As the nation moves into “Covid normal”, how worried should we be about asymptomatic cases?
These are the mysterious, little-understood instances where people have the coronavirus percolating in their system, with nary a tickle in the throat to show for it, and are thought to be as dangerously contagious as someone coughing their hearts out.
About three weeks ago, Sydney was feeling pretty good about itself – the city had enjoyed 12 days straight without a new case. And then boom, the spell was broken. Three new cases.
Mary-Louise McLaws, an epidemiologist from the University of NSW, told ABC online:
“It would appear to be unusual to start getting three cases, but up to 20 per cent of all cases are asymptomatic.”
Asymptomatic carriers, she said, were “likely the reason the virus could remain undetected in the community.”
That 20 per cent number, the same rate found in countries where the virus is out of control, demands some contemplation. For the moment, though, Australia has its fingers crossed.
China does it differently
Get this: after a single asymptomatic coronavirus case was reported in the region of Xinjiang, China, authorities ordered nearly five million to be locked down and tested.
As of October 27, 137 new cases were identified from 2.8 million tests so far. All were asymptomatic, according to Xinjiang’s regional health commission.
Not so many, you might think. But this in a country that has taken a ruthless path to suppression of the virus.
It has to be said that testing in Australia continues to be relatively robust, although in NSW it’s on the decline – and it’s largely confined to people with symptoms or people who have been in contact with known cases.
Is it enough?
Nasty news from random tests in the UK
The Imperial College of London is heading a project called REACT-1 which, according to a college statement, is tracking current cases of COVID-19 in the UK community by testing more than 150,000 randomly-selected people over a two-week period each month.
Volunteers take nose and throat swabs at home, which are then analysed in a laboratory by a technique called RT-PCR.
Out of 85,971 swabs analysed so far in the latest round of testing (between October 16 and October 25) 863 were positive, giving an estimated prevalence of 1.28 per cent.
Okay, we know Britain doesn’t have the coronavirus anywhere near under control. But the sobering point of the random testing is that the cases unearthed are either asymptomatic or people with minor symptoms.
In line with Australia and elsewhere, the researchers concluded that 20 per cent of all COVID-19 cases in the UK are asymptomatic.
When this is factored into immediate projections, the UK, in real terms, is being belted by just under 100,000 new cases every day.
These UK results, supported by earlier studies suggest a more extensive program of random testing may be warranted in Australia as we continue to see persistent mini-clusters.
Study suggests how a person becomes asymptomatic
Researchers from Aarhus University and Aarhus University Hospital in Denmark have published a new study that suggests the SARS-CoV-2 virus may hide its genomic material from being recognised in specialised immune cells called alveolar macrophages – thereby not inducing an immune response.
If the virus is hidden in your body, and no immune response is triggered, then you’re infected and, for the short term at least, asymptomatic. In other words, you’re a carrier.
Here’s the study’s logic and various terms explained:
- SARS-CoV-2 is a respiratory virus that typically infects the outermost cell layer of the lungs, the epithelial layer.
- It’s assumed that the epithelial layer is the virus’s target – and a deadly one because our blood vessels are all lined with this layer, thus allowing the virus access to the toes, the metabolic organs (liver, pancreas), the lungs and brain.
- But, it’s also assumed that when the viral particles are sucked into the lungs – when we take a breath – the first defensive cells that the virus encounters are large numbers of alveolar macrophages. Their function is to keep the lungs healthy.
- New research has shown that interferon production in the infected epithelial cells can be inhibited by the SARS-CoV-2 virus.
- Interferons are a group of signalling proteins (cytokines) that regulate the immune response and inflammation.
- This results in low interferon production and therefore also a limited activation of the immune system to fight against the virus.
- Alveolar macrophages have previously been shown to produce large amounts of interferons upon infection with respiratory viruses, such as influenza.
- In their experiment, the researchers isolated alveolar macrophages and exposed them to the SARS-CoV-2 virus.
- Contrary to the researchers’ expectations, the alveolar macrophages produced no interferons. This led to the conclusion that alveolar macrophages essentially host the virus, thereby protecting it and forestalling an immune response.
It’s a theory at this point, with “more research is needed to understand how SARS-CoV-2 can avoid being recognized by the immune system.”