Delaying the second dose would allow more people to be vaccinated faster. This could control the pandemic sooner if immunity does not fade too quickly after the first dose.
The effectiveness of vaccine over time with/without the second dose is still being studied. Both the Pfizer-BioNTech and Moderna vaccines were designed to be administered in two doses, 21 days apart for Pfizer and 28 days for Moderna.
Michael Mina and Zeynep Tupfekci advocate for investigating the possibility of delaying the second dose in the NYTimes. They describe many important considerations, including one of the key reasons for optimism about #SecondDoseDelay:
For both vaccines, the sharp drop in disease in the vaccinated group started about 10 to 14 days after the first dose, before receiving the second. Moderna reported the initial dose to be 92.1 percent efficacious in preventing Covid-19 starting two weeks after the initial shot, when the immune system effects from the vaccine kick in, before the second injection on the 28th day.
Will scientists conclude that delaying SARS-CoV2 vaccines second dose did or would have saved lives?
Question resolves "Yes" if, on 2021-12-31, more than 50% of the scientific literature supports the claim that delaying the second dose (booster) of the Pfizer-BioNTech or Moderna vaccines beyond their initial schedule probably saves or would have saved lives?
The relevant scientific literature is judged to include peer-reviewed studies nominated by Metaculus participants which explicitly compare estimated or actual mortality with longer delays for a second vaccine dose will be considered. Note: this criteria may exclude many editorials or published claims that don't include simulations or empirical data comparing counterfactual mortality with a delayed booster.
Question will resolve "No" if more than 50% of studies meeting these criteria claim that a longer delay before the second dose would probably increase mortality.
Question receives mixed resolution if neither the criteria for "Yes" or "No" are met. This would happen if, for example, most studies meeting the inclusion criteria don't indicate that either the originally scheduled or a delayed 2nd dose is likely to have saved lives overall.