There is growing concern that the Food and Drug Administration, under political pressure, could approve a Covid-19 vaccine before it has robust safety and efficacy data.

The consequences of such a decision could be significant, particularly if the vaccine is ultimately shown to be less effective than early data suggest. But an approval before the completion of large, Phase 3 trials does not have to be problematic. Experts aren’t ruling out the possibility that a vaccine could be cleared this fall if it is very effective.

“There are mechanisms by which products that have a good amount of data can be made available in a controlled way,” said Natalie Dean, an assistant professor of biostatistics at the University of Florida who specializes in vaccine study design.

But Dean also sees risks.

“If you make a decision based on promising but not convincing data, and then you discontinue your randomization,” she said, “you discontinue your evidence-generating process. You can never go backward. You can never go back and generate your evidence.”

Here is how a fast approval might play out, if all goes well — and some warning signs that a science-based process is not being followed.

The FDA has laid out clear criteria for the full approval of a vaccine: It should reduce the rate of symptomatic Covid-19 disease by 50%. Equally important is that the data should suggest it’s highly unlikely that the vaccine could possibly be less than 30% effective. Any vaccine less effective than that would be useless.

The agency also said that there should be safety data of a year or more for at least 3,000 patients. There’s no way to shorten that timeframe, and it is one of the reasons experts believe the FDA could grant a Covid-19 vaccine an emergency use authorization, rather than full approval.

It’s also possible that the studies could be ended early based on an interim analysis of data.

These early looks are handled by a data and safety monitoring board, or DSMB, that reviews the data from a study regularly to make sure that patients are not being endangered by side effects from the vaccine and that it’s still ethical to give a placebo.

“At the end of the day, if things are done according to the tradition, the DSMB looks at the data intermittently and makes one of four determinations,” explained Anthony Fauci, the director of the National Institute for Allergy and Infectious Diseases, which is working with Moderna and AstraZeneca on vaccine trials.

A study can be stopped because a vaccine is clearly effective; it can be stopped because clinicians are seeing troubling side effects. The DSMB can recommend changes to a study. Or, as usually happens, it can do nothing.

“It’s up to the DSMB, in their judgment, to balance the safety issue, the efficacy issue, and the duration of the trial issue,” Fauci said. “And that’s the reason why they’re an independent group. They are not the company because obviously the company is going to want to get their product approved as quickly as possible.”

But there are also pre-set rules as to when a trial can be stopped. “The earlier you stop it, the higher the bar,” Fauci said.

Technically, the final decision about what to do rests with the companies that are developing the vaccines, Fauci said. But, he added, “it would be unusual for the company to do something completely dissociated from or at odds with the DSMB.”

How could these interim analyses play out? Each of the Phase 3 trials is enrolling at least 30,000 volunteers. It’s expected that the trials could need about 150 cases of Covid-19 to tell whether or not the vaccine is preventing disease.