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In churches, on cruise ships and even in the White House, widespread events that can make dozens, even hundreds, of people sick have illustrated the potential for coronavirus contagion in dramatic bursts.
Experts say these large clusters are not just extreme outliers, but rather the likely primary driving force behind the pandemic.
And understanding where, when and why they occur could help us tame the spread of the virus in the period before a vaccine becomes widely available.
Research increasingly suggests that the SARS-CoV-2 coronavirus does not spread uniformly in the population, but spreads to extremes in an almost ‘all or nothing’ pattern.
Many studies now suggest that most people with COVID-19 barely pass it on to anyone else, but when infections do occur, they can be explosive and overload an epidemic.
So the virus can infect “10, 20, 50 or even more people,” said Benjamin Althouse, a researcher at the Institute for Disease Modeling.
This matches the “80/20 rule” of epidemiology, where 80% of cases come from only 20% of infected people, but Althouse said this coronavirus could be even more extreme, with 90% of cases having potentially comes from only 10% carriers.
This transmission scheme is like “throwing matches on a pile of wood,” he told AFP.
“You throw a match, it won’t light up. You throw another match, it won’t light. You throw one more match, and this time you see the flames blazing,” he said.
“For SARS-CoV-2, this means that while it is difficult to establish itself in new places, once established it can spread rapidly and far.”
Virus hallmark
Massive events have taken the headlines, looming large in the narrative of the ongoing pandemic.
In February, the Diamond Princess and her 4,000 passengers spent weeks in quarantine at the port in Japan as the number of infections on board increased to 700.
The same month, a 61-year-old woman known as “Patient 31” attended several religious services at the Jesus Shincheonji Church in the South Korean city of Daegu.
Since then, Korean Centers for Disease Control and Prevention have linked more than 5,000 infections to Shincheonji.
More recently, the virus managed to infiltrate the White House despite a series of measures to keep it out.
Political gatherings, business conferences and sports tournaments have all acted as infection incubators, but these high-profile events may be just the tip of the iceberg.
A study by US researchers, based on one of the largest contact tracing operations in the world and published in Science in September, he found that “superspansion predominated” 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 infected people accounted for 60% of new cases, while 71% of people with the virus does not pass it on to one of their contacts.
Perhaps this shouldn’t come as a surprise.
Maria Van Kerkhove, an infectious disease epidemiologist at the center of the World Health Organization’s pandemic response, tweeted in October that “spread is a hallmark” of coronaviruses.
Indeed, it has been observed in many infectious diseases.
One of the most famous super speakers was Mary Mallon, a cook who worked in New York in the early 1900s and who was the first documented healthy carrier of typhoid bacteria in the United States.
Blamed for causing the disease to dozens of people, she was given the obnoxious label “Typhoid Mary” and forced to lock up for years.
Measles, smallpox, and Ebola also see clustering patterns, as do the other coronaviruses, SARS and MERS.
Factor K.
At the start of the pandemic, much attention was focused on the base reproduction number (R0) of SARS-CoV-2.
This helps calculate the rate of spread of a disease by looking at the average number of others infected by a person with the virus.
But looking at transmission through this metric alone often “fails to tell the whole story,” said Althouse, who this month co-authored an article on R0 limits in the Journal of the Royal Society Interface.
For example, he said Ebola, SARS-CoV-2, and influenza all have an R0 value of about two or three.
But while people with the flu tend to infect two or three others “consistently,” the pattern of transmission for those with Ebola and SARS-CoV-2 is overly dispersed, meaning most are unlikely to spread it and some will give rise to dozens of other cases.
A different metric – “k” – is used to capture this clustering behavior, although it usually requires “more detailed data and methodologies,” said Akira Endo, a research student at the London School of Hygiene and Tropical Medicine.
His modeling from the first international spread of the virus, published in Wellcome Open Research, suggested that SARS-CoV-2 could be highly over-dispersed.
A telltale clue, he said, is that some countries have reported numerous imported cases but no signs of prolonged transmission – such as the match analogy – while others have reported large local outbreaks with only a few imported cases.
But even k may not give the full picture, said Felix Wong, a postdoctoral fellow at the Massachusetts Institute of Technology.
His research analyzing known COVID-19 spread events, published this month in the journal PNAS, they found that they were occurring even more frequently than traditional epidemiological models predicted.
They are “extreme events, but probable,” Wong told AFP.
Biology vs opportunity
So why does super-diffusion occur?
We don’t know for sure if biological factors, such as viral load, play an important role.
But what we do know is that people can spread SARS-CoV-2 without symptoms and in a poorly ventilated and crowded space, particularly where people are talking, screaming or singing, the virus can spread.
This could be why a studio in Nature this month it found that restaurants, gyms and cafes are responsible for the majority of COVID-19 infections in the United States.
Using cell phone data from 98 million people, the researchers found that about 10 percent of the locations accounted for more than 80 percent of cases.
That said, experts say the focus should be on these types of spaces and reducing the opportunities for the virus to access large numbers of people.
Wong said his model showed that if each individual were limited to ten transmissible contacts, “viral transmission would quickly die out.”
Trace back
Excessive spread also means that most people who test positive for the virus are likely to be part of a cluster.
This opens up another way to track infections: backwards.
“The idea is that it might be more efficient to track down and isolate super speakers than to track down and isolate individuals who, even if infected, could transmit the virus to very few people,” Wong said.
Both Japan and South Korea have used backward contact tracing, which has been credited for helping them curb their outbreaks, along with other control measures.
Masking, social distancing and reducing contact are all ways to limit transmission opportunities, Althouse said, adding that even characterizing people as “super speakers” is misleading.
“There are huge differences in biology between individuals – I may have a million times more viruses in my nose than you – but if I’m a recluse, I can’t infect anyone,” he said.
© Agence France-Presse
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