DENVER (AP) — Colorado U.S. Sen. John Hickenlooper announced Thursday that he has tested positive for a “breakthrough” case of COVID-19.

The first-term Democrat issued a statement saying he tested positive after experiencing mild symptoms and is self-isolating at the direction of the attending physician for the U.S. Congress, Dr. Brian P. Monahan. Hickenlooper said he was “feeling much better.”

“I’m grateful for the vaccine (and the scientists behind it) for limiting my symptoms and allowing us to continue our work for Colorado,” Hickenlooper said. “If you haven’t been vaccinated, don’t wait for the virus — get the shot today, and a booster when it’s available too!”

Infections and illnesses can happen even after being vaccinated. Experts say vaccination could help make any illnesses less severe. The U.S. Centers for Disease Control and Prevention notes that newer versions of the coronavirus could be a factor in “breakthrough” cases.

Hickenlooper, 69, is a former brewpub entrepreneur, Denver mayor and two-term governor who defeated incumbent Republican U.S. Sen. Cory Gardner in the 2020 election.