Dr. Scott Gottlieb told CNBC on Friday that he believes the US is significantly under counting Covid Delta infections, making it difficult to know if the highly communicable strain is causing unexpectedly high hospital admissions and death rates.

“We just don’t know what the denominator is,” said Gottlieb in an interview with “Squawk Box”. “I think we are underestimating the extent of the Delta Spread right now because I think that people who are vaccinated may develop mild symptoms or develop a breakthrough case, by and large, not going out and getting tested. has been vaccinated and you are just catching a mild cold, don’t think you have Covid. “

Coronavirus cases in the United States have increased due to the Delta variant, with the seven-day average of new infections every day a CNBC analysis of data from Johns Hopkins University at 26,448. That’s 67% more than a week ago. The weekly average of new daily deaths has increased to 273 from a week ago, according to CNBC analysis.

“There is no clear evidence that this is more pathogenic, that it causes more serious infections. It is clearly more virulent, it is clearly more contagious” than previous strains of the virus, said Gottlieb, who sits on the board of directors at Covid vaccine manufacturer Pfizer.

If younger Americans get the Delta variant at a higher level compared to earlier points in the pandemic, it is because “younger people remain unvaccinated,” claimed Gottlieb. “When vaccinated people get infected and there are breakthrough infections, they don’t get as sick. They are protected from serious illnesses.”

Delta is now the most common strain of coronavirus in the United States, accounting for more than 57% of cases in the two weeks June 20 through July 3. This is the latest available window on the CDC website.

U.S. health officials have been sounding the alarm for weeks about the potential of the variant to reduce hard-earned advances in reducing infection rates that plummeted in the spring as the American vaccination campaign took off. According to the Centers for Disease Control and Prevention, by Friday 48.3% of the country’s population had been fully vaccinated and nearly 56% had received at least one dose.

The Covid vaccination rate is higher in the most vulnerable group of Americans: the elderly. According to the CDC, more than 79% of people aged 65 and over are fully vaccinated and nearly 89% have received at least one dose.

The vast majority of US states with currently high infection rates – defined as at least 100 new cases in the last seven days per 100,000 residents – have vaccinated fewer than 40% of their residents, according to a CNBC analysis completed earlier this week.

Los Angeles County officials responded Thursday to a surge in cases by reintroducing an indoor mask mandate for those who were fully vaccinated. LA County, the most populous county in the country, had lifted its previous mask requirement about a month ago, in conjunction with the lifting of most of its remaining pandemic restrictions by the state of California.

Gottlieb said he doesn’t expect many other state or local governments to follow LA County and put in place abated mitigation measures “because there won’t be much support for mandates at this point.”

“People who are worried about Covid have been vaccinated for the most part. I understand that not everyone could be vaccinated, but most of the people who are worried about this infection have been vaccinated,” said Gottlieb, who was the FDA in 2017 in the Trump administration until 2019.

“People who remain unvaccinated are not worried about the infection or want to wear masks. The bottom line is that this will only spread to the population,” he added.

Disclosure: Scott Gottlieb is a CNBC employee and a member of the board of directors of Pfizer, genetic testing startup Tempus, health technology company Aetion, and biotechnology company Illumina. He is also co-chair of the Healthy Sail Panel of Norwegian Cruise Line Holdings and Royal Caribbean.