Washington, Dec 20 (IANS): US Senator Elizabeth Warren announced that she has tested positive for Covid-19 despite being administered a booster shot against the virus.

The Democratic Senator from Massachusetts tweeted that her regular test last week came back negative, but that on Sunday she "tested positive with a breakthrough case", reports Xinhua news agency.

Warren said she was only experiencing mild symptoms and felt "grateful for the protection provided against serious illness that comes from being vaccinated and boosted".

It's not clear whether Warren has contracted the highly transmissible Omicron variant, which is driving Covid-19 flare-ups in the US.

The country's top infectious disease expert Anthony Fauci warned that Omicron is "going to take over" the US in the winter, urging Americans to get fully vaccinated and boosted.

UPresident Joe Biden is expected to deliver an Omicron-focused speech on Tuesday.

White House Press Secretary Jen Psaki said that the administration is "prepared for the rising case levels", and the President will detail how they will respond to the challenge.