Anyone who receives the Moderna or Pfizer vaccine must get two full doses, two top US Food and Drug Administration officials said Monday.

They also dismissed other ideas for stretching the vaccine supply and said people who are speculating about the possibility of making do with just one dose or cutting doses in half are misinterpreting the data.

"We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19," FDA Commissioner Dr Stephen Hahn and Dr Peter Marks, who heads FDA's vaccine division, said in a statement.

"These are all reasonable questions to consider and evaluate in clinical trials. However, at this time, suggesting changes to the FDA-authorized dosing or schedules of these vaccines is premature and not rooted solidly in the available evidence. Without appropriate data supporting such changes in vaccine administration, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19," they added.
Operation Warp Speed's top adviser, Moncef Slaoui, told CNN Sunday that the FDA would consider giving half-doses of Moderna's Covid-19 vaccine to people 18 to 55 -- which could make the vaccine available to twice as many people in this age group.
Slaoui said earlier data show that the vaccine appeared to elicit effective antibody responses among volunteers under age 55 who received either the full 100-microgram dose or a half dose. While an FDA briefing document last month also references these "comparable" immune responses from Moderna's phase 2 study, the full data have not yet been published.
But Marks and Hahn said these findings covered only a very few people who were not followed for long to see if their immune responses held up over time.
"What we have seen is that the data in the firms' submissions regarding the first dose is commonly being misinterpreted. In the phase 3 trials, 98% of participants in the Pfizer-BioNTech trial and 92% of participants in the Moderna trial received two doses of the vaccine at either a three- or four-week interval, respectively," they wrote.

https://edition.cnn.com/2021/01/04/health/fda-coronavirus-vaccines-doses/index.html