Primary series vaccines, like the two-dose Pfizer and Moderna vaccines, were called “monovalent” because they were designed to protect against the single original virus that causes COVID-19. They also provided some protection against Omicron, but not as much as the updated, bivalent vaccine.
The updated, bivalent vaccine is called “bivalent” because it protects against both the original virus that causes COVID-19 and the Omicron variants BA.4 and BA.5. Monovalent vaccine is no longer available because updated, bivalent vaccine provides the best protection against currently circulating Omicron variants.
The CDC recommends eligible people 6 months and older receive an updated, bivalent Pfizer or Moderna vaccine that offers better protection against variants. Some people, like those who are immunocompromised or over 65, are eligible for a second bivalent dose. The CDC has guidance for everyone 6 months and older based on age and medical condition.