The World Health Organization chief has warned against suggestions by some to just allow Covid-19 to spread in the hope of achieving so-called herd immunity. Calling such an approach "unethical", Tedros Adhanom Ghebreyesus said that herd immunity has never been used as a response to an outbreak, let alone a pandemic, in the history of public health. "It is scientifically and ethically problematic," he said, addressing a virtual press briefing.