The Food and Drug Administration said Wednesday that observational studies have had mixed findings assessing whether the efficacy for BioNTech SE BNTX, +6.08% and Pfizer Inc.’s PFE, +0.13% COVID-19 vaccine is declining as a result of time or the delta variant but in general the vaccines available in the U.S. continue to protect people against severe disease and death. The FDA’s document was published Wednesday in advance of an advisory committee meeting set for Friday; at that meeting, a group of independent experts will discuss and then vote on whether they think the FDA should approve a COVID-19 booster dose for people who are at least 16 years old or older. The FDA is not required to follow the advice of the committee but often does. Pfizer’s document for the meeting was also published on Wednesday; it cited the need to look at the “totality” of clinical evidence when it comes to making a regulatory decision about a third dose of its vaccine. The FDA’s document, on the other hand, said it has not independently verified all potentially relevant studies.