In churches, on cruise ships, and even in the White House, overexpressing events that can affect dozens or even hundreds of people illustrate the potential for the coronavirus to become infected in dramatic outbreaks.
Experts say these large clusters are more than extreme endpoints, but rather the likely main driver of the pandemic’s transmission.
And understanding where, when, and why they occur can help curb the spread of the virus before the vaccine is widely available.
Studies are increasingly showing that the SARS-CoV-2 coronavirus does not spread evenly in the population, but spreads to extremes in almost “all or nothing.”
Many studies now suggest that most people with COVID-1
The virus can then infect “10, 20, 50 or even more people,” said Benjamin Althaus, a researcher at the Institute for Disease Modeling.
This is in line with the “80/20 rule” of epidemiology, where 80% of cases come from only 20% of those infected, but Althouse said this coronavirus could be even more extreme, with 90% of cases coming from potentially only 10 % of carriers.
This model of transmission is like “throwing matches on a pile of kindling,” he told AFP.
“You throw one stick, it doesn’t ignite. You throw another stick, it doesn’t ignite. You throw another stick and this time you see flames blazing,” he said.
“For SARS-CoV-2, this means that although it is difficult to establish in new locations, once it is established, it can spread quickly and far.”
“Distinctive feature” of the virus
The super-conversational events grabbed the headlines, appearing great in the story of the unfolding pandemic.
In February, the Diamond Princess and her 4,000 passengers spent weeks in quarantine in a Japanese port as the number of those on board rose to 700.
That same month, a 61-year-old woman known as “Patient 31” attended several church services at the Shinchongji Jesus Church in the South Korean city of Daegu.
Since then, the Korean Centers for Disease Control and Prevention has linked more than 5,000 Shincheonji infections.
More recently, the virus has infiltrated the White House despite numerous measures to prevent it.
Political gatherings, business conferences and sports tournaments have acted as infectious incubators, but these high-profile events can only be the tip of the iceberg.
A study by US researchers based on one of the largest contact tracking operations in the world and published in Science in September, he found that “over-reliance” prevailed on the show.
Analyzing data from the first four months of the pandemic in Tamil Nadu and Andhra Pradesh in India, the authors found that only eight percent of those infected accounted for 60 percent of new cases, while 71 percent of people with the virus did not pass it on to any of their contacts. .
Maybe this should come as no surprise.
Maria Van Kerhove, the infectious disease epidemiologist at the heart of the World Health Organization’s pandemic response, tweeted that “over-spread is a hallmark” of coronaviruses.
It is indeed observed in many infectious diseases.
One of the most famous over-distributors is Mary Malone, a cook working in New York in the early 1900s who was the first documented healthy carrier of typhoid bacteria in the United States.
Accused of spreading the disease to dozens of people, she received the unsympathetic label “Tif Maria” and was forcibly imprisoned for years.
Measles, smallpox and Ebola also see cluster patterns, as do other coronaviruses, SARS and MERS.
To the factor
At the beginning of the pandemic, much attention was focused on the primary reproductive number (R0) of SARS-CoV-2.
This helps calculate the rate at which the disease can spread by looking at the average number of other people infected with the virus.
But looking at the show on that metric alone often “fails to tell the whole story,” said Althaus, who co-authored an article on the limitations of R0 in the Journal of the Royal Society Interface this month.
For example, he said that Ebola, SARS-CoV-2 and the flu have an R0 value of about two to three.
But while people with the flu tend to infect two or three others “sequentially,” the transmission pattern of those with Ebola and SARS-CoV-2 is overly prevalent, meaning most are unlikely to spread it and some dozens of other cases.
A different metric, “k,” is used to capture this cluster behavior, although “more detailed data and methodology” are usually required, said Akira Endo, a student at the London School of Hygiene and Tropical Medicine.
Its modeling from the early international spread of the virus, published in Wellcome Open Research, suggests that SARS-CoV-2 may be highly dispersed.
Publishing evidence, he said, was that some countries reported numerous imports, but no signs of prolonged transmission – like the match analogy – while others 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 events for the COVID-19 overtime, published this month in the journal PNAS, found that they occur even more frequently than predicted by traditional epidemiological models.
They are “extreme but probable events,” Wong told AFP.
Biology versus opportunity
So why is over-reliance happening?
We do not know definitively whether biological factors, such as viral load, play a major role.
But what we do know is that humans can spread SARS-CoV-2 without symptoms, and by giving poorly ventilated, crowded space – especially where people talk, scream or sing – the virus can spread.
This may be the reason for research in Nature this month found that restaurants, gyms and cafes account for the majority of COVID-19 infections in the United States.
Using mobile phone data from 98 million people, researchers found that about 10 percent of places accounted for more than 80 percent of cases.
With that in mind, experts say the focus should be on these types of spaces – and reducing the virus’s ability to access large numbers of people.
Wong said his modeling shows that if each individual is limited to ten transmissible contacts, “viral transmission will pass quickly.”
Over-dispersion also means that most people with a positive virus test are likely to be part of a cluster.
This opens up another way to track infections: backwards.
“The idea is that it could be more effective to track and isolate over-spreaders than to track downstream and isolate individuals who, even if infected, can transmit the virus to very few people,” he said. Wong.
Both Japan and South Korea have used contact tracking, which has been attributed to helping them limit their epidemics, along with other control measures.
Masks, social distancing and reduced contact are all ways to limit transmission opportunities, Althouse said, adding that even characterizing people as “over-distributors” is misleading.
“There are huge differences in biology between individuals – I may have a million times more virus in my nose than you, but if I’m a hermit, I can’t infect anyone,” he said.
© Agence France-Presse