Here in New Zealand, we’re using what’s called quantitative polymerase chain reaction (PCR)​ to test for Covid-19. If they are a historical infection, they’d have another high CT value or may even test negative. To be sure a case is historical, health officials will also ask the person for a blood sample, so they can do serology testing. Waiting for the results of serology testing is usually why there is a delay before the Ministry of Health announces historical cases. Finding out whether someone has a new or an historical infection really matters.

Source: Stuff February 14, 2021 16:00 UTC