Centers For Disease Control

The CDC officially updated the wording of its definitions for both “vaccine” and “vaccination” on September 1, 2021. Prior definitions dating back to at least 2011 described a vaccine as “a product that stimulates a person’s immune system to produce immunity to a specific disease.” As of early September 2021, the updated definitions now refer to a vaccine as “a preparation that is used to stimulate the body’s immune response against diseases,” and vaccination as “the act of introducing a vaccine into the body to produce protection from a specific disease”

Why the change?

• The CDC explained it was a semantic adjustment aimed at improving clarity. The original wording might have been interpreted as implying that vaccines provide perfect immunity, which is not accurate for any vaccine 

• According to internal communications, the change was prompted by public and political critiques—particularly from those arguing that COVID‑19 vaccines didn't meet the original definitions—so the CDC opted for terms less likely to be misread

• Experts affirmed that the revisions did not alter the scientific meaning or intent; they simply reflect the evolving landscape of vaccine technology and immunology