When you have eliminated the impossible…

A gentleman who is Not New Zealand’s Favourite DJ has tested positive for Omicron on his 9th day in NZ, after prior negative tests.  It seems surprising that a positive test could take so long — one theory is that it’s the sort of sporadic positive you can get for a while after recovery.  On the other hand, it’s worth thinking about why it’s surprising.  New Zealand keeps seeing strange Covid occurrences: long incubation period, transmission from very brief contact, and so on. Why us? It’s us because no-one else would be able to tell.

The normal assumption if a London DJ tests positive for Omicron on December 25 is that they got it in London a few days earlier. In this example, ‘a few days earlier’ means Waiheke Island and however he got there from MIQ;  in contrast to almost everywhere else in the world, Omicron isn’t circulating in Auckland.  The week before that he was in MIQ, so the next conclusion is that he got it there; unfortunate but not unprecedented. Without genome sequencing we’d stop there, but his viral genome doesn’t match any of the three he could potentially have picked up in MIQ.  We’re now down to weird possibilities, and the least weird is that he was carrying it all the time. But if he was almost anywhere else in the world, we wouldn’t even be starting to think about the weird possibilities.

It’s fairly easy to estimate the low end of the time-to-positive-test distribution: someone goes to a party; a few days later there are twenty cases; you do the maths. To get the low end you need some cases where you’re sure they couldn’t have been infected before a specific exposure, because they weren’t exposed before then. At the start of an outbreak that’s fairly easy. To get the high end of the distribution you need some cases where you’re sure they couldn’t have been infected after a specific exposure. That’s a much less common scenario, so the data aren’t as good.