It began with a headache and sore throat one morning at the end of June. Then came a heavy chest that made breathing a struggle, while the headache became so bad that 66-year-old Mio Blagojevic describes it as ‘like having an out-of-body experience’. The property developer from Hertfordshire adds: ‘I’d never felt so ill. It was like I’d been hit by a bus.’ With symptoms like these, this time last year the instant suspicion would have been Covid. But Mio, a keen runner, and his wife Karen had been fully vaccinated and then boosted in January. Due to the severity...