It's one of the great mysteries of Australia's Covid-19 experiment: despite fears supermarkets and hairdressers could prove high risk for the spread of the virus, there have been no major outbreaks detected.

When pubs, clubs, and restaurants were shut down seven weeks ago, the fact that hairdressers were allowed to stay open seemed to go against the grain of the prevailing wisdom.

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Social distancing was impossible and hairdressers who remained open were often wearing little in the way of personal protective equipment and gloves.

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When the Prime Minister first allowed hairdressing salons to continue trading, he stipulated a 30-minute rule, which was quickly lampooned and extended.

Some hairdressing salons begged to be able to close, fearing the risk to their staff and clients was too great.

But in the weeks that followed, many hairdressers have remained open and there has been seemingly little in the way of outbreaks. In Denmark, hairdressing salons were recently reopened with little sign it is sparking a new increase in cases.

Covid19.govt.nz: The Government's official Covid-19 advisory website

Supermarkets have also stayed open without signs they are spreading the virus, despite the fact many workers are not wearing masks or gloves.

Why is this so?

Australia's deputy chief medical officer Paul Kelly admits it's been a learning curve. But the early evidence suggests it's close, prolonged contact that is the key to the virus' spread.

Hairdressers were initially told to restrict appointments for 30 minutes, but that rule was quickly dropped. Photo / news.com.au
Hairdressers were initially told to restrict appointments for 30 minutes, but that rule was quickly dropped. Photo / news.com.au

That's most likely to occur in households and aged-care facilities, which is where the greatest spread of the virus is occurring.

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It's also likely to occur at weddings and parties where large gatherings occur for extended periods.

"We learn things every day,'' Professor Kelly said.

"We know that this is an infectious disease, that is quite clear. It is a virus, it does spread from person to person and we know it is quite infectious.

"We also know that this infection is more likely to spread in households. So it is between people living in the same house, for example, family members.

"And again from adult to adult rather than from child to adult. In terms of households, it is due to the close and prolonged proximity with someone that is infectious with others that are susceptible to the virus. So that household infectiousness is the most prominent one around the world."

Australia will consider lowering their restrictions next Friday. PM Scott Morrison stressed the importance of the COVIDsafe data gathering app to their plans. Video / AP

That raises natural fears about reopening schools given large groups of children will be gathering in campuses across Australia.

But Professor Kelly is adamant the medical advice on this risk is clear.

"It is absolutely clear now that it is less infectious in children. Children are less likely to spread the virus between each other, or even from children to adults," he said.

"They are also less likely to be infected and they are less likely to have a severe infection and very, very rarely does that end up in intensive care and the more severe end of the spectrum. So children, in general, are not as much of an issue in terms of this virus."

Doctors are also learning more about when you are most likely to pass on the virus to friends, relatives and work colleagues.

"Most of that infectiousness happens in the first five days of people being sick,'' he said.

"So for — mostly the infection is when someone is quite obviously sick, they know they are sick and they are transmitting it from person to person. Much less so before they get sick. So this is now quite clear," he said.

Staying outdoors is also a lot safer than staying indoors, raising some questions about the original prohibitions on reading a book in Sydney's parks.

"There has been one well-documented case of a stadium, a Champions League soccer match in northern Italy when Italy was right in the middle of their epidemic where there was an outbreak that came in that setting," he said.

"But other than that, very little in relation to outdoors — people outdoors infecting each other.

"So these are now strengthening our resolve in terms of our general principles as to how we are going to look at reopening society, reopening the economy in a Covid-safe way."

But infectious diseases expert Professor Peter Collignon cautioned we should not regard hairdressing salons, schools, or supermarkets as magic spaces where transmission should not occur.

Supermarkets, for example, were linked to the spread of Covid-19 in Italy and the United States.

The most likely reason it has not occurred here is simply due to the low rates of community transmission in Australia. That is, cases where the infected person has no known contact with another Covid-19 sufferer.

If a second or third outbreak were to occur in Australia, there's no reason to believe infections could not occur in supermarkets and hairdressing salons.

"Anybody can get it from anybody. But it's proportionate with the amount of time you spend with someone,'' Professor Collignon said.

Deputy Prime Minister Winston Peters spoke on Tuesday about a possible bubble deal with Australia and getting the economy going again. Video / Mark Mitchell

"Hairdressers, we've had them open and they are obviously closer than 1.5 metres. But it's also a function of the fact we have very low rates of community transmissions.

"If you take out the cruise ships, the people returning from overseas and their close contacts, there's less than 10 per cent of cases in Australia that are community transmissions.

"If we were like New York, I would be closing the hairdressing salons down."