Millions of Americans have failed to take the second dose of COVID-19 vaccine on time and the number is growing, according to the latest data from U.S. Centers for Disease Control and Prevention (CDC).

The data shows that over 5 million people, or 8 percent of the population, who received their first COVID-19 vaccine shots, had missed their second shots.

It is more than double the rate among people who were inoculated in the first several weeks of the national vaccination campaign, New York Times reported on Sunday.

The CDC said 40 million people received their first shots between December 14 and February 14. Data shows 8.6 percent of those vaccinated had missed the recommended interval between doses (21 days for Pfizer and 28 days for Moderna) but were still within the allowable interval to receive it. As many as 3 percent were overdue for their second dose.

The reasons behind missing shots varies. Some are worried about the side effects and others said they felt a single shot was enough, reported New York Times.

In addition, problems with shipment or appointment also caused troubles for people to receive the second shot. Some local providers were out of stock of certain brand of vaccines, rendering many people unable to receive their second shot of the same brand as the first dose.

Massive evidence collected in the trials and campaigns shows the second dose is crucial to enhance the immunity of the vaccinated and the single shot can only provide partial protection against COVID-19, according to New York Times.

"I'm very worried, because you need that second dose," said Dr Paul Offit, a professor at the University of Pennsylvania and a member of the Food and Drug Administration's vaccine advisory panel, New York Times reported.