Just a single dose of either the Pfizer or Moderna vaccine was shown to be 80 percent effective in preventing COVID-19 in a new CDC study.

The high level of protection occurs within the first two weeks after receiving part one of the two-dose regime, researchers said.

“These interim vaccine effectiveness findings for both Pfizer-BioNTech’s and Moderna’s mRNA vaccines in real-world conditions … demonstrate that current vaccination efforts are resulting in substantial preventive benefits among working-age adults,” according to the federal Centers for Disease Control and Prevention.

At least one previous FDA study showed an even higher protection rate — more than 92 percent — after a single shot of either vaccine.

The latest study examined front-line health care workers, first responders and other essential employees in eight US cities between Dec. 14 and March 13.

While there was an 80 percent protection rate with one dose, that figure jumped to 90 percent in the first two weeks after the second injection, scientists said.

Researchers urged that the full two-dose regime still be followed — given studies showing that immunity after a double shot is more lasting and because the growing proliferation of virus variants.

“The reason that they are two-dose vaccines is that the second doses give you a titer of neutralizing antibodies, virus-specific neutralizing antibodies that is almost 10 growth fold greater than after the first dose,” Dr. Paul Offit, a member of the FDA advisory panel that reviewed both vaccines, told CNBC.

“Secondly, and more importantly, you can detect so-called cellular immunity, called T cells responses, that usually protect more durable immunity,” after two doses, he said.

The third vaccine currently on the US market is a single-dose shot made by Johnson & Johnson. It has shown to be 66 percent effective in thwarting moderate to severe COVID-19-related illness.