Australian study determines true asymptomatic COVID rate

Since the COVID-19 pandemic began, there have been concerns around asymptomatic transmission.
 
There has also been confusion, along with debate over how many people infected with SARS-CoV-2 are truly asymptomatic, and what that means for efforts to try and limit the spread of the disease.
 
However, a new systematic review and meta-analysis has provided clinicians and public health officials with the clearest indication yet of how many people that contract coronavirus remain symptomless throughout – as opposed to being pre- or mildly-symptomatic.
 
According to the paper, only around 17% of cases are thought to be truly asymptomatic, a much lower rate than the US Centers for Disease Control and previous studies have suggested. It also found transmission rates are lower by about 42% for asymptomatic cases when compared with symptomatic cases.
 
Study co-author and GP researcher Professor Paul Glasziou told newsGP the findings indicate chains of asymptomatic transmission are less common than initially thought, emphasising the importance of robust contact tracing and isolation programs.
 
‘We originally did this because the World Health Organization was interested in what the asymptomatic rate was, with the concern really being about chains of asymptomatic transmission,’ he said.
 
‘That is you had somebody who was asymptomatic, who gave it to another person who was asymptomatic, etcetera, so that its circulating in the community and no one knows about it.
 
‘What we suggest from this is that that’s pretty unlikely to happen, [but] there is still a lot of pre-symptomatic transmission.’
 
Co-author Associate Professor Katy Bell from the University of Sydney’s Faculty of Medicine and Health and School of Public Health, told newsGP the new paper has key differences to previous research conducted in the area.
 
‘What differentiates this review from some of the previous research into asymptomatic cases is that this is a systematic review rather than a narrative review – with at least two reviewers screening all abstracts, full texts, doing data extraction and critical appraisal,’ she said.
 
‘We only included studies where there was follow up to check if symptoms developed among cases – ie truly asymptomatic, rather than pre-symptomatic at time of diagnosis … [and] where the decision to do the PCR test was not influenced by whether or not the person had symptoms.’
 
Professor Glasziou said it’s important to differentiate between pre- and asymptomatic cases, as even though people are still able to unknowingly spread the disease while pre-symptomatic, they are easier to identify subsequently than those who never develop symptoms.
 
‘You’re probably infectious for a day, maybe two before you develop symptoms, and then for several days afterwards, but we don’t know the proportion of transmission that occurs then,’ he said.
 
‘But when they declare themselves as symptomatic you can break the chain, because you detect them, isolate them and also find who they’ve been in contact with.
 
‘With asymptomatic, it’s much more difficult because you can only capture them through contact tracing, you can’t capture them directly.’
 
Associate Professor Bell said as the findings indicate less than 10% of transmission is likely to stem from asymptomatic cases, it may be possible to contain the spread of infection without resorting to lockdowns, so long as other measures are in place.
 
‘The main takeaway for clinicians and public health units is that truly asymptomatic infection doesn’t appear to be a main driver of the pandemic,’ she said.
 
‘Test and trace programs are critically important. Fast identification and testing of close contacts of confirmed cases can find both asymptomatic cases – who wouldn’t otherwise be found without mass screening, which has its own problems – and pre-symptomatic cases.’
 
As a result, Professor Glasziou says it is vital that GPs communicate with patients and their close contacts about the importance of self-isolating appropriately. He also said very close contacts of confirmed cases, such as people living in the same household, should self-isolate regardless of symptoms or test results.
 
‘There’s about a 20% chance that they will get infected and even though there are very few false positives with a PCR [polymerase chain reaction] test, you can miss infected people as it’s dependant largely on the quality of the swabbing,’ he said.
 
‘It’s just being aware that just because somebody says, “Oh no, I’m feeling fine”, it doesn’t mean they’re not COVID positive. If they’ve been in close contact with someone they may well have an asymptomatic infection.’
 
Aside from potentially impacting the measures public health units employ to combat the disease domestically, Professor Glasziou said asymptomatic transmission should also influence the approach taken once international tourism resumes.
 
‘This is an important thing – working out what a sufficient testing and quarantine procedure would be,’ he said.
 
‘Whilst we maintain the two week quarantine, even if you’re coming in from an infected country, I think we’re probably fine.
 
‘But when we want to relax that, and we may be tempted sometime next year to start relaxing from certain countries, I think we need to recognise that you can’t just rely on symptomatic people or outbreaks, we need some sort of testing as people come in.’
 
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