Thousands of volunteers participating in studies of the Pfizer-BioNTech and Moderna COVID-19 vaccines continue to be involved in follow-up research, but many in the placebo group have now gotten their vaccinations, meaning it may be difficult for the companies to answer questions about the medications. 

"We don't know how long protections last," Dr. Steven Goodman, a clinical trials specialist at Stanford University, told NPR, explaining that losing control groups will make it more difficult to answer important questions about the vaccines. "We don't know efficacy against variants — for which we definitely need a good control arm — and we also don't know if there are any differences in any of these parameters by age or race or infirmity."

Volunteers signing up for studies last year did not get promises of special treatment when it comes to getting vaccines, but the companies opted to offer the shots once the Food and Drug Administration authorized them. 

Dr. Carlos Fierro, who runs the vaccine study in Lenexa, Kansas, said all participants were called back after the vaccines were authorized, and options were discussed to stay in the study without getting the vaccine, and there were "amazingly" some who said they would do that. 

However, he said that everyone else in his placebo group got the vaccine, meaning he has almost nobody left in the comparison group as the study goes on. 

"It's a loss from a scientific standpoint, but given the circumstances, I think it's the right thing to do," he said. 

Still, having a placebo group is the most definitive way to gather information about vaccine effectiveness, but Fierro said he does think that over time, "we'll get that data."

Already, scientists are collecting data from the vaccine studies that identify how people's immune systems have responded to the vaccinations, which could let them identify immune system features that strongly indicate the effectiveness of vaccines. 

Goodman said he is thinking about how to collect other data, including identifying people in groups not now eligible for a vaccine, such as children, or conducting studies in parts of the world where vaccines are not available. 

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