Marc Daalder
Marc Daalder is a senior political reporter based in Wellington who covers Covid-19, climate change, energy, primary industries, technology and the far right. Twitter: @marcdaalder.
COVID-19
We have no idea how much Covid-19 there is, but everyone agrees that the official case numbers are huge undercounts, Marc Daalder reports
The Ministry of Health will launch an investigation into random Covid-19 testing in the coming weeks to determine the true prevalence of the virus in the community.
A ministry spokesperson confirmed to Newsroom that the study will examine prevalence in the community at any time and will also include blood tests to determine the population’s immunity to Covid-19. Seroprevalence tests will be able to distinguish between antibodies from Covid-19 infection, vaccination or a combination of both.
Infection surveys involve testing a statistically appropriate sample of the population to derive the estimated true prevalence of Covid-19 infection, regardless of symptoms present or not. The design work is complete and we expect it to be completed in the coming weeks,” the spokesperson said.
“The seroprevalence survey includes antibody testing and will be added a few weeks after the start of the infection prevalence survey.”
A prevalence survey has long been requested by independent experts who say the daily number of cases is a massive undercount. Even Director-General of Health Ashley Bloomfield said on Thursday we detect only half of the cases in the community. He said prevalence surveys are heavily used abroad when community transmission is so widespread.
In the UK, the Office for National Statistics has conducted a weekly prevalence survey since the original lockdown in 2020. As testing is declining and fewer people are reporting positive results, the ONS figures are widely regarded as the most accurate estimate of the spread of the sickness. virus.
Estimating Prevalence
Without such research in New Zealand, we are left to coarser methods of estimating prevalence.
Comparing current hospitalization rates across age groups to a period when testing was higher – the onset of the first Omicron outbreak – indicates that there have been four times more cases in the last week of June than the 40,000 reported by the ministry. of Public Health have been reported.
Changes in the methodologies for reporting hospitalizations make it difficult to provide a more recent estimate of prevalence through this method.
There is also the WellKiwis survey, which randomly tests a cohort of adults and children in the Wellington area for flu, Covid-19 and other viral illnesses. In the week ending July 3, more than 2.5 percent of the cohort tested positive for Covid-19. That’s more than double what the public case numbers would suggest.
Other available data allow us to compare the prevalence with earlier stages of the Omicron outbreak.
Data from the Institute of Environmental Science and Research (ESR) also shows that calls to Healthline about flu-like illnesses have risen nationwide since late May. In the week ending July 8, as many people called Healthline about flu-like symptoms as in the second week of March, near the height of our first Omicron wave. While some of this may be related to non-Covid-19 illness circulating in the community, it’s still nearly double the historical level for this time of year.
ESR also publishes a weekly report on wastewater testing for Covid-19. The latest report, covering the week ending July 10, shows that significantly more Covid-19 viral copies were found in wastewater from the Auckland metro region and the South Island than at the height of the first Omicron wave. Northland’s wastewater prevalence was about the same as the previous peak, with the rest of the North Island yet to reach those levels.
Regional Wastewater Samples Point to Covid Rise (scale log10)
Knowing the prevalence of the virus is key to taking steps to protect yourself. Without accurate numbers, experts say, it’s difficult to make a good judgment as to whether you should attend a particular high-risk event, visit an elderly loved one, or move a gathering outside to reduce the risk of spreading the virus.
Once the prevalence of Covid-19 is more widely available, it will even be possible to calculate the probability of encountering someone with the virus – if you make certain assumptions. Mathematical biologist Kit Yates provided the formula behind the following chart, which assumes that half of all cases isolate and spread the virus evenly across the population.
BA.5 fuels rise in cases
The increase in the number of cases is undoubtedly caused by the BA.5 subvariant, which accounted for more than half of yesterday’s cases.
It will take a few more days to fully confirm that, but the latest ESR-released sequence data for variants shows BA.5 was responsible for 46 percent of cases on July 8. That was 13 percentage points more than a week earlier, so even two-thirds of our cases could be the hypertransmissible subvariant at this stage.
What we can say with certainty is that BA.5 is now the dominant variant, with the BA.2 subvariant in second place with 41 percent of the genomes.
The recent rise in the number of cases will have been fueled by BA.5 and the slightly slower rise in the comparable BA.4 subvariant. If the ESR sequences are overlaid on the total New Zealand community curve, we can see that BA.2, which drove our original Omicron wave and was the dominant variant until early July, has roughly settled around a baseline of 3000 daily cases.