Dr. Scott Gottlieb said Monday he's not yet concerned by the number of Americans who have skipped their scheduled second Covid vaccine dose.
"We're not certain that those people aren't going to eventually come back. They just didn't come back on time," the former Food and Drug Administration commissioner said in an interview on CNBC's "Squawk Box."
However, Gottlieb said receiving the second Covid shot is necessary to obtain the full protective benefits provided by the vaccines for the months ahead. The Pfizer-BioNTech and Moderna vaccines require two shots. (Johnson & Johnson's Covid vaccine, the third cleared for emergency use in the U.S., requires only a single dose.)
"My advice to anyone would be that even if you're young and there's evidence that you derive a robust immune response just from that first dose, we don't know the durability of that response," said Gottlieb, who sits on Pfizer's board. "If you really want to get a durable effect from the vaccine, you really should get the second dose."
On Friday, White House chief medical advisor Dr. Anthony Fauci said about 8% of U.S. residents who have received the first dose of the Pfizer or Moderna vaccines have not come back for the second shot.
"The number of people that so far that we've seen not coming back for the second dose is low relative to historical standards or historical norms," said Gottlieb, who led the FDA from 2017 to 2019 in the Trump administration. For example, he said, the return rate for the Covid vaccine is better than for the two-dose shingles vaccine.
Gottlieb acknowledged it's possible a higher percentage of U.S. vaccine recipients could skip the second shot as more young people get the shot. That is partly because "younger people know that they're deriving a more robust immune response just from the first dose versus an older individual, who really needs that second dose to derive the full immune protection," he said.
People who haven't returned for the second shot aren't necessarily doing anything intentionally wrong, Gottlieb added. He complimented the pharmacies that are administrating vaccines for "trying to implement reminders for those patients."
"A lot of times it's just lost to follow-up. It's not people who are deliberately not coming back," Gottlieb said. "There are some situations, I've talked to people, who are worried about the second dose, the side effects that are purportedly associated with the second dose versus the first dose. But right now, the percentage of people who've come back for that second shot is pretty high."
Nearly 105 million people in the U.S., almost a third of the population, have been fully vaccinated, according to the latest data available from the Centers for Disease Control and Prevention. About 147 million people, roughly 44% of the U.S. population, have received at least one dose, the CDC data says.
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 Inc. and biotech company Illumina. He also serves as co-chair of Norwegian Cruise Line Holdings' and Royal Caribbean's "Healthy Sail Panel."