"The definition of vaccine we have posted is problematic and people are using it to claim the COVID-19 vaccine is not a vaccine based on our own definition," Alycia Downs, a CDC official, wrote in an email on Aug. 25, 2021, to a colleague.

The definition is located on a page titled Immunization Basics.

"Vaccine" was defined since at least 2011 by the CDC as a product that triggers immunity, while "vaccination" was described as an injection that prevents a disease, according to archived versions of the page. However, a flood of inquiries on the definitions was triggered by the fact that the COVID-19 vaccines have been increasingly ineffective against infection by the virus that causes COVID-19, the emails show.

"Our question is how is the CDC and the rest of the world allowed to call the shot a vaccination when it doesn't even meet your own definition," one person wrote to the CDC.

"Right-wing covid-19 pandemic deniers are using your 'vaccine' definition to argue that mRNA vaccines are not vaccines," another said.

The Pfizer and Moderna COVID-19 vaccines are both built on messenger RNA technology. They are two of the three COVID-19 vaccines available in the United States.

Downs and colleagues Allison Michelle Fisher, Cynthia Jorgensen, Valerie Morelli, and Andrew (no last name given) worked on changing the definitions for "vaccine" and "vaccination," according to the emails.