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A curious pattern has emerged in the infection rate of COVID-19

A curious pattern has emerged in the infection rate of COVID-19 thumbnail

Coronavirus has wreaked havoc on the world, with more than 10 million infections and half-a-million deaths recorded since the pandemic began.But in some countries, there are signs that the virus is “burning out” – as if its engine is running out of fuel, experts say.Even in places that have experienced devastating outbreaks, like Sweden, the…

Coronavirus has wreaked havoc on the world, with more than 10 million infections and half-a-million deaths recorded since the pandemic began.

But in some countries, there are signs that the virus is “burning out” – as if its engine is running out of fuel, experts say.

Even in places that have experienced devastating outbreaks, like Sweden, the rate of death has begun to slow despite difficulties getting on top of the spread.

And in countries that are seeing a flare-up, like Germany, which had relatively few cases initially but has seen hotspots cropping up, the transmission rate spiked and then fell again after a brief time.

Attention is now turning to why that might be occurring.

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Since the pandemic began, scientists have been puzzled by the spread of coronavirus, with its infection characteristics difficult to predict and understand.

But two European epidemiologists, Professor Paul Franks from Lund University and Professor Joacim Rocklov from Umea University in Sweden, have observed an intriguing pattern.

“On the Diamond Princess cruise ship, for example, where the virus is likely to have spread relatively freely through the airconditioning system linking cabins, only 20 per cent of passengers and crew were infected,” they wrote in an article for The Conversation.

“Data from military ships and cities such as Stockholm, New York and London also suggest that infections have been around 20 per cent – much lower than earlier mathematical models suggested.

“This has led to speculation about whether a population can achieve some sort of immunity to the virus with as little as 20 per cent infected – a proportion well below the widely accepted herd immunity threshold (of 60 to 70 per cent).”

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What’s behind this behaviour? Is it merely an anomaly or a sign the pandemic might end sooner than initially feared?

Cause for optimism has been fuelled by additional speculation about the potential of a “immunological dark matter”, Prof Franks and Prof Rocklov said.

It’s a kind of pre-existing immunity that can’t be detected in COVID-19 antibody testing.

“Antibodies are produced by the body’s B-cells in response to a specific virus. Dark matter, however, involves a feature of the innate immune system termed ‘T-cell mediated immunity’.

“Studies show that people infected with (COVID-19) indeed have T-cells that are programmed to fight this virus. Surprisingly, people never infected also harbour protective T-cells, probably because they have been exposed to other coronaviruses.

“This may lead to some level of protection against the virus – potentially explaining why some outbreaks seem to burn out well below the anticipated herd immunity threshold.”

Young people and those with mild coronavirus infections were more likely to have a T-cell response than older people, Prof Franks and Prof Rocklov wrote.

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Despite the hypothesis, Professor Raina MacIntyre, a renowned Australian infectious diseases specialist and the head of the biosecurity research program at the Kirby Institute, isn’t convinced COVID-19 will burn out at 20 per cent infection.

“Herd immunity can be calculated mathematically based on the R0 (average transmission rate), and 20 per cent is nowhere nearly enough – it needs to be 60 to 70 per cent,” Prof MacIntyre said.

“People forget that the Spanish influenza pandemic occurred over a period of two years, and different people got infected in different waves. People who are uninfected will largely remain susceptible, and we will see second waves until an effective vaccine is available.”

She believes that behavioural change is an important factor in the examples provided in The Conversation analysis.

“At a 20 per cent infection rate, the health system will be near collapse, most people will know someone who has died, and behaviour will change – people will social distance, wear masks, and may be subject to enforced lockdowns.

“Even on the Diamond Princess, once the outbreak was apparent, quarantine was enforced on the ship and resulted in behaviour change.”

Lockdowns enforced by countless countries, coupled with effective public health messages about health, hygiene and social distancing, have no doubt slowed the spread and saved lives, Prof Franks and Prof Rocklov wrote.

“Indeed, in cases such as Sweden – where lockdown was eschewed and social distancing rules were relatively relaxed – the virus has claimed an order of magnitude more lives than in its pro-lockdown neighbours, Norway and Finland.

“But it is unlikely that lockdowns alone can explain the fact that infections have fallen in many regions after 20 per cent of a population has been infected – something that, after all, happened in Stockholm and on cruise ships.”

On the flip side, the fact that more than 20 per cent of people have been infected in other places suggests the T-cell theory isn’t the only explanation either.

“If a 20 per cent threshold does exist, it applies to only some communities, depending on interactions between many genetic, immunological, behavioural and environmental factors, as well as the prevalence of pre-existing diseases,” they wrote.

“Understanding these complex interactions is going to be necessary if one is to meaningfully estimate when (COVID-19) will burn itself out.

“Ascribing any apparent public health successes or failures to a single factor is appealing – but it is unlikely to provide sufficient insight into how COVID-19, or whatever comes next, can be defeated.”

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