safe and effective. for instance, getting the pfizer shot and then for the booster getting moderna. do we know if there are any drawbacks at all to mixing vaccine types? >> it really doesn't look like it. people have sort of speculated for some time there could be an advantage to doing that. the way thaw think about it is that you are creating antibodies to a specific part of the spike protein of the virus. moderna and pfizer very similar. but they also have some differences that might actually give you a broader immunity. so it doesn't seem like there is any drawbacks. there could be potential advantages to having more pro roe bust or at least widespread production by mixing and matching. we will get more data on that, but talking to some of these trial investigators, that's what they seem to have been seeing so far. we will see how it sort of plays out in the real world. >> from what we know now about the boosters being considered, is there one that you think is better than the others?