The U.S. Centers for Disease Control and Prevention (CDC) raised its travel warning for the UK to its highest level, level 4, on Monday.

The “very high” warning advises Americans not to travel to the UK, and if they do, the CDC said they advised travelers to be fully vaccinated against COVID-19.

“Because of the current situation in the United Kingdom, even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants,” the CDC website said.

Meanwhile, the UK relaxed nearly all restrictions over the weekend, as Prime Minister Boris Johnson ended the mask mandate and other coronavirus measures in what was called “Freedom Day.”