of the Center for Vaccine Development and Global Health at the University of Maryland in Baltimore. She is also co-chair of the US National Institutes of Health’s COVID-19 Prevention Trials Network, which arranges clinical trials for companies including Pfizer and Moderna. Participants who initially received a placebo but crossed over to get the vaccine could be monitored as a separate group, and a comparison of the vaccine’s long-term efficacy and safety could be made between those groups, she says. Neuzil used a similar set-up to determine the length of protection offered by the first shingles vaccine.
Before unblinding the trials, companies could also ask volunteers to remain in the study and receive the vaccination as soon as the trial is over, says Corey.