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Many of us wouldn’t know if we’ve had symptomless coronavirus, but an increasing number of studies suggest it could be more common than you think.
Around eight in 10 coronavirus infections are mild or asymptomatic – meaning the person had no symptoms – according to the World Health Organisation (WHO). Meanwhile 15% of infections are severe, and 5% are critical.
But that eight in 10 statistic doesn’t specify how many people experience mild symptoms versus how many have the virus without feeling ill at all.
While we don’t know exactly how many people tend to be asymptomatic, we do know that the number could range from 35-50%. It could be even higher.
The Centers for Disease Control and Prevention (CDC) in the US has estimated that just over a third (35%) of Covid-19 infections are symptomless.
But other studies suggest this figure might be conservative. One from Italy published at the end of June found that of residents who tested positive for Covid-19 in the municipality of Vo’, a small town near Padua, 42% were asymptomatic.
Another study of 9,000 people selected to take a coronavirus test in Iceland found 50% of them tested positive for Covid-19, but didn’t have any symptoms.
Studies from cruise ships suggest the figure could be higher. Almost three quarters (72%) of people on board the Diamond Princess cruise ship who had Covid-19 were thought to have been asymptomatic, according to a preliminary study by the London School of Hygiene & Tropical Medicine.
Meanwhile a study of the infection on a different (and smaller) cruise ship discovered that eight in 10 passengers and crew members who tested positive for Covid-19 were asymptomatic.
Of those testing positive for the virus, 24 (one in five) people had symptoms, but 108 (eight in 10) people didn’t.
Professor Alan Smyth, joint editor-in-chief of the journal Thorax, where the study was published, told HuffPost UK: “It’s difficult to find a reliable estimate of the number of Covid positive patients who have no symptoms. In early March, WHO suggested the figure might be only 1% – very different from the 81% figure found on the cruise ship.”
As countries progress out of lockdown, the number of asymptomatic individuals may mean that a much higher percentage of the population than expected may have been infected with Covid-19, he said.
“These individuals may have immunity to Covid-19,” he said. “This would in turn reduce the chances of a ‘second peak’ in cases, and the size of the peak should one occur.”
He noted that while the population on the cruise ship was small, every single member was tested – so this is a unique “natural experiment”, which will never be repeated elsewhere.
“There’s no reason why the passengers and crew on the ship are any more likely than the general population to be infected with Covid-19 but have no symptoms,” he said.
“It’s therefore very plausible that the rate of 81% infected but symptom-free might apply to the general population. This means that many more people may be (potentially) immune than was first thought.”
Why are asymptomatic cases a problem?
While it’s good news that some people aren’t getting ill from coronavirus, this presents a problem if those people are still going to the shops and to work, because they think they’re fine, but are inadvertently spreading it around.
Dr Jenna Macciochi, an immunologist and author of Immunity: The Science of Staying Well, tells HuffPost UK asymptomatic carriers are “one of the biggest challenges” with easing lockdown restrictions.
Studies have found that symptomatic and asymptomatic people have a similar viral load. Explaining what a viral load is, Dr Edward Parker, research fellow in systems biology at the London School of Hygiene and Tropical Medicine, said: “After we are infected with a virus, it replicates in our body’s cells. The total amount of virus a person has inside them is referred to as their ‘viral load’.”
The higher the viral load, the more infectious someone is likely to be. And with asymptomatic people being potentially as infectious as those with symptoms, one study from South Korea suggested the isolation of asymptomatic patients may be necessary to control the spread of the virus.
The current NHS advice is that people who have symptoms of coronavirus – a high temperature, new continuous cough, or loss of smell or taste – or those who have tested positive for the virus, should self-isolate.
While people at high risk of catching Covid-19 (because they live or work with someone with it, for example) can be tested for the virus even if they have no symptoms, the government advice is that people shouldn’t arrange for testing unless they develop symptoms of Covid-19.