of either pfizer or moderna be given to those who have weakened immune systems. talk to us about whether you anticipate that happening for everyone or how it might be ruled out in the fall for people beyond those who have weakened immune systems? >> let's just distinguish between what is happening with this group of people and everyone else who has been vaccinated. i don't call this a booster. this is a series. we're talking about some cases needing three or four doses like we do in childhood immunizations before you achieve immunity. booster is when you had immunity but then lost it over time. that's the question you're asking next. are we going to likely have to be confronted with that? i think right now we do have some troubling data that supports that we may be talking about booster doses.