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Paris – In churches, on cruise ships and even in the White House, widespread events that can make dozens, if not hundreds, of people sick have illustrated the potential of the coronavirus to infect with dramatic outbursts.
Experts say these large groups are more than just extreme outliers, but rather the main driver of transmission for the pandemic.
And understanding where, when and why they happen could help us tame the spread of the virus in the lead-up to vaccine availability.
Research increasingly suggests that the SARS-CoV-2 coronavirus does not spread evenly across the population, but spreads to extremes in an almost ‘all or nothing’ pattern.
Numerous studies now suggest that the majority of people with COVID-19 barely pass it on to anyone else, but when infections do occur, they can be explosive and supercharge an outbreak.
Then the virus can infect “10, 20, 50 people, or even more,” said Benjamin Althouse, a researcher at the Institute for Disease Modeling.
This fits the ’80/20 rule’ of epidemiology, where 80% of cases come from just 20% of those infected, but Althouse said this coronavirus could be even more extreme, with 90% of cases potentially coming from only 10% of cases. carriers.
This model of transmission is like “throwing matches at a pile of kindling,” he said.
“You throw a match, it doesn’t ignite. You throw another match, it doesn’t ignite. You throw yet another match, and this time you see flames igniting, ”he said.
“For SARS-CoV-2, this means that while it is difficult to establish in new places, once established it can spread quickly and far.”
Mass media events grabbed the headlines, playing a big part in the story of the ongoing pandemic.
In February, the Diamond Princess and her 4,000 passengers spent weeks in quarantine at a Japanese port as the number of infections on board rose to 700.
That same month, a 61-year-old woman known as “Patient 31” attended several religious services at the Shincheonji Church of Jesus in the South Korean city of Daegu.
Korea’s Centers for Disease Control and Prevention has since linked more than 5,000 infections to Shincheonji.
More recently, the virus has managed to infiltrate the White House despite a multitude of measures to prevent it from entering.
Political rallies, business conferences and sports tournaments have all acted as incubators for infections, but these high-profile events may well be the tip of the iceberg.
A study by US researchers, based on one of the world’s largest contact tracing operations and published in Science in September, found that “super-diffusion predominates” in transmission.
Analyzing data from the first four months of the pandemic in the states of Tamil Nadu and Andhra Pradesh in India, the authors found that only 8% of those infected accounted for 60% of new cases, while 71% of those infected. did not. forward it to one of their contacts.
Maybe that shouldn’t come as a surprise.
Maria Van Kerkhove, an infectious disease epidemiologist at the heart of the World Health Organization’s pandemic response, tweeted in October that “overdiffusion is a hallmark” of coronaviruses.
Indeed, it has been observed in many infectious diseases.
One of the most famous super-spreaders was Mary Mallon, a cook working in New York City in the early 1900s who was the first documented healthy carrier of the typhoid bacteria in the United States.
Blamed for giving the disease to dozens of people, she was given the unsympathetic label “Typhoid Mary” and was forcibly confined for years.
Measles, smallpox, and Ebola also see clustering patterns, as do other coronaviruses, SARS, and MERS.
At the start of the pandemic, great attention was paid to the basic reproduction number (R0) of SARS-CoV-2.
This helps calculate how quickly a disease can spread by looking at the average number of other people infected with the virus.
But looking at transmission through that single metric “often fails to tell the whole story,” said Althouse, who co-wrote an article on R0 limits in the Journal of the Royal Society Interface this month.
For example, he said that Ebola, SARS-CoV-2, and influenza all have R0 values of around two to three.
But while people with influenza tend to infect two or three other people ‘systematically’, the transmission pattern of people with Ebola and SARS-CoV-2 is overdispersed, which means most don’t. will hardly spread and some will give rise to dozens of other cases.
A different metric – ‘k’ – is used to capture this clustering behavior, although it typically requires “more detailed data and methodology,” said Akira Endo, a student at the London School of Hygiene and Tropical Medicine.
His modeling of the early international spread of the virus, published in Wellcome Open Research, suggested that SARS-CoV-2 could be highly overdispersed.
A telling clue, he said, is that some countries have reported many imported cases but no signs of sustained transmission – like the match analogy – while others have reported large local outbreaks with only a few cases. imported.
But even k may not give the full picture, said Felix Wong, postdoctoral fellow at the Massachusetts Institute of Technology.
His research analyzing known events of the spread of Covid-19, published this month in the journal PNAS, found that they occur even more frequently than expected by traditional epidemiological models.
These are “extreme, but probable events,” Wong said.
So why is over-spreading happening? We are not sure whether biological factors, such as viral load, play a large role.
But what we do know is that people can spread SARS-CoV-2 without symptoms and in a poorly ventilated and crowded space – especially where people talk, scream or sing – the virus can become rampant.
Perhaps that is why a study in Nature this month found that restaurants, gyms, and coffee shops are responsible for most COVID-19 infections in the United States.
Using mobile phone data from 98 million people, the researchers found that about 10% of the sites accounted for more than 80% of the cases.
Given this, experts believe the focus should be on these types of spaces – and reduce the opportunities for the virus to reach large numbers of people.
Wong said his modeling showed that if each individual was limited to 10 transmissible contacts, “the virus transmission would die quickly.”
Overdispersed spread also means most people who test positive for the virus will likely be part of a cluster.
This opens up another way to track infections: backwards.
“The idea being that it might be more efficient to trace and isolate super-spreaders than to trace downstream and isolate individuals who, even if they were infected, could transmit the virus to very few. people, ”Wong said.
Japan and South Korea have both used reverse contact tracing, which has helped them curb their outbreaks, as well as other control measures.
Masks, social distancing and reduced contact are all ways of limiting the opportunities for transmission, Althouse said, adding that even calling people “super-diffusers” is misleading.
“There are big differences in biology between people – I have maybe a million times more viruses in my nose than you do – but if I am a recluse I can’t infect anyone,” he says.
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