What is the epidemiological indicator of dispersion “k” and why scientists are constantly monitoring it

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The R0 reproduction/transmission index has become famous – and in Greece – it shows how quickly an epidemic disease such as Covid-19 spreads to the population, based on the calculation of how many other people a virus carrier infects. The lower than one (1) R0 is, the better (in Greece today it is very low, around 0.37).

But another major epidemiological indicator has gone relatively unnoticed, and that is the “dispersion factor k”, which shows how much Covid-19 and any other epidemic tends to base its spread not on a wide population spread, but on a few outbreaks. -transmission. The lower the k index, the more likely it is that the transmission of corona comes from a small number of people who transmit it to others (in Greece the k index is still unknown).

After months of experimenting with the behaviour of the new SARS-CoV-2 corona, scientists around the world have found that only a few people do all the “damage” by infecting many others and many others. – perhaps most – not only do they have no symptoms, but they don’t even transmit it. There are plenty of examples of hearth transmitters on ships, buses, restaurants, nursing homes, hospitals, prisons, churches, choirs, immigrant centres, etc., where a unique man infected dozens of others.

Other infectious diseases are spreading from a few outbreaks, but the new coronavirus – like the related coronaviruses of SARS and MERS – seems particularly prone to spreading to groups of people close to each other. However, this is considered encouraging by scientists, because restricting gatherings indoors or taking precautions (two meters away and wearing a mask) can “slow down” the virus.

“If you can predict the circumstances that cause such events, mathematics shows that it is really possible to very quickly reduce the possibility of Covid-19 disease spreading,” said Jamie Lloyd-Smith, a professor of infectious diseases at the University of California, Berkeley. – Los Angeles (UCLA). He stressed that “the most common R0 number is zero, as most people do not transmit the infection at all.”

That’s why, along with R0, scientists are also turning to the dispersion index k. In a 2005 scientific report on Nature, Dr. Lloyd-Smith and his colleagues estimated that in Serious Acute Respiratory Syndrome (SARS) the k was only 0.16 (i.e., over-transmission by a few played a key role. ), while in the Respiratory Syndrome of the Middle East (MERS) the k was 0.25. In the pandemic of “Spanish” flu in 2018 (which was so badly christened as a great inconvenience to the Spaniards) the k is estimated at about one, so the transmission was through a wider dispersion.

The k of the new corona is unknown

It is also under discussion how much is the k of the current SARS-CoV-2 corona. Researchers at the University of Bern have found – based on data from China – that it is slightly larger than the SARS and MERS k, which was agreed upon by epidemiologist-modelling Gabriel Leung of Hong Kong University, who noted that ” Certainly what we’re seeing is a lot of outbreaks, where a small percentage of people are responsible for a large percentage of infections. ”

But according to another recent scientific prediction by Dr. Adam Kurtzarski of the School of Health and Tropical Medicine in London, the k for Covid-19 may be lower than SARs, perhaps only 0.1. In other words, he claims, “probably about 10% of cases lead to 80% of the spread.”

This – is true – may explain why the virus did not “take off” faster around the world after its initial appearance in China, and why some very early cases elsewhere (eg in France at the end of December 2019) failed. to trigger a wider outbreak. In fact, k is 0.1, then most transmission “chains” go out on their own. According to Kutsarski, in this case, the new colonist must be introduced unnoticed in a country at least four different times until he finds the opportunity to establish himself there.

The cause of coronae tending to create more single outbreaks than other microbes “remains a really open scientific question,” according to Christopher Fraser of the University of Oxford, who has studied the over-transmission of Ebola and HIV. . One possible explanation is that some people secrete far more virus particles for longer than others, perhaps due to differences in their immune systems. Some also have a lot of social contacts, while others don’t. A very sociable person, whose body easily secretes the virus through the droplets of saliva, is ideal for over-transmission times.

Be careful indoors

Scientists have found that Covid-19 over-transmission foci are more likely to occur indoors. “Clearly there is a much greater danger indoors than outside,” said Christian Althaus of the University of Bern. Chinese researchers studying the spread of the corona outside Wuhan and Hubei Province (the epicentre of the pandemic) have identified 318 outbreaks with at least three cases, only one of which has started in the open. A Japanese study has found that the risk of Covid-19 infection is almost 19 times higher indoors than outdoors.

Indoor spaces e.g. In gyms where intense dance aerobic programs are performed (Zumba, Latin, etc.), they increase the risk of transmitting the corona, as opposed to calmer programs (eg pilates or yoga). “Perhaps slow gentle breathing is not a risk factor, unlike heavy, deep or fast breathing and voices,” according to Dr. Guan Nate of the London School of Health.

Scientists are aware that countries that have drastically reduced coronary heart disease (eg Greece) must be vigilant about possible new outbreaks of hyper-transmission in order not to lose their hard-earned profits (eg N). Korea rushed to relax the restrictive measures and showed a new escalation). The big challenge, according to Kurtzarski, is for health authorities to focus on where a new transmission is likely to emerge and – if necessary – to move only to local and not national restrictive measures, thus avoiding hitting the whole society.

Source: RES-EIA

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