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More Americans are vaccinated against Covid, but that's not the only reason coronavirus cases in the U.S. keep falling, Dr. Scott Gottlieb told CNBC on Friday.

The former Food and Drug Administration commissioner said additional factors contributing to declining infection levels include warming weather and the fact that a portion of the unvaccinated population has already been infected with Covid.

The percentage of Americans who have some immunity against coronavirus is higher than the vaccination rates, Gottlieb said, estimating that at least one-third of the population has been infected. 

"So, if you assume that the percent of prior infection among the unvaccinated population is more than that third, and it probably is, and you assume that we've given at least one dose to around half the population right now, we're approaching pretty high levels of immunity," said Gottlieb, who led the FDA from 2017 to 2019. He now serves on the board of vaccine maker Pfizer.