Live updates: follow the latest news on Omicron
US Representative Alexandria Ocasio-Cortez has tested positive for Covid-19, according to a statement from her office.
Ms Ocasio-Cortez, who is vaccinated and received her booster shot, is "experiencing symptoms and recovering at home", her office said on Sunday. The Democratic congresswoman added that she encourages everyone to receive their booster shot and follow guidance from the US Centres for Disease Control and Prevention.
With the nation dealing with another surge of the virus since the emergence of the highly contagious omicron variant, numerous members of Congress have reported breakthrough cases despite being fully vaccinated and boosted.
The seven-day average positivity rate in the US Capitol has risen from 1 per cent to 13 per cent, according to a memo sent to US lawmakers last week from Capitol physician Brian Monahan. Mr Monahan warned that positive cases can rise significantly in the next few weeks.
The breakthrough in cases has prompted Congress to ramp up its Covid-19 protocols with the Senate returning to session on Monday.
Mr Monahan has encouraged congressional offices to work from home and limit the amount of in-person meetings. Those inside Capitol Hill have also been urged to wear N95 or KN95 masks rather than cloth or surgical masks.
The House of Representatives had already reinstated its mask mandate regardless of vaccination status back in the summer, when the Delta variant was raging through the US.
The Associated Press contributed to this report