A game staff holds a sign urging fans to wear mask during a game between the LSU Tigers and the Mississippi Rebels at Tiger Stadium on Dec 19, 2020 in Baton Rouge, Louisiana. (Photo: Agencies)

Scott Gottlieb believes the US is significantly undercounting the number of COVID delta infections, CNBC reports.

The former FDA chief said this made it difficult to know whether the highly transmissible strain is causing higher-than-expected hospitalization and death rates, according to CNBC.

"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 US 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 percent from a week ago. The weekly average of new daily deaths is up 26 percent from a week ago, to 273, according to CNBC's analysis.