Washington, United States --- The United States on Thursday authorized an extra dose of COVID vaccine for people with weakened immune systems, as the country struggles to thwart the Delta variant.

Emergency use authorization for a third injection of Pfizer-BioNTech and Moderna vaccines was granted by the US Food and Drug Administration (FDA) regulator.

"The country has entered yet another wave of the COVID-19 pandemic, and the FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease," said acting FDA Commissioner Janet Woodcock in a statement.

The FDA said the additional dose was for solid organ transplant recipients or those with equivalent weakened immune systems.

US health authorities had been debating whether a third dose may be required, following a similar move by Israel.

Some United States media reports suggest that one million Americans may have had unauthorised third doses in an attempt to increase protection against COVID.

"Individuals who are fully vaccinated are adequately protected and do not need an additional dose of COVID-19 vaccine at this time," Woodcook said.

Booster moratorium call

Earlier this month, the United States rejected an appeal from the World Health Organization for a moratorium on booster shots to help ease the drastic inequity in dose distribution between rich and poor nations.

More than 619,000 people have died in the United States from COVID-19, with case numbers increasing sharply in recent months due to the spread of the Delta variant.

The country's rapid vaccination program has slowed particularly in politically conservative regions in the South and Midwest, and among younger people, those with lower income and racial minorities.

"Right now at this moment, other than the immune-compromised, we're not going to be giving boosters to people," top US COVID advisor Anthony Fauci told NBC on Thursday.

"But we will be following them very carefully, and if they do need it, we'll be ready to give it to them... inevitably there will be a time when we will have to give boosts."

COVID-19 vaccines are free and widely available in the United States, yet only half the population is fully vaccinated.

President Joe Biden announced in July that all federal employees must attest to being vaccinated or face tough restrictions such as regular testing amid a surge in hospitalizations.

Propelled by the highly contagious Delta variant, coronavirus infections have soared to a daily average of more than 100,000, a level not seen since the winter surge.