Washington Post reports on the matter
This adds to the earlier story by the New York Times earlier here.
The report above cites multiple individuals familiar with the situation in saying that Pfizer has told the Trump administration that it cannot provide substantial additional doses of its coronavirus vaccine until late June or July.
This is because other countries have already rushed to buy up most of its supply.
Again, this is a bit of a setback to the timeline in getting things back to "normal" in the US as it may mean that the virus situation may persist for quite some time yet unless they are able to secure larger quantities from other vaccine providers.