In an email Friday, Dr. Robert Kadlec said the administration was accelerating production of a safe and effective vaccine.
But Kadlec's statement counters President Trump's recent comments on the vaccine's schedule. He's claimed at rallies, debates and press conferences that the vaccine could be ready much earlier.
"So we've manufactured all of the necessary supplies so that as soon as the FDA approves the vaccine and as you know, we're very close to that, we'll be able to distribute at least 100 million vaccine doses by the end of 2020 and a large number much sooner than that."
Kadlec says those doses could be available by the end of this year, but there would still be a long process to get emergency authorization to administer it, and it will take time to distribute.
Kadlec isn't the only one to go against President Trump's timeline. Dr. Anthony Fauci says he's cautiously optimistic the U.S. will have a vaccine by the end of this year.
"We can make a reasonable projection, that we will know by sometime in November or December whether we have a safe and effective vaccine. It is conceivable, though unlikely, that we will know before then, like in October."
Pfizer, the company behind this vaccine, is the only major developer that says it could know before Election Day whether its drug formula is effective for the virus. It says its fast progress has been driven by science. But FDA is demanding strict standards to support the company's data from clinical trials.