WASHINGTON: President Joe Biden hailed the "exciting" announcement Saturday that US regulators had authorized a third Covid-19 vaccine but warned the hard-hit nation could not let its guard down.

"This is exciting news for all Americans, and an encouraging development in our efforts to bring an end to the crisis," Biden said in a statement after the Johnson & Johnson vaccine got the green light.

"But we cannot let our guard down now or assume that victory is inevitable."