Dr. Scott Gottlieb on Thursday warned coronavirus vaccine recipients against letting their guards down right away, telling CNBC they should adhere to public-health measures such as wearing masks.

The former U.S. Food and Drug Administration commissioner said the advice is particularly important for elderly Americans who are at higher risk of death or serious illness from Covid-19.

“I think for an older individual who is vulnerable to this virus, certainly .. wait for a period of time after the second shot until you’re likely to have full protective immunity,” Gottlieb said on “Squawk Box.” “I don’t think people should feel completely secure after the first shot.”

Both Covid vaccines that have received emergency use authorization from the FDA require two doses. Pfizer and its German partner BioNTech developed one of the vaccines, while Moderna makes the other. Gottlieb sits on the board of Pfizer.

Roughly 10.3 million Americans have received their initial Covid shot, as of Wednesday morning, according to the Centers for Disease Control and Prevention. About 29.4 million doses have been distributed.

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