Nine months later, those advisers are meeting to discuss booster shots, and the situation is very different.

When this advisory committee meets on Friday, it will be presented with dueling data, some of it suggesting there's a need for boosters, but other pieces of data suggesting there is no such need.

Schaffner has been following the FDA deliberations closely, as he serves as member of an advisory committee to the US Centers for Disease Control and Prevention that will review booster shots if the FDA green lights Pfizer's application.

Monday, those FDA staffers -- two of the agency's top vaccine experts -- co-authored an article stating that the data does not appear to support a need for booster shots for the general public right now.

Schaffner, an infectious disease expert at Vanderbilt University, searched for the right word to explain the entire set of circumstances around Friday's meeting.

One study looked at data from 13 states and counties, another looked at data from nine states, and the third looked at data from five Veterans Affairs medical centers.

Looking at illnesses in the second half of July, that study found that those who'd received their second dose of Pfizer's vaccine in March were 70% more protected against severe disease than those who received the second shot in January.

Eric Rubin, a member of the advisory committee and an infectious disease expert at Harvard.

The second set of data will be the Israeli study suggesting that over time, two shots of the vaccine stopped being as effective at keeping people out of the hospital.

Israel has data showing that people over age 60 who received a booster were more than 10 times less likely to become severely ill with Covid-19.

Israel felt so strongly about a booster program that they didn't even wait for the FDA and the CDC to weigh in on whether it's a good idea.