Explainer: masks or face-shields... which is better?

In the early days, the scientific view was that masks did little to protect the wearer though they might prevent mask-wearing infected people from spreading the virus.

Now, people in Melbourne have been asked to wear masks.

So what do the experts say?

Scientists now think masks can help in crowded places where people are infected.

A number of studies have shown they are effective - not 100 per cent effective but a help.

The spread of COVID-19 slowed in parts of the United States when masks were introduced, according to research publicised by the University of California in San Francisco.

"Another study looked at coronavirus deaths across 198 countries and found that those with cultural norms or government policies favoring mask-wearing had lower death rates."

And two cases may clinch it: "In one case, a man flew from China to Toronto and subsequently tested positive for COVID-19. He had a dry cough and wore a mask on the flight, and all 25 people closest to him on the flight tested negative for COVID-19.

"In another case, in late May, two hair stylists in Missouri had close contact with 140 clients while sick with COVID-19. Everyone wore a mask and none of the clients tested positive."

Every authority says: keep your distance and wash your hands frequently.

And cover your mouth and nose when coughing and sneezing (just like your mother told you to).

The federal government puts it less clearly than your parents did. It talks of "cough etiquette and respiratory hygiene" but it's the same thing - "coughs and sneezes spread diseases" as the Spanish Flu slogan put it a century ago.

On masks, in places apart from the hot-spots of uncontrolled infection (Melbourne), the federal government's advice remains that the routine use of face masks is not recommended:

According to research done in Canada, cloth can be effective in preventing small droplets which carry the virus getting through to your nose and mouth - but the mask has to be constantly changed and washed.

"The filtration of cloth is quite variable and single layers of scarf, sweatshirt and t-shirt may be in the 10 to 40 per cent range. But multiple layers increase efficiency, and modern studies have confirmed that some combinations of cloth, for example, cotton-flannel, block more than 90 per cent of particles," Dr. Catherine Clase, professor of medicine at McMaster University, said.

Professor Peter Collignon, an infectious diseases doctor at the the Canberra Hospital, is a fan of face-shields in areas where there is a substantial outbreak.

"The advantage is that they last," the professor at the Australian National University said. They also protect the full face, including the eyes.

"It's bi-directional protection" by which he means that they protect the wearer from infection by others and others from infection by the wearer.

They are also much easier to clean - masks need replacing every few hours. Shields can be cleaned easily with detergent.

And they are easier to wear than masks. They are cheap and available at hardware stores.

A group of 239 scientists recently sent a letter to the World Health Organisation suggesting that it's not just small droplets we need to worry about but even smaller particles.

There may be tiny infected particles - microdroplets - called "aerosols" which float in the air and which can linger there for someone to breathe in much later.

The WHO responded that there was evidence of these droplets but more research had to be done.

"The possibility of airborne transmission in public settings, especially in very specific conditions - crowded, closed, poorly ventilated settings - cannot be ruled out. However, the evidence needs to be gathered and interpreted," Dr Benedetta Allegranzi, the WHO's technical head for infection prevention and control, said.

If these microdroplets do exist, the consequence would be that even more care would have to be taken. Social-distancing would not be enough to ensure you didn't catch the virus where it was prevalent because the virus would remain in the air and on surfaces after the infected person had left the area.

And the consequences for health care workers would be large. It would mean that full protective gear would have to be worn in hospitals because of the risk of these floating, lingering, infectious droplets.

It is but that will probably be a long wait, perhaps an indefinite one.

Professor Collignon reckons there's a 60 to 70 per cent chance that one will be developed but it's not a hundred per cent certain.

And he thinks it might take 18 months to two years before one is developed and in production. The task would then be to inoculate pretty well everyone on the planet - not a small task.

In the meantime, keep your distance, wash your hands frequently, wear a mask in infected regions. Get used to it.