https://www.scientificamerican.com/article/should-we-change-covid-vaccine-doses-to-reach-more-people-what-the-data-say1/

The first COVID-19 vaccines have arrived, and the race is on to get them into as many arms as possible as soon as possible. But vaccination rates have fallen frustratingly short of what is needed to turn the tide of this pandemic, which continues to claim thousands of lives every day. The currently authorized vaccines are meant to be given in two doses. To speed things up, some scientists have floated the idea of delaying the second dose, only giving people only a single dose or halving the dose size. U.K. regulators have prioritized giving as many individuals their first dose of the Pfizer and AstraZeneca vaccines as possible—even if that means waiting up to 12 weeks for the second shot. Other countries are also considering changes to their vaccine dosing regimens.

Some experts say there are not enough data on the amount and length of protection these altered dosing schedules provide, however. They argue that the bigger problem is distributing the existing vaccine supply and that changing the schedules without rigorous evidence could compromise public trust in vaccines.