Operation Warp Speed chief adviser Moncef Slaoui said there is evidence that two half doses in people between the ages of 18 and 55 gives “identical immune response” to the recommended one hundred micorogram dose, but said the final decision will rest with the FDA.

“It will be based on facts and data to immunize more people,” Slaoui said on CBS’ “Face the Nation,” adding, ”of course we continue to produce more vaccine doses.”

His remarks came amid intensifying concern that the Covid vaccine distribution has been slower than expected, with the Trump administration missing its initial goal of vaccinating 20 million Americans by the end of 2020. So far, just over 4 million Americans have received their first of two Covid shots, and another 13 million doses have been delivered to states, according to the CDC.