The agency’s full approval is more than just a formality. Yes, the vaccines are a tremendous success. But although early, publicly available data have now been thoroughly scrutinized, the agency has access to enormous amounts of more recent information that needs to be combed over for any surprises or signs of possible undetected issues. On a press call about the approval announcement, Peter Marks, the director of the FDA’s Center for Biologics Evaluation and Research, said that the agency reviewed hundreds of thousands of pages of new data from Pfizer. Even for an agency as well resourced as the FDA, thorough analysis and consideration of so much information is a heavy lift, especially for multiple vaccines at once; Moderna’s application for full approval arrived hot on the heels of Pfizer’s, on June 1. (In response to questions on the call, Marks declined to comment on the timeline of a full approval for Moderna.)

And it’s not just a lot of data from the clinical trials of the vaccine. The trials and other studies by the manufacturers established the vaccine’s efficacy and basic safety, along with common and not-so-common possible reactions. Now that it’s been rolled out, though, the regulators have to look for signals of very rare issues that wouldn’t show up even in trials of tens of thousands of people.

We’ve already seen how crucial this effort can be. When Pfizer submitted its application for full approval, the link between the currently available mRNA vaccines and some cases of heart inflammation hadn’t been established. The CDC informed us about this potential risk in mid-May. A safety issue particularly affecting younger people was a serious development in the regulatory process of a vaccine approval that includes 16- and 17-year-olds. The small amount of early data for that age group had been a sticking point back at the emergency-use-authorization stage. Now investigating whether the incidence of this potential side effect might be higher for Moderna’s vaccine than for Pfizer’s is reportedly the reason Moderna still hasn’t received emergency use authorization for adolescents.