The New Jersey-based company says this extra dose will increase protection up to 94%.
"The difference with Johnson & Johnson is it was only one shot initially. It only achieved about 70% protection with that one dose," said epidemiologist Dr. Marci Drees of ChristianaCare in Delaware.
On Tuesday, eligible recipients came to Deliverance Evangelistic Church in North Philadelphia to get their booster.
SEE ALSO: J&J seeks US clearance for COVID-19 vaccine booster shots
"I'm immunocompromised and wanted to be safe, keep my family safe," said Angela Harrelson of West Philadelphia.
Harrelson, who is eligible because of her medical condition, received her Moderna booster vaccine. She's looking forward to what an additional level immunity would mean for her.
"We can come together as a family for the holidays. Last year we didn't see one another," said Harrelson.
Action News asked Dr. Drees, who also serves on the CDC Advisory Committee on Immunization Practice: who is eligible for a third dose right now?
"People who are immunocompromised; transplant patients, people with cancer, people undergoing chemotherapy, or on medications that suppress their immune system. They're currently recommended to get a third dose of either Pfizer or Moderna," said Dr. Drees.
In addition, certain patients who received their second Pfizer shot at least six months ago qualify for a booster shot. They include nursing home residents and those who live in congregate settings, those in the 50 to 64-year-old age range with underlying health conditions, 18 to 49-year-olds with underlying health conditions, and frontline workers at risk of contracting COVID-19.
Third doses of Pfizer or Moderna were approved last month for anyone with a compromised immune system, 28 days after their second shot.
"We're not going to boost our way out of this pandemic. It really has to be vaccinations," says Dr. Drees.
While COVID-19 booster shots are making headlines and garnering attention, Dr. Drees says what's still most important are initial vaccinations.
"The big difference is how many susceptible people we have," said Dr. Drees. "When there is so much transmission, we're going to see a lot more mutations occurring."
As for the frequency of vaccinations and possible boosters, she says the more of the population immunized the less frequent need for a booster.
"It may be something more akin to a Hepatitis B vaccine, where you get your two doses one month apart, then your third six months apart and you're done. It may be more of that model," said Dr. Drees.
Next week, the FDA will meet to discuss green-lighting a Johnson & Johnson vaccine and the possibility of expanding eligibility for a Moderna booster shot.
The Associated Press contributed to this report.