Immune responses, particularly neutralizing antibody responses, to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection have been observed to decay rapidly in the first few months after infection but the rate of decay seems to slow with time3. a | In an ideal scenario, immunity remains above a protective threshold for a prolonged period, providing strong immunity from reinfection. b | Immunity may drop below the threshold required to prevent infection but still provide sufficient protection against severe illness. If viral levels after reinfection are high enough to enable transmission, the endemic spread of infection may boost memory responses to maintain immunity at protective levels82. c | Weak immune memory, rapidly decaying immunity or viral antigenic variation may result in the level of immune memory dropping below the threshold required to protect against severe disease, thereby exposing the patient to a similar risk to that seen in primary infection.