COVID-19: How common is it to be asymptomatic and should we be worried?

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Two new studies have found a significant number of positive coronavirus cases that presented with zero symptoms. Experts weigh in on whether we should all be getting tested - symptoms or not.

By now we’ve all been told what COVID-19 symptoms to look out for -- from the high fever to the dry cough, difficulty breathing or even fatigue. 

There’s been studies in multiple countries exploring the phenomenon of asymptomatic cases of COVID-19 from Iceland, Japan, China and here in Australia. 

In a JAMA Network Open paper, researchers in China found that out of 78 patients, 42.3 per cent presented no symptoms. 

A different study by Australian researchers, published by Thorax, found of the 217 people on the Greg Mortimer cruise ship, more than eight out of the 10  who tested positive for COVID-19 were asymptomatic. 

What are pre-symptomatic and asymptomatic transmission? 

The World Health Organisation have charted the difference between the two. 

Pre-symptomatic is the period after exposure but before the virus’ symptoms arrive. 

But even without symptoms, it  doesn’t mean there’s no potential transmission. There have been recorded studies of people testing positive one to three days before they contract any symptoms. 

Asymptomatic transmission is when the virus spreads from a person who doesn't present with symptoms of COVID-19.

So what do experts think about the recent studies?

Professor Raina MacIntryre, the Head of the Biosecurity Program at the Kirby Institute, says there is a "substantial body of evidence that asymptomatic and presymptomatic infection is common with COVID-19."

She references the studies into aged care and other outbreaks that have "also found 50 per cent or more of all positive cases are asymptomatic."

In early May, Grant Lodge, an aged care home in the Melbourne suburb of Bacchus Marsh, saw an asymptomatic employee test positive.

"We should not be debating this any longer. High-risk contacts in outbreak situations, whether family contacts or in a closed setting outbreak, should be tested regardless of symptoms or cases will be missed," Prof MacInyrte said.

"People take 10-14 days to develop antibodies, so it is no surprise that the antibody-based rapid test was of limited use in an acute outbreak."

Some experts aren't so sure about the nature of asymptomatic COVID-19 cases.

Sanjaya Senanayake is a specialist in infectious diseases at the Australian National University.

He refers to the study from China where authors conclude that asymptomatic cases may not have isolated themselves because they didn't have symptoms.

"[Researchers] didn't look at whether any secondary cases arose from them" not self isolating, he says.

In February, the WHO China Joint Mission report found most asymptomatic patients during the time of testing ended up developing symptoms. Prof Senanayake is uncertain whether those patients in the study went on to get sick or not.

"Another limitation here, which the authors accept, is how accurately the assessment was of the cases having no symptoms. Is it possible that though they weren't overtly unwell that they still didn't feel 100 per cent right e.g. they were okay at rest, but didn't feel up to exercising etc?"

He cites the "varying proportions of asymptomatic cases in differing studies" for the limitations of understanding the nature of asymptomatic COVID-19 cases.

Iceland found 50 per cent of people tested positive without symptoms, 30.8 per cent were found in Japan while there was 80 per cent in a different study from China.

"It's hard to know which is right. And although we are getting closer to understanding the proportion of asymptomatic cases with COVID-19 , we still don't know for sure the magnitude of the impact that they have on further transmission of cases i.e. do they generate lots of secondary cases or only a small proportion?"

"In other words, there were four asymptomatic carriers for every ill passenger."

Professor Ivo Mueller is an epidemiologist at the Walter and Eliza Hall Institute of Medical Research.

He explains that understanding how asymptomatic infections are contributing to transmission of COVID-19 isn't only important to gaining a clearer picture of those exposed to the virus but "will also affect our predictions on how the COVID-19 pandemic may develop over the coming months."

"And what interventions are most important to prevent the second wave of cases and deaths," he said.

The research into the Greg Mortimer cruise ship, which had 96 Australian passengers, found out of the 217 people on board, 128 tested positive for COVID-19. Of those who contracted the virus, 104 people showed no symptoms -- which was 81 per cent of cases who tested positive.

"In other words, there were four asymptomatic carriers for every ill passenger. If the same pattern is repeated elsewhere, this means that in countries that only test symptomatic cases, the true burden of infections may be five times higher than currently reported," Prof Mueller said.

Prof Mueller says figuring out the “true infectiousness of asymptomatic carriers of all age must now be an urgent priority.” 

People in Australia must stay at least 1.5 metres away from others. Check your state’s restrictions on gathering limits. Testing for coronavirus is now widely available across Australia.

If you are experiencing cold or flu symptoms, arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.

The federal government's coronavirus tracing app COVIDSafe is available for download from your phone's app store.

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