At a May 30 birthday party in Texas, a man reportedly infected 18 friends and family with the coronavirus.
By reading reports like these, you can think of the virus as a forest fire, instantly triggering epidemics wherever you go. But other reports tell another story entirely.
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In Italy, for example, scientists looked at stored wastewater samples to detect the first trace of the virus. Last week they reported that the virus was in Turin and Milan on December 18. But it would be two months before hospitals in northern Italy began to fill with Covid-19 victims. So those December viruses seem to have run out.
Oddly enough, these reports do not contradict each other. Most infected people do not transmit the coronavirus to another person. But a small number passes it on to many others at so-called superpreparation events.
“You can think about throwing a match into the ignition,” said Ben Althouse, senior research scientist at the Institute of Disease Modeling in Bellevue, Washington. “If you throw a match, it may not turn on the ignition. You throw another match, it may not light the firewood. But then a party hits the right place, and suddenly the fire goes up. “
According to scientists, understanding why some parties cause fires, while others do not, will be crucial in stopping the pandemic. “Otherwise, you’re in the position that you’re always one step behind the virus,” said Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine.
When the virus first emerged in China, epidemiologists were quick to understand how it spread from person to person. One of his first tasks was to estimate the average number of infected people for each sick person, or what epidemiologists call the reproductive number.
The new coronavirus turned out to have a reproductive number somewhere between two and three. It is impossible to specify an exact figure, since people’s behavior can facilitate or hinder the spread of the virus. Upon entering the closure, for example, Massachusetts reduced its number of breeders from 2.2 in early March to 1 at the end of the month; It is now at 0.74.
This average figure can also be misleading because it masks the variability of spread from one person to another. If 9 out of 10 people don’t transmit a virus at all, while 10 transmit it to 20 people, the average would still be two.
In some diseases, such as the flu and smallpox, a large proportion of infected people transmit the pathogen to a few more. These diseases tend to grow steadily and slowly. “The flu can really move forward,” said Kristin Nelson, an associate professor at Emory University.
But other diseases, such as measles and severe acute respiratory syndrome, or SARS, are prone to sudden outbreaks, and only a few infected people transmit the disease.
Epidemiologists capture the difference between outbreaks and progression with something known as a dispersion parameter. It is a measure of how much variation there is from person to person in the transmission of a pathogen.
But James Lloyd-Smith, a UCLA disease ecologist who developed the dispersion parameter 15 years ago, cautioned that just because scientists can measure it doesn’t mean they understand why some diseases outperform others. “We just understand its parts,” he said.
When Covid-19 exploded, Kucharski and his colleagues tried to calculate that number by comparing cases in different countries.
If Covid-19 were like the flu, I’d expect the outbreaks in different places to be mostly the same size. But Kucharski and his colleagues found wide variation. They found that the best way to explain this pattern was that 10% of infected people were responsible for 80% of new infections. Which meant that most people transmitted the virus to a few, if any others.
Kucharski and colleagues published their study in April as a preprint, a report that has not been reviewed by other scientists and published in a scientific journal. Other epidemiologists have calculated the dispersion parameter with other methods, ending with similar estimates.
In Georgia, for example, Nelson and his colleagues analyzed more than 9,500 Covid-19 cases from March to May. They created a model for the spread of the virus across five counties and estimated how many people each person infected.
In a preprint published last week, the researchers found many widely publicized events. Only 2% of people were responsible for 20% of the transmissions.
Now researchers are trying to discover why so few people transmit the virus to so many. They are trying to answer three questions: Who are the super-spreaders? When does the overlearning take place? And where?
Regarding the first question, doctors have observed that viruses can multiply in greater numbers within some people than others. It is possible for some people to become virus stacks, expelling clouds of pathogens with each breath.
Some people also have more opportunities to get sick and then get other people sick. A bus driver or nursing home worker can sit in a social network hub, while most people are less likely to come in contact with other people, especially in a block.
Nelson suspects that the biological differences between people are less significant. “I think the circumstances are much more important,” he said. Lloyd-Smith agreed. “I think he is more focused on events.”
A large amount of transmission appears to occur in a limited period of time that begins a couple of days after infection, even before symptoms arise. If people are not around many people during that window, they cannot get past it.
And certain places seem to lend themselves to supersension. A busy bar, for example, is full of people speaking loudly. Any one of them could vomit viruses without coughing. And without good ventilation, viruses can stay in the air for hours.
A study conducted in Japan this month found clusters of coronavirus cases in health centers, nursing homes, daycare centers, restaurants, bars, workplaces, and music events such as live concerts and karaoke parties.
This pattern of over-grasp could explain the puzzling delay in Italy between the arrival of the virus and the rise of the epidemic. And geneticists have found a similar delay in other countries: the first viruses that appear in a given region do not lead to epidemics that occur weeks later.
Many countries and states have struggled with outbreaks with blockages, which have reduced the reproductive number of Covid-19. But as governments move toward reopening, they should not be complacent and forget about the virus’s potential to over-signal itself.
“You can really go from thinking you have things under control to having an out-of-control outbreak in a matter of a week,” Lloyd-Smith said.
Singapore’s health authorities were praised from the start for containing the epidemic by carefully tracking Covid-19 cases. But they did not appreciate that the huge dormitories where migrant workers lived were prime venues for high-profile events. Now they are struggling with a resurgence of the virus.
On the other hand, knowing that Covid-19 is a super broadcast pandemic could be a good thing. “It bodes well for control,” said Nelson.
Since most transmissions occur only in a small number of similar situations, it is possible to devise smart strategies to prevent them from occurring. It may be possible to avoid crippling crashes, across the board, by targeting overcoming events.
“By curbing activities in a fairly small proportion of our lives, we could reduce most of the risk,” said Kucharski.
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