(AP) — The U.K. has opened seven mass vaccination centers as it moves into the most perilous moment of the COVID-19 pandemic, with exhausted medical staff reeling under the pressure of packed hospitals and increasing admissions. England’s chief medical officer has warned that the coming weeks will be the worst of the pandemic for the National Health Service, as he appealed to the public to more closely follow prevention measures. Dr. Chris Whitty said Monday that the new variant of COVID-19, which spreads the virus more quickly, has aggravated an already difficult situation. Hospitals are overflowing and exhausted medical staff are under strain. Britain has approved three different coronavirus vaccines and plans to inoculate 15 million people by mid-February.