【明報專訊】With the increase in the proportion of people vaccinated against the coronavirus, more and more countries, such as the UK, US, and Singapore, have gradually shifted their anti-epidemic strategies from ''clearing individual cases'' to ''coexisting with the virus''. Physicians always remind us of a remarkable fact: the body remembers the pathogens it has encountered, sometimes for life. But that does not mean that you either have or do not have immunity, because while the body recognises old intruders, there is a whole host of factors that determine whether it can stay perfectly immune. Immunity, in other words, is not a clear-cut biological state.