As waves of new coronavirus variants circulate the globe, one thing has become clear: human immunity against the virus fades over time....But some of the nuances around why and how immunity against Covid-19 fades remain a mystery. The steepest drops in immunity – which come about four to five months after vaccination and up to eight months after infection, but can vary – are against Covid-19 symptoms, getting infected and getting sick. Protection against severe outcomes, hospitalization and death remains much higher for a longer period of time, but even this decays to some degree, especially for the elderly and...