Vaccine advisers to the FDA meet Thursday and Friday of this week to tackle the next round of questions about booster vaccines for Covid-19 -- and they'll see the first trickle of data on mix-and-match boosters. On Thursday, they'll focus on Moderna's application to add a booster dose to its two-dose vaccine schedule. On Friday, they'll consider Johnson & Johnson's request for boosters. Only after they've decided those two questions will members of the FDA's Vaccines and Related Biological Products Advisory Committee, or VRBPAC, turn to mix-and-match boosters.