Coronavirus: Questions raised over efficiency of NZ's rapid surveillance testing

Last-minute random testing of asymptomatic New Zealanders has ramped up as the Health Ministry tries to improve its COVID-19 data. 

But a professor of infectious diseases says in epidemiological terms, such an approach to sampling is known as "quick and dirty".

A testing centre was rapidly set up in a Christchurch supermarket car park on Friday so that anyone who wanted a swab could get one. 

The aim was to survey the general population and find out if people are carrying the virus unknowingly, according to Pegasus Health chief executive Vince Barry.

"What we're doing here is running surveillance, so this is just looking at a random group of Cantabrians who are coming shopping," Barry told Newshub.

Some took up the opportunity as they left with their groceries. 

"It's the only way to find out how many people may or may not have it. It also gives you comfort in yourself," said one Christchurch resident Eliza Hamilton.

Another, Narlene Smith, said the testing is good in case someone who hasn't shown symptoms has caught COVID-19 unknowingly.

But the question is - how good exactly is this method of surveillance?

According to Professor Philip Hill from Otago University's Global Health Institute - not very. 

"I don't think I would describe this as community surveillance. I think it's what we would call a quick and dirty process from an epidemiological point of view that might give some help to the decision making next week."

On Thursday 343 shoppers and supermarket goers were swabbed in Queenstown. 

Half have been processed - all were negative. 

"What I think you could get from that is that there's no unexpectedly high rate of community transmission at least in the population that's represented by the people that turn up at a supermarket on that day," Prof Hill told Newshub. 

He says this type of testing is not truly representative of the population. 

In Iceland for example, several thousand were tested in the community, but it focused on specific demographics. 

"You can get a sample that would represent the whole country. And you can do specific sampling within target groups you're interested in like Māori or Pacific communities." 

Professor Michael Plank from the University of Canterbury's school of maths and stats agrees, saying the most useful data is targeted and specific. 

"For the modelling, every bit of data we can get helps. I think just testing randomly might not be the best way of doing it, it is likely to have a pretty low hit rate." 

Professor Plank says as we have low numbers of cases, it's better to be focused, like for example targeting essential workers. Not just supermarket staff, but groups like police and health workers.

"The better you can target it to higher risk groups, the better the quality will be." 

So with a rush on random testing there remains questions about just how useful this process will be and how much it should be relied on come the decision to lift out of lockdown on Monday.