Why the difference between a COVID vaccine ‘booster’ and ‘third dose’ matters


RICHMOND, Va. (WRIC) – Thousands of Virginians have already received their COVID-19 booster shots. While people head into pharmacies and vaccination clinics to get their shots, some getting the Moderna vaccine may be offered a third dose instead of a booster shot -– and there is a difference.

New vaccine information is flooding in every week and it can get confusing.

“I think a lot of the information around boosters has been changing. It’s been confusing with the different types,” said Dr. Danny Avula, Virginia’s vaccine coordinator.

He says boosters of the Johnson & Johnson and Pfizer vaccines are the same shots that have been given out for months.

On the other hand, the Moderna booster is a half dose.

“So if people are getting the Moderna booster then that should automatically be a 50 microgram dose as opposed to the 100,” Dr. Avula said.

However, when going to get another shot of Moderna, you could be offered a full third dose. That’s because back in August, the Food and Drug Administration (FDA) authorized the third shot of Pfizer or Moderna for certain immunocompromised people.

Joanna Cirillo with Richmond’s Health Department says boosters are for people whose immune response may have weakened over time. A third dose is for people who may not have had a strong enough immune response from the first two doses.

“It depends on lots of different things. If you take certain medications that might affect your immune system,” she said.

When in doubt, talk to a doctor.

“They’re gonna know your medical history. They’re gonna know all your medical risks so the best thing to do is talk to them about it,” Cirillo said. “That same advice applies to people who are considering which booster to get now that mixing and matching is a reality.”

If you’re getting a third dose because you’re immunocompromised, experts say those three doses should all be the same type.

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