Experts continue to debate whether to delay or change dosing to extend the supply of COVID-19 vaccines as more transmissible strains spread globally.

The World Health Organization’s vaccine panel, the Strategic Advisory Group of Experts on Immunization (or SAGE), said Tuesday during a media briefing that it has created a provision for countries to delay second doses — a move already being adopted in the U.K.

SAGE chair Dr. Alejandro Cravioto said Tuesday that while he acknowledges a lack of evidence to support the idea, a provision has been made “for countries, in exceptional circumstances of vaccine supply constraints and epidemiological settings to delay the administration of the second dose for a few weeks in order to maximize the number of individuals benefitting from a first dose.”

While delays in vaccine boosters aren’t seen as dangerous, according to some experts, the studies focused on two full doses so there is no data to support any other actions. For this reason, the U.S. Food and Drug Administration (FDA) has warned against any changes to the dosing that has been authorized.

Over the New Year weekend, Operation Warp Speed’s Moncef Slaoui suggested the federal government would consider half of the two doses of Moderna’s (MRNA) vaccine, anticipating the same level of protection as two full doses.

Prior to the emergency use authorization of both Moderna’s and Pfizer (PFE)/ BioNTech’s (BNTX) vaccines, the advisory panel debated the idea of relying on strong protection from a single dose until a majority of Americans received vaccines before looking at a second dose.

Late Monday, the FDA released a statement negating the idea from Slaoui.

FDA Commissioner Stephen Hahn and vaccine expert Dr. Peter Marks stated, “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.”

The duo added that they understand the discussions are based on the idea that lower doses or delays can help get more vaccines out, but “making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health. Until vaccine manufacturers have data and science supporting a change, we continue to strongly recommend that health care providers follow the FDA-authorized dosing schedule for each COVID-19 vaccine.”