Around 50 percent of people who contract COVID-19 might not show any symptoms, worrying new figures out of Iceland suggest.
The small island nation, which has a population similar to Christchurch, has reportedly tested about 5 percent of its entire population - around 18,000 people.
While only 1 percent of them tested positive for SARS-CoV-2 - the virus behind the pandemic - around half of them weren't showing any symptoms.
"Because we are screening the general population, we are catching people early in the infection before they start showing symptoms," Kári Stefánsson of Reykjavik-based biopharmaceutical company deCODE genetics told CNN.
That's a problem, because scientists have in recent weeks concluded the virus can be spread by asymptomatic people, contrary to what was believed early in the pandemic. One study, looking at data from China, suggested around 10 percent of person-to-person transmissions came from people unaware they had the virus, on average about four days after being infected.
Young people are also more likely to be asymptomatic and were the target of Prime Minister Jacinda Ardern's ire at her daily media conference on COVID-19 on Wednesday.
And a new study, published this week in Nature, suggests those with only mild symptoms - which are unpredictable but can resemble far less dangerous common diseases like the flu or common cold - are more contagious than those who fall seriously ill.
Researchers looked at nine COVID-19 patients, seven of whom only showed mild symptoms.
"Peak concentrations were reached before Day 5 and were more than 1000 times higher" than levels of the virus seen in SARS patients back in 2003, the study said. SARS was caused by a similar coronavirus to COVID-19.
Viral loads slipped from day five, to the point where they were probably no longer infectious by day 10. But for the two patients who unfortunately had the virus progress into their lungs and fell much sicker, viral 'shedding' peaked around day 10.
NZ's testing regime 'balanced' - expert
Experts have said testing as much as possible, combined with social distancing and isolation, is the key to stopping the virus. But questions have been raised whether New Zealand is doing enough testing - around 2000 a day, but in a population about 14 times bigger than Iceland's.
Massey University public health researcher David Hayman says while there is a "lot of angst" amongst the public, he's not as concerned as some other experts.
"Ideally... we'd be testing everyone who had any sort of upper respiratory tract type of infection," he told Magic Talk on Thursday.
"I think it's balanced. Yes, we'd like to see more, but right now, given that there's a global shortage of reagents, I think they're trying to take a precautionary approach."
Reagents are chemicals used in the testing process. There are other tests available, but they're not as reliable, Prof Hayman says. Most look for antibodies produced by the immune system in response to infection - meaning the person has been exposed at some point in the past, but not whether they're currently hosting the virus.
"I think the restrictions that have been put in place, along with the tracing and testing, together are painting a picture that New Zealand can potentially manage. Not long ago I was getting increasingly anxious about the approach we were taking. I didn't think it was necessarily quite strong enough, but that changed.
"Part of me thinks right now if there is unseen community transmission, as long as people are behaving appropriately if we miss a few it may not matter because people are not being able to transfer it to each other it will disappear anyway."
Even if New Zealand manages to eliminate local transmission of the virus, as long as it's out-of-control overseas, there's always a risk someone could bring it in if we loosen border restrictions.
Prof Hayman said quarantining new arrivals would sort that out, and calls for mandatory testing were misguided.
"Testing is slightly problematic, because what do you test? You could look at people's temperatures and you could take swabs. One of the problems with that is that the temperature approach, people may be pre-symptomatic... the swab approach, if you test and they're negative, that doesn't mean they're not incubating the infection.
"That's where that quarantining and isolation becomes really important... if they're quarantined, that solves the problem."