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Coronavirus: what is the reproduction number R?

Again and again there is confusion about the reproduction number R. For example at the beginning of November 2020. The number of registered new infections in Germany continued to rise and most recently reached around 20,000 cases per day. It seemed paradoxical that the 4-day R-value fell from over 1 to just 0.79 in the same period. The 7-day R-value was still 0.93 (as of November 5th, 2020).

The RKI explains such apparent contradictions in numbers with the "nowcasting" method. The R value is estimated taking into account the delay in diagnosis, reporting and transmission. So it is not a daily or hourly calculation.

This is how the number of reproductions is determined

The spreading potential of a virus is hidden behind the reproduction number R. If the number of reproductions is greater than 1, each infected person transmits the disease to at least one other person - the virus spreads. If the number is less than 1, fewer and fewer people are infected and the number of infected people decreases. In order to curb the spread of a virus, its reproduction number must be less than 1. Expressed mathematically: R <1.

The epidemiologist Adam Kucharski explains which factors play a role here. He works on mathematical models of infectious diseases in order to better understand their course. This understanding, in turn, can help politicians to make decisions that are intended to curb the spread of a virus and sometimes make responsible citizens tremble about their rights to freedom.

More: Coronavirus Numbers: What do all the numbers mean?

Four adjusting screws

In the past, Adam Kucharski has already researched diseases such as Ebola, SARS and influenza, now on COVID-19. In his book "The Rules of Contagion: Why Things Spread - and Why They Stop "he names four parameters that describe the contagion potential of a disease. In English they start with the letters D-O-T-S (dots, in German: points).

  • Duration: Corresponds to the duration of the infectiousness. The longer a person is sick, the longer they can infect other people. The sooner a sick person is isolated from others, the less time they have to pass the virus on to others. Problematic with SARS-CoV-2: Those affected seem to become infectious two to three days before the onset of symptoms. Time in which you can infect other people unnoticed.
  • Opportunity: How many people does an infected person have contact with in such a way that the virus can get from one person to the next? According to Adam Kucharski, this happens on average about five times a day under normal circumstances. The variable quasi depicts our social behavior. It can be reduced if we increase the social distance, for example waving to each other in greeting rather than hugging.
  • Transmission probability: How likely is it that the virus will actually be transmitted from one person to the next when two people meet? Adam Kucharski and his team assume that this could happen on every third opportunity.
  • Susceptibility: Given the time, opportunity, and likelihood of transmission, how likely is it that a person will receive a virus and become ill with it? Since there is currently no vaccine and the infection rate (and thus the rate of potentially immune) is quite low, this variable is almost 100%. It will decrease with increasing immunization. However, it can only be decisively reduced by vaccination across the board.

More on this: Coronavirus: FAQs that move the world

Yardstick for political decisions

The rest is math: Multiply D, O, T and S give the reproduction number. All four parameters are adjusting screws to prevent the virus from spreading. Vaccinations in particular are usually effective for this. Since these do not yet exist, you can only work on D, O and T: isolate the sick, avoid social contacts, cough in the crook of your arm, wash your hands.

The effective reproduction number R must be distinguished from the basic reproduction number R0. While R indicates how many people infected on average after containment measures have been taken or a part of the population is immune, R0 describes how many people an infected person infects without countermeasures. R0 assumes that no one is vaccinated, no one had the disease and is therefore immune and there is no way to contain the spread.

The Robert Koch Institute, which is responsible for monitoring infectious diseases in Germany, assumes that the basic reproduction number of SARS-CoV-2 is between 2.4 and 3.3. Without countermeasures, each infected person would infect around two to three other people. In other words: in order to bring the epidemic under control (i.e. R <1), around two thirds of all transmissions must be prevented.

The currently applied measures aim to "flatten the curve" (English: "to flatten the curve"). The number of sick cases should not exceed the capacity of health systems. So that doctors do not have to decide which patients they can treat and which not.

More on this: The immune system in the fight against corona

This article from April 28, 2020 was last updated on November 5, 2020

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    It's about the type of germ

    Not all germs are equally dangerous. In the case of salmonella, which is transmitted through rotten eggs, a healthy person has to swallow a good 1000 to get sick. In the case of legionella, which arise in the warm, humid climate of hot water systems, fewer than 100 inhaled germs are sufficient. Even clean air already contains hundreds of bacteria and fungal spores.

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    Silent place is usually cleaner than expected

    On average, there are fewer than ten germs on a square centimeter of toilet seat. This makes the toilet one of the cleanest places. Even some window panes are more contaminated with germs and fungi, because they are only cleaned once every six months. The toilet, on the other hand, usually several times a week.

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    Keymagnet keyboard

    It looks much worse at the workplace: an average desk contains over 3000 microbes per square centimeter - 400 times more than a toilet bowl. The computer keyboard is the most contaminated. Because here the dirt has the best conditions to get stuck between the keys and in the cracks. Over 10,000 germs per square centimeter are not uncommon on keyboards.

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    Money stinks

    Bills and coins go from hand to hand. New York researchers have genetically identified up to 3,000 different germs on banknotes. With sensitive measuring methods, you can even find traces of cocaine on most notes - because they are often used to sniff. In any case, sellers should never touch food and money one after the other.

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    Incubator for molds

    Even the supposedly clean refrigerator contains a large number of germs. In any case, the humid climate and the fat and sugar that are present are ideal for mold. Even with regular cleaning, they can still find a niche somewhere - for example behind the rubber seals on the doors.

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    Hospital risk area

    Hand hygiene must be meticulous, especially in hospitals. Because here the spread of resistant bacteria can quickly become fatal. That is why many hospitals have dispensers with an antibacterial solution in addition to the wash basin with soap. Before visiting the patient, it is important to disinfect your hands.

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    Dangerous culprit

    "Methicilin-resistant Staphylococcus aureus" (MRSA) is the name of this highly dangerous germ. The known antibiotics no longer work here. Even without food, the stubborn pus can survive for seven months - on the floor, the table, the bed, on the skin and of course on the many door handles.

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    Copper against germs

    A clinic in Harburg has now started a promising experiment to reduce the germ load on door handles. The bacteria do not like copper. The number of germs was reduced by about half. But this should not be a substitute for hand washing, because there are still enough germs left.

  • Ugh, ugh, uh, germs everywhere!

    Please don't be too rude

    Shall we stop shaking hands now? This might be a good idea in the hospital, but otherwise this precaution would be excessive. It is better to ensure a good level of hygiene: wash your hands again and again, clean the mouse and keyboard regularly, do not touch the food immediately after paying and wipe the refrigerator more often!

    Author: Fabian Schmidt