Could you get COVID-19 and not even know it?

The simple answer is yes — but you're more likely to have some symptoms, even if they're mild.

While the World Health Organisation says asymptomatic cases of COVID-19 are "rare", emerging research is challenging that statement.

Of course, having no symptoms is a better experience of COVID-19. But asymptomatic people could be spreading the virus in the community without realising they're infected. And while they may not show symptoms, the people they transmit it to could have a response anywhere from mild to fatal.

It's for this reason that the US Centers for Disease Control and Prevention (CDC) has advised all Americans to wear masks in public.

But first — just how common is symptom-free COVID-19?

Almost one in five cruise ship cases are asymptomatic

When a passenger who disembarked from the Diamond Princess cruise ship was found to have COVID-19, the majority of the remaining passengers were tested for the new coronavirus (SARS-COV-2).

From the results of those tests, researchers concluded that 17.9 per cent of people with COVID-19 are asymptomatic.

But the researchers behind this study say the number could be higher for two reasons:

Firstly, symptomatic people were prioritised for testing after the ship docked in Japan (as they were more likely to require medical care). This means there might be more asymptomatic people than the study accounts for.

Firstly, symptomatic people were prioritised for testing after the ship docked in Japan (as they were more likely to require medical care). This means there might be more asymptomatic people than the study accounts for. Secondly, the cruise was largely older people, who are more likely to show symptoms.

The real figure could be closer to 30 per cent, one of the researchers, Dr Gerardo Chowell, told the New York Times.

"The substantial asymptomatic proportion for COVID-19 is quite alarming," said Dr Chowell, an epidemiologist with Georgia State University.

There might be more still

Health authorities in several countries are casting doubt on the WHO's early estimation that just 1-3 per cent of cases are asymptomatic.

That estimation was based on February's figures from the Chinese Centre for Disease Control and Prevention, which claimed 1.2 per cent of its then 72,000 cases were asymptomatic.

But more recently, the Chinese Government revealed to the South China Morning Post that a further 43,000 people tested positive but were not included as confirmed cases because they showed no symptoms.

That puts the asymptomatic rate in China at around 30 per cent, although there is likely a lot more asymptomatic cases who were not tested.

Iceland has been testing anyone in the population who volunteers, along with people who are showing symptoms. Early results suggest roughly half of the cases in the general population are asymptomatic, but the sample size was still small when Iceland's chief epidemiologist reported these findings.

South Korea's CDC director Jeong Eun-kyeong told reporters in mid-March that "Korea currently has a significantly higher rate of asymptomatic cases than other countries, perhaps due to our extensive testing".

In the US, the director of the CDC told NPR the proportion of silent carriers "could be as high as 25 per cent".

So, are these people infectious?

The answer is "probably", in the same way people with COVID-19 can be contagious a couple of days before developing symptoms.

On the one hand, asymptomatic people are less likely to be spreading droplets through coughing. On the other, they're more likely to be mixing with others as they don't realise they're sick, let alone infectious.

Researchers are still working to understand exactly how contagious asymptomatic cases may be. In one study, a 26-year-old Chinese man dubbed "Patient Z" felt nothing despite being a close contact of a confirmed case. But by day seven, the virus had bloomed in his nose and throat in exactly the same way as those who showed symptoms.

"The viral load that was detected in the asymptomatic patient was similar to that in the symptomatic patients, which suggests the transmission potential of asymptomatic or minimally symptomatic patients," the authors wrote.

There's also debate among researchers as to whether asymptomatic people truly have no symptoms or just fail to notice them as they are so mild.

The bottom line

There are a few key takeaways here.

Firstly, the outbreak is probably significantly larger than the cases we know about. Australia's Chief Medical Officer, Brendan Murphy, said the global tally was likely "five to 10 times higher" than the 1 million cases currently confirmed.

In Australia, we've prioritised testing people with symptoms to conserve tests. While this approach returns a higher proportion of positive tests, we may be missing silent carriers and the opportunity to quarantine those people.

For most of March, Australians were told to stay home or keep their kids home "if they had symptoms", but this advice likely overlooked some asymptomatic carriers. With this in mind, the New Zealand Government told all its citizens in a text message to: "Act as if you have COVID-19. Doing so will save lives."

Asymptomatic cases also throw into question WHO advice that healthy people don't need to wear masks in public. As the US overrides this advice, will Australia follow suit?

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