Covid vaccines are increasingly likely with vast parts of the world unprotected, and rich countries should hold back on booster doses until others catch up, according to a special envoy to the World Health Organization. “Variants that can beat the protection offered by vaccines are bound to emerge all over the world in the coming months and years," David Nabarro, the WHO envoy, said in an interview Monday with Bloomberg Television. “This is an ongoing battle, and we need to work together." Nabarro issued the warning as some countries such as the UK prepare to give an extra dose to people already vaccinated.