What Do These New Vaccination Terms Mean?

The US Centers for Disease Control and Prevention (CDC) published updated terminology for COVID-19 vaccine dosing schedules on October 25, 2021.

These new CDC terms are as follows:

• Primary series: 2-dose series of an mRNA COVID-19 vaccine (Pfizer-BioNTech and Moderna) or a single dose of Janssen vaccine
• Additional dose after an initial primary series: a subsequent dose of vaccine administered to people who likely did not mount a protective immune response after primary vaccination in order to optimize vaccine-induced protection. An additional mRNA COVID-19 vaccine dose is recommended for moderately to severely immunocompromised people who received an mRNA vaccine primary series.
• Booster dose: a subsequent dose of vaccine administered to people with protection from primary vaccination is likely to have waned over time.
• Homologous booster dose: a subsequent dose of vaccine that is the same product as the primary series
• Heterologous booster dose (mix-and-match booster): a subsequent dose of vaccine that is a different product than the primary series

The CDC stated these terms could be updated when additional information becomes available or if different vaccine products are approved or authorized.