A high-level scientific review has explored the vexed issue of face masks – finding there's not enough evidence to recommend people wear them to protect against Covid-19.
That's despite a top epidemiologist urging New Zealand to adopt "mass masking" when it drops down to alert level 2.
The review, commissioned by the Ministry of Health's chief science advisor Dr Ian Town, explored a range of approaches that countries have taken.
It found there was evidence of potential benefits - but also potential harm - around masks and that the science wasn't conclusive.
Director general of health Dr Ashley Bloomfield said the review had been referred to the ministry's technical advisory group for guidance on whether there should be any change on the Government's stance.
"We are aware that the issue of the general public wearing masks continues to generate attention," Bloomfield said.
The review noted that there weren't yet any clinical trials on the efficacy of public face coverings to protect against Covid-19, so information had to be pulled from other research - most of it around influenza or wearing masks in clinical settings.
While the World Health Organisation had also concluded there wasn't enough information to make a recommendation, a non-systematic review carried out by the UK Royal Society's Delve Initiative came out in support of their use.
Yet the new review noted the Delve Initiative report had been criticised by other scientists – with the University College London's Dr Antonio Lazzarino dismissing it as "no more than opinion" that over-stated the available evidence.
The US Centres for Disease Control and Prevention recommended face masks – yet also could not find any data to quantify what difference they made.
The review also found that, while Singapore had ordered its citizens to wear masks in public, the daily number of Covid-19 cases was still higher than before the new rule kicked in on April 15.
It cited one Chinese study which reviewed three types of masks – N95 respirator masks, medical masks and home-made ones – which found that wearing one, along with instant hand wiping, could help slow the exponential spread of the virus.
Yet other research had suggested that the physical properties of cloth masks – and notably the effectiveness of cleaning them, and the potential for them to retain moisture – could even increase the risk of infection.
"The majority of studies have not demonstrated benefit in cluster randomised controlled trials evaluating the effect of members of the general public wearing masks in non-healthcare settings to prevent the acquisition of viral respiratory infections," the review found.
"If masks are not used appropriately, and not combined with meticulous hand hygiene, there is a theoretical risk of increased infection risk through self-contamination."
In any case, any potential benefits of wearing masks were likely to be less effective than washing hands or physical distancing.
However, prominent Otago University epidemiologist Professor Michael Baker has advocated "mass masking" in confined spaces as a key public health measure of level 2.
"I don't know about you, but I'd feel much more confident in a bus if everyone else had a mask on - that seems like a very small sacrifice to make," he told the Herald this week.
Baker felt that mass masking could encourage those worried about infection not to keep themselves at home, and wanted to see the Government promote the concept.
"Mass masking is universal in Asia and is being introduced more widely in other countries."
A recent Ipsos poll showed the countries where most people had adopted the practice were Vietnam (91 per cent), China (83 per cent), Italy (81 per cent), Japan (77 per cent) and India (76 per cent).
Meanwhile, those in developed nations are least likely to do this – they included United Kingdom (16 per cent), Germany (20 per cent), Australia (21 per cent), Canada (28 per cent) and France (34 per cent).
Other Otago University scientists have pointed out that the main way the Sars-CoV-2 virus spread was likely through small droplets – often just via talking – and these could sometimes travel much further than the two metres used in distancing guidelines.
They cited one recent Chinese paper that suggested that a non-fitted surgical mask could block up to 100 per cent of droplets containing coronavirus, while another, focused on cotton mask use on infected patients, found viral spread from a cough was cut down by 96 per cent.