US Senator Cory Booker speaks as US Senator Elizabeth Warren listens at the first US 2020 presidential election Democratic candidates debate in Miami, Florida, US, June 26, 2019.

Three Democratic US lawmakers have announced that they have breakthrough coronavirus infections amid a nationwide surge in cases.

Sens. Elizabeth Warren of Massachusetts and Rep. Jason Crow of Colorado as well as Cory Booker of New Jersey said on Sunday they tested positive for COVID-19.

In a tweet, Warren, 72, said she is vaccinated and boosted and regularly tested for COVID-19, and while I tested negative earlier this week, today I tested positive with a breakthrough case.

As cases increase across the country, I urge everyone who has not already done so to get the vaccine and the booster as soon as possible - together, we can save lives, she added.

Crow, 42, tweeted that he just returned from an official congressional delegation visit to Ukraine and tested positive for a breakthrough COVID infection.

Booker, 52, a New Jersey Democrat, said in a tweet he had learned Sunday of his positive test after first feeling symptoms on Saturday.

He added he had relatively mild symptoms and recently had received a vaccine booster. I’m certain that without them I would be doing much worse.

This comes as the spread of the Omicron variant across the United States has thrust the nation and the White House back into an uncertain pandemic reality.

As of Saturday, the country was averaging 126,967 new cases per day, showed data from Johns Hopkins University, up from an average of just over 70,000 new cases per day at the beginning of November.

Meanwhile, Washington state Sen. Doug Ericksen, a staunch conservative, died at age 52 on Friday. His death came weeks after he said he had tested positive for the coronavirus while in El Salvador, though his cause of death was not immediately released.

Top US infectious disease expert Dr. Anthony Fauci has warned of a new high in the number of deaths over Omicron.

We are going to see a significant stress in some regions of the country on the hospital system, particularly in those areas where you have a low level of vaccination, which is one of the reasons why we continue to stress the importance of getting those unvaccinated people vaccinated, he said during an interview with CNN.