The U.S. government has not approved booster shots against the virus, saying it has yet to see evidence they are necessary. But Welch and an untold number of other Americans have managed to get them by taking advantage of the nation's vaccine surplus and loose tracking of those who have been fully vaccinated.

Welch, a graduate student from Maine who is studying chemical engineering, said she has kept tabs on scientific studies about COVID-19 and follows several virologists and epidemiologists on social media who have advocated for boosters.

"I'm going to follow these experts and I'm going to go protect myself," said Welch, a 26-year-old with asthma and a liver condition. "I'm not going to wait another six months to a year for them to recommend a third dose."

While Pfizer has said it plans to seek U.S. Food and Drug Administration approval for booster shots, health authorities say that for now, the fully vaccinated seem well protected.

Yet health care providers in the U.S. have reported more than 900 instances of people getting a third dose of COVID-19 vaccines in a database run by the Centers for Disease Control and Prevention, an Associated Press review of the system's data found. Because reporting is voluntary, the full extent of people who have received third doses is unknown. It's also unknown if all of those people were actively trying to get a third dose as a booster.

"I don't think that anyone really has the tracking" in place to know how widespread it is, said Claire Hannan, executive director for the Association of Immunization Managers.