It is premature to consider delaying the second doses of COVID-19 vaccines from Pfizer Inc. and Moderna Inc. or cutting the shots in half to vaccinate more people, top officials at the U.S. Food and Drug Administration said.

Changes to the dosing schedules of the two-dose shots is "not rooted solidly in the available evidence," said FDA Commissioner Stephen Hahn and Peter Marks, director of the agency's Center for Biologics Evaluation and Research.

The regulators' Jan. 4 statement came in response to discussions in recent days and news reports about reducing the number of doses, extending the length of time between shots, cutting the doses in half or mixing and matching vaccines to immunize more people against COVID-19.

While changing the dosing schedules may be "reasonable questions" to consider and evaluate in clinical trials, not having the data to support those ideas now runs the risk of putting public health in jeopardy and undermining the historic vaccination efforts to protect the population from COVID-19, Hahn and Marks said.

"The available data continue to support the use of two specified doses of each authorized vaccine at specified intervals," they said.

The second dose of Pfizer's vaccine, which it developed with BioNTech SE, must be given 21 days after the first injection. For Moderna's product, which it developed with the U.S. National Institute of Allergy and Infectious Diseases, or NIAID, the second dose must be given 28 days after the first shot.

Officials in the U.K. decided to not hold back second doses in reserve so that it could vaccinate as many people as possible with the first shots, saying they could wait as long as 12 weeks to get the second dose.

Some had suggested the U.S. should do the same because the Trump administration has fallen well behind its commitment to distribute 40 million COVID-19 vaccine doses — enough to fully vaccinate 20 million people — by the end of 2020.

About 15.4 million doses of Pfizer's and Moderna's vaccines have been distributed in the U.S. but only 4.5 million first-dose shots have been administered, the Centers for Disease Control and Prevention reported Jan. 4 on its website. Over 2.5 million of the distributed doses were provided to U.S. long-term care facilities, though only 365,294 of those shots have been administered, the CDC said.

Moncef Slaoui, chief scientific adviser for the Trump administration's Operation Warp Speed, told CBS "Face the Nation" Jan. 3, that vaccinations would move quicker if doses of Moderna's vaccine could be cut in half, with people ages 18 to 55 years receiving two 50-microgram shots rather than the two 100-microgram doses authorized by the FDA.

Slaoui said that strategy would work better than waiting longer to give the second shot than the 28-day period studied by Moderna.

Misinterpreting data

Hahn and Marks said that the data in the Pfizer's and Moderna's emergency use authorization applications about the first dose is "commonly being misinterpreted."

In the phase 3 trials, 98% of participants in the Pfizer-BioNTech trial and 92% of volunteers in the Moderna study received two doses of the vaccine at either a three- or four-week interval, respectively.

"Those participants who did not receive two vaccine doses at either a three-or four-week interval were generally only followed for a short period of time, such that we cannot conclude anything definitive about the depth or duration of protection after a single dose of vaccine from the single dose percentages reported by the companies," Hahn and Marks stated.

While some of the discussions about changing the dosing schedule or dose are based on a belief that such steps would help get more vaccine to the public faster, doing so without adequate scientific evidence "may ultimately be counterproductive to public health," the regulators said.

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," Hahn and Marks said.

Fixing the delivery system

The idea of stretching out the period between doses so that more people can get the vaccine is not the problem in the U.S. right now, NIAID Director Anthony Fauci told NBC's "Meet the Press" Jan. 3.

"We have vaccine. We need to get it into people's arms," Fauci said.

Peter Hotez, dean of Baylor University's National School of Tropical Medicine, said he agreed with the FDA officials about keeping the two-dose schedule and "not doing reckless things."

"The way to fix a broken vaccine delivery health system is to fix the system, not to provide vaccines of questionable efficacy," Hotez tweeted Jan. 4.

"We can't even get doses we have of two approved COVID-19 vaccines distributed properly, why on Earth are we talking splitting doses or delaying second dose?" tweeted Gregg Gonsalves, assistant professor of epidemiology at Yale University. "There is no reliable data to support these strategies, just isn't. People are grasping at straws, doing policy by panic."

The federal government needs to ensure the states and territories are getting the doses they were promised and have the resources they need to administer the shots, Fauci said Jan. 4 on MSNBC's "All in with Chris Hayes."

"We've got to do better in just logistically and mechanically getting it going to get it into people's arms," Fauci said. "We've got to correct that."

"One thing you don't want … is you don't want the vaccine sitting in a refrigerator or sitting on a shelf," Fauci added.