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In churches, on cruise ships, and even at the White House, high-profile events that can sicken dozens, even hundreds, of people have illustrated the potential for the coronavirus to become infected in dramatic explosions.
Experts say these large clusters are more than extreme outliers, but the likely main driver of the pandemic transmission.
And understanding where, when and why they occur could help us control the spread of the virus in the period before a vaccine becomes widely available.
Research increasingly suggests that the SARS-CoV-2 coronavirus is not distributed evenly across the population, but rather spreads at extremes in a nearly “all or nothing” pattern.
Many studies now suggest that most people with COVID-19 barely transmit it to anyone else, but when infections do occur, they can be explosive and fuel an outbreak.
So the virus can infect “10, 20, 50 or even more people,” said Benjamin Althouse, a research scientist at the Institute for Disease Modeling.
This corresponds to the “80/20 rule” of epidemiology, where 80 percent of cases come from just 20 percent of those infected, but Althouse said this coronavirus can be even more extreme, with 90 percent of cases potentially coming from only 10 percent of cases. carriers.
This transmission pattern is like “throwing matches on a pile of firewood,” he told AFP.
“You throw a match, it doesn’t light. You throw another match, it doesn’t light. You throw another match, and this time you see the flames burning,” he said.
“For SARS-CoV-2, this means that while it is difficult to establish itself in new places, once established, it can spread quickly and far.”
‘Stamp’ virus
The events that have spread widely have made headlines and taken center stage in the unfolding pandemic narrative.
In February, the Diamond Princess and her 4,000 passengers spent weeks in quarantine in the port of Japan as the number of infections on board increased, reaching 700.
The same month, a 61-year-old woman, known as “Patient 31,” attended various religious services at Shincheonji Church of Jesus in the South Korean city of Daegu.
Since then, the 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 meetings, business conferences, and sports tournaments have acted as infection incubators, but these high-profile events might just be the tip of the iceberg.
A study conducted by American researchers, based on one of the largest contact tracing operations in the world and published in Sciences in September, it found that “over-spread” prevailed in the broadcast.
Analyzing data from the first four months of the pandemic in India’s Tamil Nadu and Andhra Pradesh states, the authors found that only eight percent of infected people accounted for 60 percent of new cases, while 71 percent of people with the virus did not. pass it on to any of your contacts.
Perhaps this should not be a surprise.
Maria Van Kerkhove, an infectious disease epidemiologist at the heart of the World Health Organization’s pandemic response, tweeted in October that “over-spread is a hallmark” of coronaviruses.
In fact, it has been observed in many infectious diseases.
One of the most famous super-spreaders was Mary Mallon, a cook who worked in New York in the early 1900s and was the first documented healthy carrier of typhoid bacteria in the US.
Blamed for transmitting the disease to dozens of people, she was given the unsympathetic label “Typhoid Mary” and forcibly locked up for years.
Measles, smallpox, and Ebola also see clustering patterns, as do the other coronaviruses, SARS and MERS.
K factor
Early in the pandemic, a lot of attention was paid to the basic reproduction number (R0) of SARS-CoV-2.
This helps estimate the rate at which a disease can spread by looking at the average number of people that a person infects with the virus.
But looking at transmission through this metric alone often “fails to tell the whole story,” said Althouse, co-author of a paper on R0 limitations in the Journal of the Royal Society Interface this month.
For example, he said that Ebola, SARS-CoV-2, and influenza have an R0 value of around two to three.
But while people with the flu tend to infect two or three others “consistently”, the transmission pattern of people with Ebola and SARS-CoV-2 is over-dispersed, meaning that most will hardly spread it and some will lead to to dozens of other cases.
A different metric, “k”, is used to capture this clustering behavior, although it generally requires “more detailed data and methodology,” said Akira Endo, a research student at the London School of Hygiene and Tropical Medicine.
Their model of the early international spread of the virus, published in Wellcome Open Research, suggested that SARS-CoV-2 could be highly over-dispersed.
One telling clue, he said, was that some countries reported numerous imported cases, but no signs of sustained transmission, such as the coincidence analogy, while others reported large local outbreaks with only a few imported cases.
But even k may not give the complete picture, said Felix Wong, a postdoctoral fellow at the Massachusetts Institute of Technology.
His research looking at known COVID-19 super-spread events, published this month in the journal PNAS, found that they were occurring even more frequently than predicted by traditional epidemiological models.
They are “extreme but probable events,” Wong told AFP.
Biology vs opportunity
So why does superpropagation occur?
We don’t know for sure if biological factors, such as viral load, play a role.
But what we do know is that people can spread SARS-CoV-2 without symptoms and in a crowded and poorly ventilated space, especially where people are talking, yelling, or singing, the virus can spread rampant.
This could be the reason why a study in Nature This month it found that restaurants, gyms and cafes account for the majority of COVID-19 infections in the United States.
Using data from the mobile phones of 98 million people, the researchers found that about 10 percent of places accounted for more than 80 percent of cases.
Given this, experts say that attention should focus on these types of spaces and reduce the opportunities for the virus to access large numbers of people.
Wong said his model showed that if each individual were limited to ten communicable contacts, “viral transmission would quickly die out.”
Tracing
Overly widespread spread also means that most people who test positive for the virus are likely to be part of a group.
This opens up another way to track infections: backwards.
“The idea is that it could be more efficient to track and isolate super-propagators than to track downstream 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, credited with helping them curb their epidemics, along with other control measures.
Masks, social distancing and reducing contacts are all ways to limit transmission opportunities, Althouse said, adding that even characterizing people as “super-spreaders” is misleading.
“There are big biological differences between individuals, I may have a million times more virus in my nose than you do, but if I am a recluse, I cannot infect anyone,” he said.
© Agence France-Presse
