Austin is one of the most senior members of President Biden's administration to test positive.
Reuters file
US Defence Secretary Lloyd Austin said he tested positive for Covid-19 on Sunday with mild symptoms and would attend meetings virtually as he quarantined at home for the next five days.
News of Austin's positive test comes after the Pentagon last week tightened restrictions at its headquarters over concern about the highly transmissible Omicron variant that has led to a sharp increase in Covid-19 infections throughout the world.
The military is faced with the challenge of maintaining military readiness for troops, often in close quarters on ships and planes.
Austin, who is fully vaccinated and received a booster in early October, said in a statement he last met President Joe Biden on December 21, more than a week before he started experiencing symptoms.
"As my doctor made clear to me, my fully vaccinated status — and the booster I received in early October — have rendered the infection much more mild than it would otherwise have been," Austin added.
Austin said he requested a test earlier on Sunday after having symptoms while at home on vacation. He was last at the Pentagon on Thursday.
Austin is one of the most senior members of President Joe Biden's administration to test positive for Covid-19.
In October, Homeland Security Secretary Alejandro Mayorkas tested positive for the virus.
US authorities registered at least 346,869 new coronavirus on Saturday, according to a Reuters tally. The US death toll from Covid-19 rose by at least 377 to 828,562.
Austin said he planned to attend meetings virtually when possible and he would retain all authorities in running the Defence Department and overseeing military activities around the world.
His deputy, Kathleen Hicks, would represent him in some matters, he added.
"I continue to encourage everyone eligible for a booster shot to get one. This remains a readiness issue," Austin said.
Roughly 98 per cent of active-duty troops have received the Covid-19 vaccine, which is now mandatory for them.
Top US infectious disease expert Anthony Fauci said on Sunday there was still a danger of a surge in hospitalisation due to a large number of coronavirus cases even as early data suggests the Omicron Covid-19 variant is less severe than other variants.
The Omicron variant was estimated to be 58.6 per cent of the coronavirus variants circulating in the United States as of December 25, according to data from the US Centers for Disease Control and Prevention (CDC).
Fauci added that the CDC will soon be coming out with a clarification on whether people with Covid-19 should test negative to leave isolation, after confusion last week over guidance that would let people leave after five days without symptom.