• "I think we're vastly underestimating the level of delta spread right now," Dr. Scott Gottlieb told CNBC on Friday.
• The former FDA chief said vaccinated people who may become infected likely aren't seeking out testing due to mild symptoms.
• "There's no clear evidence that this is more pathogenic, that it's causing more serious infections. It's clearly more virulent, it's clearly far more contagious," he said.

Dr. Scott Gottlieb told CNBC on Friday he believes the U.S. is significantly undercounting the number of Covid delta infections, making it difficult to know whether the highly transmissible strain is causing higher-than-expected hospitalization and death rates.

"We don't know what the denominator is right now," Gottlieb said in an interview on "Squawk Box." "I think we're vastly underestimating the level of delta spread right now because I think people who are vaccinated, who might develop some mild symptoms or might develop a breakthrough case, by and large are not going out and getting tested. If you've been vaccinated and you develop a mild cold right now, you don't think you have Covid."

Coronavirus cases in the U.S. have been rising due to the delta variant, with the seven-day average of new daily infections standing at 26,448, according to a CNBC analysis of Johns Hopkins University data. That's up 67% from a week ago. The weekly average of new daily deaths is up 26% from a week ago, to 273, according to CNBC's analysis.

"There's no clear evidence that this is more pathogenic, that it's causing more serious infections. It's clearly more virulent, it's clearly far more contagious" than earlier virus strains, said Gottlieb, who serves on the board of Covid vaccine maker Pfizer.

If younger Americans are becoming sick with the delta variant at higher levels compared with previous points in the pandemic, it's because "younger people remain unvaccinated," Gottlieb contended. "When people who are vaccinated do get infected, and there are breakthrough infections, they don't get as sick. They have protection against severe disease."

Delta is now the most-common coronavirus strain in the U.S., making up more than 57% of cases in the two weeks from June 20 to July 3. That's the latest available window on the CDC's website.

U.S. health officials have sounded the alarm for weeks about the variant's potential to cut into hard-earned progress in reducing infection rates, which plummeted in the spring as America's vaccination campaign hit its stride. As of Friday, 48.3% of the country's population was fully vaccinated and nearly 56% had received at least one dose, according to data from the Centers for Disease Control and Prevention.

Covid vaccination coverage is higher among the most-vulnerable group of Americans: the elderly. More than 79% of people age 65 and up are fully vaccinated and nearly 89% have had at least one dose, according to the CDC.

The vast majority of U.S. counties with high infection rates right now — defined as at least 100 new cases over the last seven days per 100,000 residents — have vaccinated under 40% of their residents, according to a CNBC analysis completed earlier this week.

In Los Angeles County, officials on Thursday responded to an uptick in cases by reinstating an indoor mask mandate, even for fully vaccinated people. LA County, the nation's most populous, had lifted its previous mask requirement about a month ago, in conjunction with the state of California ending most of its remaining pandemic restrictions.

Gottlieb said he does not expect many other state or local governments to follow LA County and begin putting in place already-lifted mitigation measures "because there's not going to be a lot of support for mandates at this point."

"People who are worried about Covid have largely been vaccinated. I realize not everyone has been able to get vaccinated, but most people have been vaccinated who are worried about this infection," said Gottlieb, who led the FDA from 2017 to 2019 in the Trump administration.

"People who remain unvaccinated aren't worried about the infection and don't want to be wearing masks either. Now, the bottom line, that means this is just going to spread through the population," he added.

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."