New blood test to check for undetected Covid cases

The number of confirmed Covid-19 cases across New Zealand could rise - thanks to a new blood test.

• This story was first published by RNZ

Scientists expect the test to reveal some cases of Covid-19 that were missed because of initial rules around swab testing, and potential community transmission.

The study will target healthcare workers, close contacts of confirmed cases or those considered at "higher-risk" of contracting the virus across hotspots in the Southern Region.

The research by Southern Community Labs alongside Southern District Health Board, and Auckland and Otago universities uses a new test to see if a person was infected with Covid-19 but was missed during swab testing.

The test - named the Abbott antibody test - can check for past exposure of Covid-19 by indicating if a person has antibodies to the virus from previous infection.

While the test is not useful for diagnosis of active cases, it shows whether someone has been exposed to the virus previously.

The research is the first of its kind in New Zealand and could provide vital information about community spread and how to manage if a second wave hits.

A clinical microbiologist leading the study, Dr Arlo Upton, said the Southern DHB region was specifically chosen because of its high infection rate per population.

She said they were targeting people who had already been tested rather than randomly sampling the region's population.

"We're going to look at the people who were known cases and the probable cases and their household contacts and also their broader contacts in the community, as well as high risk people defined by being healthcare workers, as well as tourism workers around Central Otago, and offer those people to enroll in the study to have antibody testing for Covid-19."

Dr Upton said the research could give a more accurate picture of the spread of the virus in the region.

She expected there would be additional cases found in people who were not able to get tested under earlier case definitions - the threshold at which people would have been tested.

"I'm expecting to find that we will see, possibly, some limited seropositivity suggesting limited spread in our community, but I'm not expecting to see anything as high as what some of the seroprevalence studies have found in other parts of the world."

Clinical microbiologist and associate professor at the University of Otago James Ussher agreed there was likely more cases to be found.

"I think we will see some additional spread, but I think that will largely be restricted to close contacts and among some of the probable cases I think will be confirmed by this. Although, we can't technically confirm them from a public health perspective."

However, Dr Ussher said he did not expect to see an increase in cases from healthworkers.

The goal of the study is to validate the Abbott test, to show it works how it should, to get a clearer picture on the virus spread, and to better understand how the body's immune system responds to the virus.

Dr Upton and Dr Ussher said the test was not designed to show someone's immunity to Covid-19.

The use of serology testing is increasing internationally to understand the true infection rate over the pandemic.

Auckland University's senior lecturer in immunology Nikki Moreland said in a country like New Zealand, where the virus was not widespread, there were concerns about false positives which could cause problems.

To combat this, she would use a different test she had developed with colleagues to double check every positive result found by the Abbott test.

That test looks for different antibodies connected to the virus than the Abbott one - allowing a second check of the initial results for accuracy.

"It is particularly important in New Zealand, because of our low prevalence at the moment and there's always a chance of false positives, but by double checking all the positive results on another test, we can minimise the risk of false positives," Moreland said.

Dr Upton said the results of this study could either undermine or give confidence to New Zealand's eradication strategy.

"Because obviously the concern is that there may be people with very minimally symptomatic infection in the community who have never been diagnosed," she said.

With people like essential workers continuing to have some contact with others during lockdown, Dr Upton said "there's a concern that there may be ongoing low symptomatic spreads."

The study is due to start over the next few weeks, with results expected in a couple of months.