With the rollout of vaccines and the uncertainty of their status, volunteers could decide to drop out once they are eligible to get one. They might stay in the study if they're told what they got, said Dr. Ana Iltis, a bioethicist at Wake Forest University.
"Participants could leave in droves. They could say, 'If you don't tell me what I got, I'm out of here,'" said Iltis. "You cannot force people to stay."
In an ideal world, participants could hold off to discover whether they received the dummy shot or the vaccine. But experts agree the current circumstances are extraordinary.
Still, unmasking participants would undoubtedly affect the trials' scope and results.
If someone learns they've already been vaccinated, for example, they may stop social distancing or wearing masks — increasing their potential exposure to the virus and possibly spreading it. It's not yet known if vaccinated people can still carry and transmit the virus.
On the flip side, if a person finds out they only received the dummy shot, they might take precautions they wouldn't otherwise.
Either outcome, Goodman said, "means the trial has basically come to an end."
Before granting its emergency use approval, the FDA required Pfizer and Moderna to provide two months of follow-up data. If studies are cut short, it becomes harder to get long-term effects, including how long immunity lasts.
"There's a reason we do clinical research in a certain way," Iltis said. "We should not abandon our norms and our principles. Are we going to be happy with short-term evidence in a year?"