Biden previously pledged that the US will have enough vaccines for every adult in the US by the end of May. Getting all of those vaccines into arms will require a distribution boost: At the current rate of 2.5 million shots a day, only about 180 million adults, of roughly 255 million, will be fully vaccinated by the end of May. The US has to do more than 4 million shots a day, on average, by then to fully vaccinate every adult in the US before June.

That will be a challenge, with lots of potential factors involved: whether drug companies can ramping up manufacturing, whether the federal government can ship those vaccines out, whether local and state governments can turn those doses into shots in arms, and whether vaccine hesitancy is sufficiently addressed to get all adults to want the vaccine.

That’s a lot that could go wrong. Biden, for his part, has vowed to get ahead of these issues, dedicating more money to vaccine distribution and public education and awareness efforts, funded in part by the recently enacted Covid-19 relief package.

Now Americans waiting for a shot will have to wait and see if Biden can turn those promises into reality.