WASHINGTON, Dec. 19 (Xinhua) -- U.S. Senator Elizabeth Warren announced on Sunday that she has tested positive for COVID-19 despite a booster shot.

The Democratic senator from Massachusetts said on Twitter that her regular test earlier in the week came back negative, but that on Sunday she "tested positive with a breakthrough case."

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 United States.

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

U.S. President Joe Biden is expected to deliver an Omicron-focused speech on Tuesday. White House Press Secretary Jen Psaki said Saturday on Twitter that the administration is "prepared for the rising case levels," and the president will detail how they will respond to the challenge. Enditem