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As experts try to get a handle on how the coronavirus outbreak might play out, they are making comparisons to past outbreaks like SARS, MERS and swine flu.
Some credible experts are even likening the COVID-19 outbreak to the 1918 Spanish flu pandemic.The WA head of the Australian Medical Association, Andrew Miller, recently said: "The truth is, we probably haven't seen a virus like this one since 1918, with the Spanish flu."
And the Government's health sector emergency response plan for coronavirus outlines several scenarios, the worst of which says, "the level of impact may be similar to that of the 1918 H1N1 'Spanish flu'".
It's a grim comparison…
Doctors commonly refer to the Spanish flu outbreak as "the greatest medical holocaust in history".At the end of World War I, about 500 million people were infected with the H1N1 Spanish flu.
"It left its mark on world history," says University of Melbourne professor James McCaw, a disease expert who mathematically modelled the biology and transmission of the disease.
"It killed 40 to 100 million people across the world in a very short period of time."There's no expert consensus on where it first broke out — it became known as the Spanish flu, but only because wartime censorship in other countries meant Spain was the first place it received widespread press attention.
Many believe it probably started in army training camps on the Western Front. Soldiers returning from the war brought it back to their home countries.
Others, though, think it broke out in the US state of Kansas and was spread around the world by American soldiers.
In Australia, which had a population of 5 million, about 15,000 people died.
Are Spanish flu and coronavirus similar?
Spanish influenza and COVID-19 are both infectious respiratory illnesses, and they share some symptoms.Both can cause fever, coughing and aches, and both can lead to pneumonia. Both vary from mild to fatal.
Biologically, the two diseases are very different — but the comparison persists because "novel" — or new — viruses are rare.
Novel viruses can be particularly dangerous because immunity in human populations is low, and it takes time to understand new viruses and develop vaccines and treatments.
"Whenever we see a pandemic, we are seeing something similar to other pandemics so that's a fair enough comparison to make," says Ian Mackay, a virologist from the University of Queensland.
"But this is a very different virus to a pandemic influenza. It is a totally different virus and it spreads differently; it causes a different disease; it seems to target different groups."
The world is responding with similar social controls
A century ago, medical knowledge was obviously well behind where it is today."People had never even heard of influenza as a virus," says Kirsty Short, an influenza virologist at the University of Queensland who has studied the pandemic.
"So how on Earth could you deal with that and contain it when you didn't even know of the pathogen?"
But governments knew enough to introduce travel bans, quarantine rules and social distancing measures — similar to those today's governments have brought in around the world.
"Cinemas and festivals were closed down," Professor McCaw says.
"People were asked to not congregate in shops and places like that.
"So there were widespread efforts to limit contact among people — and they were highly successful in reducing transmission."
Those controls could prove even more successful this time
Professor McCaw says we can take heart from what's happened in China — where the Hubei province has been locked down, school and businesses closed, factories ground to a halt and people encouraged to stay at home.
"What's happened in China gives very clear evidence that we can get what's called the 'reproduction number' under one," Professor McCaw says.
"So at the moment in China, on average, each person infected with coronavirus is passing that infection on to fewer than one other person.
"If people hadn't changed their behaviour, we would have expected somewhere around the millions of cases in China by now instead of the comparatively small number of around 100,000."
So, he says, it looks like the transmissibility of coronavirus can be significantly modified through social distancing and good hygiene.
"We could do that for flu in 1918 but not to the same extent as this one."Largely, he says, that is because people with influenza are generally infectious for a day or two before they are aware they are sick — whereas people with coronavirus do not appear to be as infectious before they develop symptoms.
"And this is really good news — it means that as long as someone who has developed symptoms is vigilant [and] reduces their contact with other people, they won't infect as many other people as they may well have otherwise," Professor McCaw says.
Spanish flu and coronavirus appear to target different age groups
Dr Short says one of the unusual features of the 1918 pandemic was it particularly hit those "in the middle ages of life — so around 35-40 or so"."Normally when you see a flu outbreak, it typically affects the very young and the very old," she says.
"But in this particular outbreak of [Spanish] flu, the elderly weren't nearly as susceptible and that's probably because they had some sort of cross-protective immunity from a pre-existing infection."
In contrast, coronavirus causes a mild illness for most people, but the elderly are much more susceptible to being severely affected.
The clinical outcomes of the two diseases are also very different for children, adds Professor McCaw.
"For influenza, children are a hub for infection and transmission, and all parents are aware of the infectious potential of their young children," he says.
"Strangely, and surprisingly perhaps, for coronavirus, children seem to be largely unaffected and those who are infected have less severe illness."
Many who died in 1918 were not killed by the Spanish flu
A lot of the flu's victims actually died from secondary bacterial infections, Dr Short says."So it wasn't necessarily the virus itself that was killing people, but it was making them more susceptible to other infections that were ultimately fatal," she says.
"We don't seem to see the same thing happening with this novel coronavirus."
In 1918, doctors did not know how to best treat bacterial infections.
"This was in the pre-antibiotic era," Dr Short says.
"So there weren't really many treatment options available for people who had those secondary bacterial infections."
So, could we be in for a similar pandemic?
Dr Short says the 1918 pandemic was a real worst-case scenario."We're not going to see that sort of level of mortality, that mortality was driven by the social context of the outbreak," she says.
"We had a viral outbreak, at the same time as the end of a world war, hitting at a time when people didn't really have any knowledge of virology or infectious diseases."
Your questions on coronavirus answered:
Modern medicine means much better care is available now than it was then.
"We've already got a lot of scientists working on novel therapies and novel vaccines to try to protect the general population," Dr Short says.
"In 1918, there were some homemade remedies like opium, but they weren't overly effective, as you can imagine. So it is a very, very different scenario."
But both Dr Short and Professor McCaw stress it's early days, and predictions remain difficult to make.
"Exactly how the coronavirus spreads through the Australian population and what the consequences will be, it's too early to predict that," Professor McCaw says.
"So how eventually it compares to the 1918-19 flu, in terms of overall infection numbers and fatalities is, at the moment, impossible to predict."
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