Face masks for all - the science

Should all New Zealanders wear face masks in public to reduce the spread of Covid-19? Farah Hancock looks at the science. 

The success of Taiwan, Hong Kong and Singapore in keeping Covid-19 levels low has seen a call for widespread mask-wearing.

In the United States, the message from officials has switched from asking people not to wear them, to suggesting people make their own. The CDC has even published a web page with DIY instructions of how to sew your own, chop up a t-shirt to make one, or fashion one from a bandana and hair ties. 

The science behind public face mask use isn’t conclusive and in New Zealand, while community transmission is low, the call hasn’t been made for the public to rev up sewing machines, or adopt a hacked t-shirt, bare midriff, covered-face appearance.

DIY t-shirt mask instructions. Image: CDC website

How is Covid-19 spread?
To stop a virus you need to stop it spreading to another person. Understanding how it spreads helps us understand what measures we need to take to stop it.

The World Health Organisation (WHO) stance is the virus is predominantly spread by respiratory droplets, not airborne transmission.

Respiratory droplets are the bigger bits of mucous or substances an infected person may spread through coughing or sneezing. To get sick from respiratory droplets you either need to be close to them, or transfer them - perhaps from a surface they’ve landed on - to your face.

Airborne transmission involves smaller droplets which can travel further and linger in the air for longer. Measles is one example of a virus which is airborne. 

The WHO says there are some instances where Covid-19 can be airborne. These are normally during medical procedures that generate aerosols. 

One recent study found the virus on air outlet fans from a room housing a patient in Singapore. The study pointed out the viability of the virus found wasn’t tested.

What’s not known yet is how these airborne virus particles could lead to infections and how different factors such as concentration, distance and environment would affect the infection risk.

What’s the purpose of a mask?

It may seem like a silly question, but masks have different roles depending on who is wearing them. 

If you think of masks like a front door to a house, they can either stop things entering, or stop things exiting.

When you’re sick, a mask helps you stop infecting others by blocking respiratory droplets. If you’re not sick, it can help stop you getting sick - if you are wearing it correctly. 

It’s important to remember there are other ways in and out of a house. A mask alone won’t protect. Hands still need to be washed and eyes remain vulnerable.

Does the widespread use of face masks help in an epidemic?

University of Auckland associate professor Siouxsie Wiles said studies on influenza show mask-wearing doesn’t have much effect, but studies done during the SARS epidemic showed it could help. 

“What is most likely is that wearing a surgical mask won’t do much to prevent someone contracting Covid-19 but may help reduce the amount of virus-laden droplets being spread by someone with the virus. Clearly more good quality research is needed in this area.”

One 2008 study published in BMJ reviewed papers on respiratory viruses. Papers on SARS showed masks were 68 percent effective as an intervention. Add gloves, a gown and frequent handwashing to the mix and you increase effectiveness to 91 percent.

Another study looking at influenza and the use of face masks in Thailand found they did nothing to stop the spread within households. 

A recent paper published in Nature Medicine looks at the other side. How well do masks prevent a sick person exhaling and coughing out virus? Droplets containing the virus were detected in 30 percent of patients not wearing a mask and aerosol particles in 40 percent. For patients wearing a mask, no droplets or aerosol particles were detected.

This in particular is what's seen as a reason for widespread mask wearing. People who are ill, but might not yet realise, may be less likely to spread virus particles if they're wearing a mask correctly.

Do different masks do different things?

N95 respirator masks filter out around 95 percent of airborne particles as well as stop respiratory droplets. These are fitted tightly on the face

Surgical masks - the ones with elastic that hooks behind your ears - stop some of the larger droplets. They were initially created to protect surgeons during surgery, but are now widely used by healthcare workers. 

These can vary in quality and don’t fit as snugly on the face as an N95 mask, and should be discarded after use.

Homemade cloth masks will vary in effectiveness and won’t filter out airborne particles but should stop some respiratory droplets. They need washing regularly.

What about New Zealand, should we mask up?

The idea of adopting a widespread use of face masks was raised during Saturday’s press conference. Director of Public Health Caroline McElnay said officials were looking to WHO advice on the issue due in the next few days. 

At present she said the idea of masks for the public was at the bottom of the list for strategies to control the spread of the virus. 

Director-General of Health Ashley Bloomfield pointed out the United States has a different issue to New Zealand. 

"They have clearly widespread infections and widespread community transmission." 

New Zealand’s community transmission at present is 1 percent - or around 10 people. 

More cases are under investigation and may later be classified as community transmission. With relaxed testing criteria, numbers emerging in the coming days might indicate if community transmission is widespread. 

What are the risks to wearing a mask?

University of Auckland’s Siouxsie Wiles cautions against complacency.

“I’ve been reluctant to recommend everyone in New Zealand wear a mask because I’ve worried that people will feel a false sense of security and then not do all the things that will help protect them.”

There’s also the question of supply.

“A bit like hand sanitiser, I’ve also worried that if everyone buys masks then those who really need them won’t be able to access them any more.”

It’s also important they are worn correctly, a fact pointed out by both Wiles and University of Otago senior lecturer Dr Ayesha Verrall.

“They are totally ineffective if not worn correctly. So if this is part of the public health response, there needs to be education on how to use them,” said Verrall.

Verrall also worries about supply. There are 41 million masks on order, but these are disposable and once used are like a “dirty filter”.

“We will face problems with supply of personal protective equipment for several years and will need to use our stock in a considered way, and high-risk essential workers should be prioritised. At the moment essential workers, like me, are the biggest vulnerability in our lockdown system because we have to leave our bubbles every day."

Verrall's take on the issue is there is little benefit in wearing a mask while community transmission is low. If that changes, the topic should be revisited.

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