How many Covid infections are really out there?

From the day the ‘elimination strategy’ ended, it was inevitable that the number of daily reported cases would stray from the reality of true infections. Now, new analysis reveals exactly how wide the gap is. Sapeer Mayron explains.

Right now, the real number of daily Covid infections is about twice as high as the daily reported cases.

This is according to an analysis from scientists at the Institute of Environmental Science and Research (ESR), alongside researchers from the University of Oxford, University of Canterbury, University of Auckland, and Massey University.

Cases have been growing steadily since mid-September, to a 14-day-average of around 4700 cases a day, but the reality is that the real number is much higher, with fewer people reporting their positive tests. Or simply not realising they have the virus.

By comparing the presence of Covid in our wastewater with the daily reported cases, scientists estimate the true number of infections could be almost twice the reported numbers. That means more we have more like 10,000 to 11,000 new infections a day.

University of Canterbury post-doctoral research fellow Dr Leighton Watson and University of Oxford PhD student Nicholas Steyn built the model to calculate the ‘‘ascertainment rate’’ of cases – the relationship between reported cases and actual infections.

A high ascertainment rate – closer to 100% – means the reported cases more closely match the true number of infections. So a 50% ascertainment rate suggests there are twice as many infections as reported cases.

In the peak of our first major Omicron wave in early March, case numbers hit peaks of about 24,000. Researchers believe those cases accounted for about 58% of actual infections. Apply the case ascertainment rate and that means there were some 40,000 infections a day back in March.

The latest ascertainment rate we have is from November 23, when it stood at 44%. On that day there were 4288 daily cases, suggesting 9745 infections.

This is still substantially fewer infections than we were likely seeing during the March or July wave even though people are less likely to report.

The mathematical model – that uses a number of assumptions about the spread of Covid-19 – shows how all year, they have been straying further and further away from reality. Although the past month has bucked the trend. On November 3 the rate was 33% but on November

23 it was 44%.

‘‘I was somewhat surprised to see the rate went up over the past few weeks,’’ Watson said. ‘‘It could be due to increased case numbers meaning that there is increased awareness. This is just speculation – but it does agree with my own anecdotal experience. Last week I went to a conference and several attendees later tested positive for Covid. Several people at my wife’s work recently tested positive as did a teacher at my daughters’ daycare. All of this means that we are much more aware of Covid and have been testing a lot more than we would have otherwise – thankfully we are all still negative.’’

‘‘Several people at my wife’s work tested positive as did a teacher at my daughters’ daycare. All of this means that we are much more aware of Covid and have been testing a lot more than we would have otherwise.’’ Dr Leighton Watson

How it all works

The case number totals you read about all the time are informed by largely selfreported Covid-19 cases: people out in the community who have swabbed at home using a rapid antigen test (RAT) and typed in their results on their phone to the government.

This requires two things. People to actually test. And people to then upload their tests.

This is where the wastewater comes in.

All around New Zealand wastewater is tested for how much Covid is swilling around it after being shed by infected people.

Specifically, scientists measure the amount of the virus SARS-CoV-2, which causes the illness we call Covid19.

When the samples are processed, they can also tell researchers which Covid-19 variants are in the community.

If the amount of Covid in the water versus the number of reported cases both rise at a similar pace, that suggests the reported cases are a realistic representation of what’s really happening.

But if the amount of Covid in the water suddenly balloons compared to reported cases, that suggests there is much more virus out there than is being reported.

This is how Watson and his team work out the ‘‘ascertainment rate’’.

And while knowing the exact number of infections matters less today than it did when New Zealand was running an elimination strategy against the virus, it can still inform policy decisions.

The year of Omicron and its variants

This year’s worth of data shows the ascertainment rate peaking in early April (not surprisingly given the first huge peak), decreasing until July and then stabilising, Watson said.

The BA.5 Wave in July may have contributed to that stabilisation, thanks to increased awareness of the virus in the community meaning more testing.

The ascertainment rate began dropping again in August and September after cases plummeted and the government dropped all restrictions.

‘‘The case ascertainment rate decreased in August and September, again likely due to decreased public awareness/ focus on Covid-19 but has plateaued in October,’’ Watson said.

As we’ve outlined, in early November the case ascertainment rate was about 35%. It has crept back up.

So while both July’s and the current wave are resulting in fewer infections than the first Omicron wave in March, the true story is a little murkier.

Eventually, the team will report the ascertainment rate on a weekly basis, in line with the analysis of wastewater data.

Watson said even RATs are not perfect, and their result depends on when you test versus where you are at in your infectious period.

‘‘Even if everyone tested once a week and reported that test, the case ascertainment rate would only be 70-80%,’’ he said.

‘‘This percentage could increase if people test (or re-test) once they have symptoms.’’