How many Americans have gotten an additional COVID-19 vaccine, including booster shots?

Through late October, 13.7 million Americans have received an additional dose of a COVID-19 vaccine. Additional shots account for 26% of all doses administered since the Centers for Disease Control and Prevention (CDC) started tracking them in late August.

While the CDC defines additional doses and booster shots differently, the agency’s data combines them into a single metric.

The regular vaccination process may not provide the same protection against the virus for immunocompromised people as it does for the general population. In August, the Food and Drug Administration approved the use of an additional dose of two COVID-19 vaccines — manufactured by Pfizer-BioNTech and Moderna — for people who have weakened immune systems. The CDC recommends an additional dose for this group of people at least four weeks after getting a second dose.

Booster shots are intended to increase immunity after the effects of full vaccination weaken. In September, the CDC approved a third shot of the Pfizer-BioNTech vaccine for people 65 and older, any adults who live in a long-term care setting, those with underlying medical conditions, or people who work or live in a high-risk setting.

In October, the CDC approved booster shots for people in the same groups who received the Moderna vaccine. The eligibility requirements are the same as the Pfizer-BioNTech booster. The CDC also approved boosters for anyone vaccinated with the single-shot Johnson & Johnson vaccine. The CDC recommends the booster shot six months after the second dose of the vaccine.

Sixteen percent of people 65 and older, about 8.5 million people, have gotten additional shots as of October 12.

The data on additional vaccine doses is another key metric to track in the ongoing pandemic. Track case and death data or vaccination rates on the COVID-19 tracker.