COVID-19: WHEN CAN WE STOP WEARING MASKS?

Professor Trish Greenhalgh of Oxford University has published a thread about why it is important to continue wearing masks.  Here are the most salient points.

Trish asks 6 questions:

• Do masks work?
• Why do some people claim they don’t work?
• Do they cause harm?
• What kinds of masks should we wear?
• How does masking need to change now we know that Covid is airborne?
• When can we stop wearing them?
  

Let’s focus on what Trish says about masks for the general public, starting with what problem are masks aiming to solve?

HOW DOES THE VIRUS SPREAD?

Covid-19 is caused by a virus called SARS-CoV-2.

There are two issues:

• It is estimated 40-50% of all people who catch Covid-19 get it from someone who has no symptoms at the time
• There is now a huge body of evidence that the SARS-CoV-2 virus is airborne.  That is that it spreads via ‘aerosols’. Tiny droplets of water bearing even tinier viruses, in our normal breath.  

So not just coughs and sneezes.  Indeed people with such symptoms stay at home.  Transmission is simply by anyone infected breathing and talking, and especially laughing and singing such that people nearby breath in the virus-laden aerosols.


HOW CAN MASKS HELP REDUCE TRANSMISSION?

There are two ways masks and other face coverings can in theory help reduce transmission:

• Catching droplets and aerosols breathed out by people not realising they are infected and infectious.  Termed ‘Source Control’
• Reducing the amount of virus breathed in by a mask wearer

But do masks and other face coverings work in one or both ways?

The materials used for masks and face coverings have tiny holes to let air in and out, and let you breathe.  The advice is that there should be at least three layers, as commonly found in the typically light blue ‘medical masks’.  That increases the chance of catching aerosols and larger droplets, both breathed out and breathed in.

Mathematically, only a small reduction in transmission makes a massive difference in the transmission over time.  Currently UK Covid-19 rates are doubling every 9 days. If they increased by 1.9 every 9 days, only 5% down:

• After 180 days (six months) daily cases would be down by 60%.  
• After three months down by 33%.

If face coverings are more effective, the decrease in case rates is more dramatic.  The fewer people are infected the better.  It reduces strain on the NHS, reduces incidence of Long Covid, and of course reduces deaths.

Countries that introduced mandated masking within 30 days of the first case, mostly Asian, had dramatically fewer Covid-19 deaths than those that delayed beyond 100 days, mostly Western, according to this study: 

SO WHY SHOULDN’T WE USE MASKS?

There is no evidence from Asia that anyone has come to harm from wearing masks.

For drugs and vaccines, it’s important to carry out RCTs (Random Controlled Trials) to assess effectiveness and safety.  With masks, this is very difficult to do, and in short periods can lead to misleading results.  

Better to use the precautionary principle, let’s all wear masks, just in case.  Provided there are no serious problems.

Some people have been concerned about two key issues:

• Self-infection from touching one’s own mask.  But then if you are infected, it’s unlikely to make much difference.  If anything people touch their face less, and therefore are less likely to spread the virus by touch
• If you wear a mask, you’ll feel protected and take more risks.  ‘Risk compensation’.  But as yet there is no evidence of this.

So without serious problems, it makes sense to use masks and other face coverings.


WHAT KINDS OF MASKS SHOULD WE WEAR?

Masks are most effective in crowded, indoor settings where ventilation is poor.  
There are three factors as to how effective a mask or face covering is:

• How well it filters
• How well it fits
• Whether you actually wear it, which in turn depends on how comfortable it is

As noted above, medical masks have three layers, and that should be the minimum.  Then making sure it is tight, by bending the metal strip over the nose and knotting the ear loops to make them tighter.

Double-masking, with another cloth covering on top, helps too.  

Provided you can breathe easily and are comfortable.  This is especially important for public facing people, such as shop staff and hairdressers, who will need to wear a mask for long periods.

Note that face visors, which have large gaps around them, are intended to stop large droplets such as someone coughing, and are ineffective for the aerosol issue in general life.

WHEN CAN WE STOP WEARING THEM?

As we’ve seen above, masks and face coverings work in two ways:

• ‘Source control’, reducing the amount of viruses from an infected person
• Protecting the wearer

Masks are most effective if everyone wears one, not knowing who is infected and infectious.  That’s primarily in indoor settings, especially where crowded.  Including public transport, shops, theatres, workplaces and schools.

Conversely, one infected person without a mask can create a super-spreader event.  Continuing regulation is needed, not leaving wearing to individual discretion.

People who are Clinically Extremely Vulnerable cannot take the risk of being near someone infectious without a mask, and so are being confined to home.  Those at medium risk will also not be inclined to mix with strangers as much as they can now.

As Trish says “When, then, will it be safe for the public to stop wearing masks in indoor spaces? The answer is when there is no longer uncontrolled spread in the community.”