Nearly 1 million Covid-19 booster shots have already been administered in the U.S. since health officials authorized administering extra shots of Pfizer or Moderna’s vaccines to people with weakened immune systems on Aug. 12, according to data compiled by the Centers for Disease Control and Prevention.

So far, about 955,000 fully vaccinated people have received an additional dose of a Covid vaccine, according to data collected by the CDC. That number includes those who had previously received two doses of Pfizer or Moderna‘s vaccines or one dose of Johnson and Johnson‘s, the agency said.

It’s unclear if all of those people were considered immunocompromised.

In approving the booster shots in people with weak immune systems, which includes cancer, HIV patients and organ transplant patients, CDC officials cited several small studies that showed they didn’t produce an adequate immune response after receiving two doses of Pfizer or Moderna’s vaccines. FDA and CDC officials didn’t approve booster doses of J&J’s vaccine earlier this month, saying they were waiting on more data.

Some Americans were already finding ways to get additional doses of the Covid vaccines on their own prior to the formal approval with some even going as far as receiving the extra shots from different companies – a practice known as “mixing and matching.”

The CDC and Food and Drug Administration are currently reviewing whether to administer booster shots to the general population.

President Joe Biden said the U.S. plans to widely distribute Covid booster shots beginning the week of Sept. 20, pending clearance from FDA and CDC scientists.

The CDC’s Advisory Committee on Immunization Practices debated the need for boosters Monday, saying the data supporting wide distribution was limited.

Dr. Helen Keipp Talbot, a voting member of the CDC advisory group, told the committee Monday that many hospitals in the south are already administering third doses in health-care workers and patients.

New Jersey officials said Monday more than 36,000 extra doses have been administered to the immunocompromised, frontline health-care workers and seniors so far.

State health commissioner Judy Persichilli said the state is looking to identify more people who qualify for a third dose.

“Nationally we think about 3% of the population is immunocompromised, so we’re really asking particularly medical directors in long-term care to do a deep analysis of those medical records and identify individuals who should be queued up to get that third dose right now,” she said.

–CNBC’s Bob Towey and Nate Rattner contributed to this report.