In an interview on "Closing Bell," 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.
Gottlieb's comments Friday came as the country's seven-day average of daily new coronavirus infections fell below 30,000 for the first time in almost a year; in late March, that figure was around 66,000.
The case decline has coincided with an expansion of vaccine availability. As of Friday, nearly 50% of the U.S. population has received at least one Covid vaccine dose, according to the Centers for Disease Control and Prevention. In late March, that number was slightly less than 30%.
However, 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. The U.S. has had around 33 million total confirmed Covid cases, but Gottlieb has repeatedly said the official tally is an undercount.
"We don't have data on this, but my guess would be that the infection level among the unvaccinated population is probably higher because a lot of people probably aren't getting the vaccine because they knew they were previously infected," Gottlieb said.
People who have recovered from Covid do have natural antibodies, but the CDC and other experts recommend they also get the vaccine. In fact, people who had the disease and receive the Covid shot may develop stronger protection against virus variants.
People who have yet to be vaccinated might have been less concerned about the virus during the pandemic and spent less time at home as a result, Gottlieb added.
"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 in the Trump administration. He now serves on the board of vaccine maker Pfizer.
And while states are lifting many pandemic-era restrictions, such as capacity limits at restaurants, Gottlieb said, some people have not returned to their pre-Covid behavior, which is helping with case reductions.
"People are being more cautious, generally, even though we're starting to take masks off and go out and about," Gottlieb said. "People are more cautious about their interactions, so some of that is still having a downward effect on transmission."
Gottlieb predicted the country's case counts will continue to decline in the coming weeks, while cautioning the pandemic is unlikely to be declared "over." He added, "I think we're going to have a very quiet summer with respect to coronavirus spread and then have to contend with it again as we head into the winter."
Disclosure: Scott Gottlieb is a CNBC contributor and is a member of the boards of Pfizer, genetic testing start-up Tempus, health-care tech company Aetion and biotech company Illumina. He also serves as co-chair of Norwegian Cruise Line Holdings' and Royal Caribbean's "Healthy Sail Panel."