Community Covid Testing

For the past couple of years I’ve been arguing against Covid testing for people who don’t have symptoms and aren’t at high risk of exposure: they’ll have only a minute chance of testing positive, so we won’t learn anything, and we have better uses for the testing resources.  The only country that’s been doing systematic surveillance of Covid has been the UK, where the background prevalence has been, let’s say, somewhat higher than it had been here.

New Zealand is now getting a substantial Covid outbreak.  We’ll be over 1000 new cases some day soon, and it will start to matter for hospital planning purposes whether we’re detecting 20% of infections or 10% or 1% — because hospital numbers follow infection numbers with a long enough lag that the information is useful.

We’ve got two possible approaches to estimating the population Covid burden. One is wastewater testing, the other is random sampling.  Both approaches will keep working no matter how high the Covid prevalence is and no matter what fraction of infections are diagnosed and reported.  Sampling is more expensive, but has the advantage that it actually counts people rather than counting viruses and extrapolating to people.  Using both would probably help balance their pros and cons.

Sampling doesn’t have to be ‘simple random sampling’. If we know there’s more Covid in Auckland than in Oamaru, we can sample at a higher rate in Auckland and a lower rate in Oamaru.  We can also do adaptive sampling, where you take more samples in places where you find a hotspot.  Statistical ecologists trying to count plant and animal populations have studied this sort of problem quite a lot over the years — and statistical ecology is, fortunately, an area where NZ has expertise. But even simple random sampling would work, and would give us an estimate of infections and symptomatic cases across the country, and help plan the short to medium term response.