COVID-19: Ministry of Health to test sample of Kiwis to understand 'true prevalence' of infections

The Ministry of Health will conduct prevalence testing to understand the "true" scale of the COVID-19 outbreak in New Zealand.

COVID-19 infections have been increasing in recent weeks as the highly transmissible BA.5 Omicron subvariant becomes dominant. The seven-day rolling average on Monday was 9689 cases, up from 9281 last Monday and 7046 two weeks ago.

But Director-General of Health Dr Ashley Bloomfield said on Thursday that it's likely we're only detecting about half the number of cases through testing. Others with COVID-19 may be asymptomatic or have very mild symptoms, he said, meaning they aren't taking a test, while others may simply not be uploading their results. 

Dr Bloomfield said he expected a prevalence survey would be launched soon, noting that they are used widely in other countries to determine "what level of infection there is in the community". 

A spokesperson for the ministry on Tuesday said it would undertake "an infection prevalence survey to identity the number of New Zealanders who have COVID-19 at a specific point in time and across six months". Serological surveys will also be used "to understand the level of immunity against COVID-19 in the population". 

"Infection surveys will include testing a statistically-appropriate sample of the population, in order to deduce the estimated true prevalence of COVID-19 infection irrespective of symptoms present or not. Design work is complete and we expect to have something in place over the coming weeks. 

"The seroprevalence survey involves antibody testing and will be added a few weeks after the Infection Prevalence Survey commences."

Serology testing involves looking for antibodies which signal if someone has had a previous infection. It can also detect if someone has been vaccinated.

Dr Bloomfield said on Thursday test results for border workers as well as wastewater testing showed there had been a "significant increase in case rates across all regions in recent weeks".

The latest ESR wastewater surveillance report for the week ending July 10 shows the COVID-19 virus was detected at all sites with samples analysed, except one in Taranaki.

"There are increases in SARS-CoV-2 levels in wastewater across almost all sites," the report said. 

"Using a threshold of an increase of 30 percent or more, 77 percent of sites have an increase compared with the previous week, 84 percent of sites have increased from levels measured two weeks ago, and 88 percent of sites have increased compared with one month ago. 

"Compared with levels one month ago, a third of sites have levels of SARS-CoV-2 in wastewater which have increased 5-fold or more."

Dr Bloomfield will provide a COVID-19 update at 12pm on Tuesday. Newshub will livestream it.