Through early October, 8.5 million Americans have received an additional dose of a COVID-19 vaccine. Additional shots account for 20% 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.

In October, more people received additional vaccines than people starting or completing vaccination.

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The regular vaccination process may not provide the same protection against the virus for immunocompromised people as it does 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. The CDC recommends the booster shot at six months after the second dose of the vaccine. Both Moderna and Johnson & Johnson are waiting for FDA approval for booster doses for their vaccines.

Sixteen percent of people 65 and older, about 8.5 million people, have gotten a third shot as of October 12.

Though the CDC does not have state-level data on additional doses administered, 20 states and Washington, DC, made that data public. Of those states, Vermont has the highest share of its population getting third shots at 4.6%. Vermont leads all states in vaccinations with 70% of its population fully vaccinated.

In states where data is available, Vermont has the highest share of its population receiving additional vaccine shots.

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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.

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