COVID Vaccine Booster: Everything You Need to Know

Medically Reviewed by Dan Brennan, MD on January 03, 2023
3 min read

Most people who are fully vaccinated against the coronavirus are relatively well-protected from serious illness and death from COVID-19.

But even highly effective vaccines often become less so over time. Getting another jab of the vaccine several months after the first round, called a “booster shot,” can help supercharge the vaccine’s effectiveness.

The CDC recommends booster shots for fully vaccinated people. The timing depends on which vaccine you receive at first.

Pfizer-BioNTech. After you’ve got the two jabs required for the primary series of the mRNA vaccine, people 6 months or older are eligible for a booster shot 2 months after the last dose.

Moderna. After the first series, anyone 6 months or older can get a booster shot 2 months after the last dose.

Novavax. People 12 and older can get 2 shots, three to eight weeks apart. Teenagers ages 12 through 17 who had the Novavax primary series are eligible to get either a Moderna or a Pfizer-BioNTech bivalent booster.

Johnson & Johnson. Those 5 and over are eligible for a booster shot if it's been 2 months since their last primary dose.

The CDC recommends an additional booster shot for certain people to amp up their immune system against potential COVID-19 variants.

If you got the J&J Janssen vaccine as your first vaccine and first booster dose, the CDC recommends a second booster using an mRNA COVID-19 vaccine. You may get this at least 2 months after your last dose.

Boosters are a type of vaccine, and scientists already know that vaccines are currently the best defense against COVID, especially its most serious effects. That’s why it’s important to get vaccinated right away if you qualify,

Early boosters were called “monovalent” because they were designed to protect against the original virus that causes COVID-19. They also provide some protection against Omicron, but not as much as the new updated boosters.

The newest boosters are called “bivalent” because they protect against both the original virus that causes COVID-19 and the Omicron variant BA.4 and BA.5.

Pfizer and Moderna both have bivalent boosters.

Research also has found that:

  • In general, antibodies from vaccines decline over time.
  • Higher antibody levels from vaccines seem to make the average person less likely to get sick from the coronavirus (vaccine efficacy).
  • Booster shots of mRNA coronavirus vaccines (Pfizer and Moderna) can increase antibodies up to 10 times.
  • Higher levels of antibodies seem to be especially important against newer variants of the coronavirus. Infections from the variants are surging, and it seems to be more contagious, especially among children.


The CDC already suggests an extra series of mRNA vaccine for people with weaker immune systems, especially those who are “moderately to severely immunocompromised.” This includes those who are getting cancer treatment, who have had a stem cell or organ transplant, who have advanced or untreated HIV, or who are taking certain medications.

Though not technically a booster shot, many people still call it that.

This extra dose, typically given a month or so after the second dose, is meant to increase the first immune response because:

  • People with weaker immune systems are more likely to have serious, long-term illness from COVID-19.
  • A weaker immune system may not respond as strongly to the vaccine and so may not make enough antibodies to fight off infection and serious illness from COVID-19.
  • Even with a good vaccine response, people with weaker immune systems may benefit from extra protection against COVID-19.

If you have a weakened immune system and it has been at least 2 months since your last vaccine, you’re eligible for an mRNA bilavent booster shot 3 months later. And the CDC recommends a second booster shot.