While vaccination against COVID-19 confers obvious and substantial benefits in preventing serious illness, booster shots have a complex immunology that means they come with a risk and must be considered with great care. Dr Rochelle Walensky, a physician-scientist who directs the US Centers for Disease Control and Prevention, and Dr William Powderly, an infectious diseases physician from Washington State, expressed reservations about the current enthusiasm for Covid vaccination: “So many of us wanted to be hopeful: this [vaccination] is our ticket out.” Looking back, they say there was “too little caution and too much optimism” and the possibility of resistant…