A ski trip, a wedding, a choir practice: what these events have in common is that they all have cases of coronavirus super-spread. This happens when someone transmits the virus to a large number of people. While there is no widely agreed definition of a super-spreading event, it is sometimes taken as an incident in which someone passes the virus on to six or more other people. Understanding why these clusters occur may be essential to gaining control of the covid-19 pandemic, as well as stopping a second wave of cases.
For months, we have heard that the number R, or reproduction number, is the most needed to measure the spread of covid-19. This is the average number of people that each infected person transmits the virus to. Prior to isolation in the UK, the R number for coronavirus was estimated to be somewhere between 2 and 3.
It is now estimated that there is a great deal of variability in the number of new cases that each infected person creates. This can be described by the "number K" of the epidemic - the distribution parameter - where a lower K value indicates more variability. You need to know both, R and K, to have a good picture of how the virus is spreading through a community.
According to an analysis of how covid-19 had spread to other countries from China, by the end of February, the K number was 0.1, an extremely low value. The researchers estimated that 80 percent of cases were caused by about 10 percent of infected persons. These 10 percent can cause a hotbed of infections, while most others would not pass the coronavirus to anyone else, and some would only transmit it to another person.
In other words, superpopulation is integral to the pandemic, says Quentin Leclerc of the London School of Hygiene. 52 others. Doctors followed close contacts of each secondary case, about three or four each, and were able to find only 10 further infections, says Lea Hamner, a public health official in Skagit County.
It is as if something qualitatively different was happening that night. Outbreaks have been reported in other diseases, including HIV, TB and typhoid, where a famous super-spreader was a New York-based chef in the early 20th century who became famous as Mary Typhoid. In these cases, there seemed to be something biological that made the person more predisposed to transmit an infection, probably because they had a high number of pathogens.
With coronavirus, the burden of the virus may also play a role, but this has not been investigated and we do not have a simple way to do it, says Benjamin Couling at the University of Hong Kong in China. "If we measure the viral load in saliva, it is not the same as the amount of virus they exhale. "Some kind of air sampling should be done."
The circumstances of the spread event also appear to be significant, and some common cases have emerged. Couling's team tracked the contacts of the first 1037 coronavirus cases in Hong Kong. The team found a slightly higher K value than the previous estimate of 0.45, but that means only 20 percent of infected individuals caused 80 percent of the cases in the country.
Mass broadcasts
Super-diffusion events tend to occur indoors, with people staying close by. Social activities caused more outbreaks than exposure in the workplace or at home - mass broadcasts took place at weddings, temples, bars and karaoke parties, for example.
The risk seems to be higher if people raise their voice in some way, such as when they sing or shout. "It has to do with the volume of air coming out of your lungs," says Couling. Understanding superpopulation is becoming even more important now that coronavirus cases are declining in many countries, says Adam Kleczkovski at the University of Strathclyde in the United Kingdom. When case numbers increase exponentially, superproliferation is probably less important, he says, as many outbreaks spread and merge.
But avoiding clusters is essential at the beginning of an epidemic before case numbers increase too much, or when case numbers fall, and a second wave must be avoided. "When you have very few cases, it is these [superpower] activities that need to be taken care of," says Leclerc.
How can we reduce the chances of further superpower events? "Our guiding principles now are: outside is safer than inside, fewer people is safer than many," says Hamner. She sees the closed premises as a special danger.
Awareness of people about super-proliferation activities can strengthen the need to avoid dangerous situations. It could also help to understand how to alleviate restrictions, says Leclerc, whose team has already set up a database of superproliferation activities. Only eight of the 201 outbreaks identified by the team occurred in schools, which at the beginning of the pandemic were seen as a source of infections.
Also, spotting real-time hotspots is key to the functionality of contact tracking, she says. / NS - Bota.al
