WASHINGTON - More than 5 million Americans, or nearly 8 percent of those who received a first jab of their COVID-19 vaccine, have missed their scheduled second dose, US media reported.

The number of those skipping their important second dose has increased from about 3.4 percent in March, more than double the rate among people who got inoculated in the first several weeks of the nationwide vaccination campaign, CNN reported, citing data from the Centers for Disease Control and Prevention.

The inoculation of the Pfizer and Moderna vaccines requires people to return for a second shot three or four weeks after the first dose to be fully effective, said health authorities.

Anthony Fauci, top US infectious disease expert, told CNN on Sunday that he was "not surprised" about this, saying it happens frequently with multi-dose vaccines.

"Obviously whenever you have a two-dose vaccine, you're going to see people who for one reason or other -- convenience, forgetting, a number of other things -- just don't show up for the second vaccine," CNN quoted Fauci as saying.

Experts have attributed the sluggish vaccine demand to factors including vaccine hesitancy, but added that further analysis is needed, the report said.