Sen. Elizabeth Warren, D-Mass., announced that she has tested positive with a breakthrough case of COVID-19.

"I regularly test for COVID & while I tested negative earlier this week, today I tested positive with a breakthrough case," Warren announced on Twitter Sunday. "Thankfully, I am only experiencing mild symptoms & am grateful for the protection provided against serious illness that comes from being vaccinated & boosted."

 Sen. Elizabeth Warren. (AP Photo/Patrick Semansky)

Warren's announcement comes as cases of COVID-19 have started to rise again throughout much of the country, with health officials worrying that the emergence of the Omicron variant could continue to make the situation more dire.

"The cases are going up," Dr. Anthony Fauci said last week. "We have an average of about 117,000 cases. We have an increase in the percentage of hospitalizations. Deaths are still over a thousand. Then you have, looking over your shoulder, the Omicron variant, which we know, from what's going on in South Africa and in the UK, is a highly transmissible virus.

FILE - In this March 3, 2021 file photo, a vial of the Johnson & Johnson COVID-19 vaccine is displayed at South Shore University Hospital in Bay Shore, N.Y. (AP Photo/Mark Lennihan, File)

"That's the reason why we are encouraging people, if they haven't been vaccinated, to get vaccinated but, as importantly, for those who've been fully vaccinated to get a booster."

CLICK HERE TO GET THE FOX NEWS APP

Despite having a positive test while being fully vaccinated, Warren encouraged Americans in a subsequent tweet to get vaccinated or to get a booster.

Democratic presidential candidate Sen. Elizabeth Warren, D-Mass., speaks during a campaign event, Monday, Jan. 20, 2020, in Grimes, Iowa. (AP Photo/Patrick Semansky)

"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," Warren said.