With New Zealand on the brink of zero known active cases of Covid-19, the clamour for an immediate move to alert level one is growing among politicians, media commentators, and many others. That’s understandable, but there are very serious reasons to be careful.

We’ve had a string of days with no new Covid-19 cases and we’re down to just one known active case. One! The question I’m now being asked is: why are we still at alert level two? Surely we’re good to move down to alert level one now? Not so fast.

There are several reasons why holding at alert level two for a little longer is the right thing to do. The main one is that a run of several days with no new cases doesn’t mean that there are no undetected active cases of Covid-19 out there. Recent modelling by Professor Nick Wilson and his colleagues at the University of Otago estimated it would take anywhere between 27 and 91 days of no new cases for there to be a 95% probability that the virus is gone from New Zealand.

The lower estimate was based on the assumption that most people showing symptoms would go and get tested. The higher estimate was based on fewer people getting tested. In other words, those estimates are the difference between shrugging off that runny nose as an allergy or going to get tested for Covid-19 just in case.

Let me be clear. Things are looking really promising for us. There is no sign so far that there are undetected cases of Covid-19 in New Zealand. But there are two problems with assuming that is the case and jumping straight to alert level one. The first is the fact that while some people end up in hospital for weeks with Covid-19, many others may just feel a little under the weather for a day or might not have any noticeable symptoms at all. I’ve written before about the lack of good studies investigating how many people get a mild case of Covid-19 and how infectious these people are. That’s what makes this so frustrating. If it turns out there are lots of people like this and they are infectious then we are likely to see cases start to rise again.

And you don’t need to look too far to see how real the risk is of just that happening. South Korea. Singapore. Australia.

Which leads me to the main reason why holding a little longer at alert level two is for the best. The incubation period of the virus. Because it takes roughly two to ten days for people to develop symptoms, our string of no cases reflects the beginning of level two. We’ve only just opened up our bars and nightclubs and increased our gathering sizes. If the virus is out there, we’ve just increased its opportunities to show us where it’s at. Now we just need to wait to see what happens.

I get why so many people want to move to level one, I really do. These last few months have turned our lives and our economy upside down. Just as they have right around the globe. We’ve made big sacrifices and we feel we’ve earned it. But surely none of us wants to risk going back to alert level three. Alert level one will come. Let’s not squander what we’ve achieved.