Ultimately, we are unlikely to have much of a choice except to mix-and-match. Immunity to the vector backbone will make homologous boosting with adenovirus vaccines unlikely to work well. So those who initially receive the Johnson & Johnson or AstraZeneca vaccine will likely need to be boosted with an mRNA vaccine. Among the mRNA vaccines, it is unlikely that there would need to be vendor ‘lock in,’ such that someone who receives Moderna vaccines this year could only receive Moderna vaccines in the future. Since boosters, like mobile phones, are likely going to be continually improved for the foreseeable future, requiring the same vendor each year wouldbe like saying that someone who buys an Android phone would never be able toswitch to iOS in the future, or vice versa, as technologies improve.