WASHINGTON (REUTERS) - US President Joe Biden said he and first lady Jill Biden would receive a third dose of the Covid-19 vaccine to boost their immunity, as his administration announced booster shots would be offered to Americans in September.

"We will get the booster shots," Mr Biden told ABC News in an television interview that aired on Thursday.

His comments were taped on Wednesday (Aug 18) as health officials unveiled their plan to make third doses of the approved two-dose Covid-19 vaccines available for US adults starting Sept 20.

The booster program is being launched even as millions of Americans have yet to adopt initial vaccination and as many around the world are waiting for vaccine supplies.

US health officials defended the administration's plan to offer Americans' additional protection against the virus with another vaccine dose, saying they could still donate millions of doses worldwide and continue encouraging people to get their first shot.

"When we see evidence that immunity is waning, especially in the face of this Delta variant... we have an obligation to act to protect people at home," US Surgeon General Vivek Murthy told MSNBC's "Morning Joe" program.

US Centers for Disease Control and Prevention Director Rochelle Walensky said data showed a need for further protection amid the Delta variant and waning vaccine effectiveness over time, although it was unclear how often Covid-19 booster shots would be needed in the future.

"We know we need a boost now... but I don't think it's a given that we will be doing this continuously," she told NBC News' "Today" program.

Both Pfizer Inc/BionNTech SE and Moderna Inc's U.S.-approved vaccines require two doses to provide adequate immunity, and regulators must sign off on a third shot. Regulators are still awaiting data on any follow-up action needed for those given Johnson & Johnson's one-shot vaccine.

Mr Biden, 78, noted that he and his wife received their first doses of the Pfizer Inc/BionNSech SE vaccine back in December, when vaccines were just being rolled out in the United States.

"It's past time," he told ABC.