The development of the Covid-19 vaccines happened with great urgency, for obvious reasons.

One of the timesaving techniques by Moderna and Pfizer involved scheduling the two vaccine doses fairly close together — just three or four weeks apart — during the research trials. The companies did not test multiple gaps between the two shots to see which was the most effective. They each chose a short gap to finish the trials as quickly as possible.

The decision made a lot of sense. It allowed the U.S. mass vaccination program to start in December, rather than pushing it back a few months. Many lives have been saved as a result.

But the approach means that nobody knows what is the most effective gap between the two shots. Maybe it really is three to four weeks. Maybe a longer delay is just as effective (or, for that matter, even more effective).

And the short delay does come with a large downside.

The U.S. is choosing to give millions of people a second shot while making millions of others wait for their first. That’s happening even though a single shot provides a high degree of protection and even as a more severe, contagious coronavirus variant is sweeping the country. Both cases and hospitalizations have risen in recent days, and deaths have stopped declining.

are calling on the Biden administration or governors to change policy and prioritize first doses: