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