Covid 19 coronavirus: Studies show where you are more likely to be infected

Now people are reconnecting with family and friends, there's a new risk people should be aware of when having people over.

During a Facebook Live this month, one news.com.au reader asked a question of Australia's Prime Minister Scott Morrison that highlights an important feature of how the coronavirus is spread.

What Kerry Porter wanted to know was: "Why do you think workers in Coles, Woolies and Bunnings [who] have been in contact with everyone ... haven't seen that many cases?"

Despite employing tens of thousands of people across thousands of stores, which have continued operating during the coronavirus shutdown in Australia, there have been very few cases among workers in stores like Coles, Woolworths, Bunnings and Kmart.

Morrison didn't have a solid explanation for why this is, saying he hadn't seen the specific statistics on that.

"What I would say is that employers in places like this have put in place social distancing and procedures to keep their patrons safe," Morrison said.

"And that's where I've seen businesses do amazing things to adapt to this new environment and we'll see them adapt more so into the future."

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

But research may show why so few retail workers have been infected by Covid-19 despite working through the pandemic.

Physician and scientist at Scotland's University of St Andrews' School of Medicine, Muge Cevik, has examined a number of contact tracing studies and found rates of infection were higher among those in enclosed environments.

The main danger appears related to spending a long amount of time with people in a cramped indoor space. In fact, many infections occur in people's homes.

One study Dr Cevik quoted, looked at 2147 close contacts of 157 coronavirus cases in Ningbo, south of Shanghai.

It found there was a higher risk of infection among friends, family and relatives, as well as those living with the infected person or taking the same transport or dining together.

Another China study of 1286 close contacts of 391 virus cases found "household contacts and those travelling with a case were at higher risk of infection".

In a Twitter thread, Cevik said the studies indicated that close and prolonged contact was necessary for coronavirus to be spread.

"The risk is highest in enclosed environments; household, long-term care facilities and public transport," she wrote.

"High infection rates seen in household, friend and family gatherings, transport suggest that closed contacts in congregation is likely the key driver of productive transmission.

"Casual, short interactions are not the main driver of the epidemic, though keep social distancing."

Cevik said her conclusions were based on data available as of May 4 and this could change once more information became available.

She also added there could also be an increased risk in crowded office spaces, packed restaurants and cafes, cramped apartment buildings and other crowded and connected indoor environments, although this was based on limited data.

Canberra Hospital infectious disease physician Professor Peter Collignon said among patients researchers had followed up, evidence indicated the longer they were inside with someone, the bigger the risk of infection.

"I think it makes a lot of difference," he told news.com.au.

He said research found that those living in the same household were up to 20 per cent more likely to be infected than someone who had only been in contact with the person for an hour or so.

This increased risk could have implications for Aussies opening up their homes to family and friends now state and territory authorities have eased restrictions.

'This may be an issue for people in their own home, where they are having longer contact with others – they may be there for three to four hours," Prof Collignon said.

Prof Collignon previously told news.com.au that social distancing shouldn't go out the door now authorities were allowing people to have family and friends over.

Prof Collignon said adults should still stay 1.5m apart, even in their own homes.

"When you sit at a table, try and space people 1.5m apart," he said.

"Don't all crowd in the kitchen while someone's preparing food.

"Or ask people to come over for lunch and to bring their own sandwich."

Outdoors is safer than indoors when it comes to stopping clusters of Covid-19 forming.

Prof Collignon suggests having people over for brunch or lunch so you can sit outside, in the sunshine if possible.

"Sit out in the sunshine as UV light does seem to be better at killing the virus."

While places like supermarkets are also considered to be enclosed spaces, Prof Collignon believes the reason why shops like Coles have not seen large outbreaks is because of strong hygiene measures including perspex screens, good crowd control, hand sanitation stations and other measures to keep customers from getting too close to staff.

He said the hygiene measures had probably reduced the risk of all respiratory disease, not just Covid-19, by 90 per cent compared to previous years.

"Customers are also in the stores for a short period of time," he said.

While customers don't seem to be getting the disease from their visits to supermarkets, Prof Collignon said the cases that had appeared linked to places like Coles were spreading among staff.

"The risk seems to be staff members spreading to other staff members," he said.

This again points to time spent together among staff being a factor.

A lot of the outbreaks around the world and even in Australia had been linked to places where people were in an enclosed space for long periods, with less than optimal ventilation. Alcohol may also have played a part.

Prof Collignon pointed to foreign workers in Singapore who had been living in dormitories, bars and nightclubs in Korea and the abattoir in Melbourne.

"All of those things reinforce that if you are close together, without adequate fact protection, you are more at risk," he said.

The good news is that almost every cluster of infection in Australia seemed to be linked to a person with symptoms, Prof Collignon said.

"We have had very little community transmission with only around 10 per cent of cases where we can't find a cause. The other cases can be tracked to cruise ships or direct contact with those with the virus."

Prof Collignon said there were so few cases in Australia it was difficult to do research that could help researchers understand exactly what was going on, so assumptions were based on overseas data.

"But I'm fairly confident in saying that outdoors is safer than indoors, and if you keep 1.5 to 2 metres away, the risk is substantially decreased," he said.

Prof Collignon said the number of coronavirus cases was so low in Australia that this also helped keep community transmission under control.

"The risk is not zero but it's very, very low," he said.

"It will only stay low if we keep basic measures going, like physical distancing, washing hands, decontaminating surfaces that are used frequently and not going to work if you are sick."