The new coronavirus has been worrying the world for three months. The real-time updated data plus various professional terms can't help but dazzle. How should these data be interpreted? Why are they so important?

(Voice of Deutschland China) What lies behind the number of basic infections is the potential of viral infection. When the number of basic infections is greater than 1, it means that each infected person will spread the disease to at least one person, and the virus will spread. When this number is less than 1, it means that fewer and fewer people are infected and the number of patients is gradually decreasing.

To control the spread of the virus is to make the number of basic infections less than 1.

And what factors will affect the number of basic infections? Mathematician Adam Kucharski will answer in the article. The Robert-Koch-Institut, which is responsible for the prevention and control of infectious diseases in Germany, estimates that the basic infection number of the new coronavirus (SARS-Cov-2) is between 2.4 and 3.3. This means that each infected person will continue to infect two to three people.

In other words, to control the epidemic, approximately two thirds of the infection chain must be cut off. Since no effective vaccine has hitherto existed, people cannot fully protect themselves from infection. Therefore, it will be difficult for the virus to spread after 60-70% of the people are infected with the new coronavirus.

Incubation period

The incubation period of new crown disease is 1-14 days, but the average is five to six days. The reason is not clear. Experts currently believe that patients with new crowns can begin to spread the virus 24 to 48 hours before symptoms appear. A study from Shenzhen, China, found that a quarter of infections were transmitted by those who had not yet developed symptoms.

Once the infected person has symptoms, the milder the infectivity lasts for 7 to 12 days. If the symptoms are severe, the infectivity can be more than two weeks. The new coronavirus is mainly spread by droplets. The study found that the new coronavirus in respiratory mucus is longer active than the new coronavirus in saliva. There is no evidence to prove that the new coronavirus can be transmitted through feces.

Fatality rate

The case fatality rate reveals the risk of death in patients with new crowns. It is not easy to calculate the mortality rate of the current pandemic. The reason is that the case fatality rate is closely related to the time and place of the outbreak. Whether it is China, Italy, or the United States, the countries that have recruited the New Corona Virus have different health systems, population age structures, patients with basic diseases, and social forms and lifestyles.

Various factors, including the above reasons, determine the susceptibility of people in various countries, so simple data comparisons cannot be made.

In addition, what kind of data people use as a denominator has a great influence on the calculation of the case fatality rate. For example, if the number of deaths on the first day of an outbreak is divided by the number of patients on that day, the number of patients on that day must be large, and the number of deaths is relatively small. The resulting fatality rate is very low. This is what we can observe from the beginning of the outbreak of China.

Another situation is that many infected people may not be found or included in the statistics. As a result, the proportion of deaths among patients will be quite high. Iran is such an example.

Kuhalsky, a mathematician and epidemiologist at the London School of Hygiene and Tropical Medicine, believes that these two effects will offset each other in the development of a pandemic. He estimated that the actual case fatality rate should be between 0.5% and 2%, that is, one or two of the 100 patients died.

Actual number of infected

In short: this data is currently unknown. Although the World Health Organization, Johns Hopkins University and the Robert Koch Institute have been monitoring and publishing confirmed cases, the emphasis here is on "confirmed" cases. We can only roughly estimate the actual number of people infected and the speed of virus transmission from these data. And it depends on a country's testing capabilities and how many people are willing to be inspected.

Based on the latest death toll, one can estimate the actual number of infections. Thomas Götz, a mathematician at Koblenz-Randau University, calculated that Italy should have 40,000 infected people by the end of February. This is 50 times the 800 cases diagnosed at the time.

This algorithm is not applicable to Germany, "because the number of deaths in Germany is currently relatively low, it is not suitable for international comparison." Germany also lacks the latest data on hospital attendance.

index increase

It is not easy to understand the non-linear growth trend. When it comes to growth, we instinctively think of something that continues to increase: one today, two tomorrow, and seven after a week. The spread of a virus shows an exponential growth (geometric growth) rather than a linear growth: one patient infects another person, and each of these two people infects another two people. Each of these four people passed on two more people. And so on.

Filling a chessboard with rice grains helps to visualize this way of growth: we imagine that we want to fill a chessboard with rice grains. From the chessboard angle A1 on the left side of white chess, put a grain of rice in a chessboard every day, if it grows in a straight line, 64 There will be 64 grains of rice after days, and if it increases exponentially, there will be an amazing 9.223.372.036.854.775.808 grains of rice on the chessboard after 64 days.

Sometimes people can't help but compare the confirmed cases of different countries. But this does not explain anything, because the relevant figures are growing rapidly, one at a time. If you want to track the spread of the virus, you must pay more attention to the speed at which the number of patients doubles. The time required to double the number of patients is getting shorter and shorter. Once the patient's rate of doubling slows, although some people will still be infected, the rate of virus spread is slowing.

Factors affecting the number of basic infections

Kuhalski understood the virus transmission process by studying mathematical models of infectious diseases. The results of these studies can help politicians and medical experts formulate relevant policies to slow the spread of the virus.

This scientist has done mathematical models about Ebola, SARS and influenza, and now he is focusing on the research of new coronavirus. In his new book "The Rules of Contagion: Why Things Spread-and Why They Stop", Kuhalsky believes that four parameters can be used to describe the spreading potential of a disease. Their English abbreviation is DOTS.

Infectious period (Duration): The longer a person is sick, the longer time it can infect others. The sooner you isolate this person, the less chance they will infect others.

Communication opportunities (Opportunity): the virus have much chance to spread from one person to another person? This variable basically describes our social behavior. Kuhalski estimates that each person makes physical contact with others about five times a day. If we increase social distance, such as saying hello and no longer have physical contact, the chances of virus transmission will also decrease.

Propagation probability (Transmission probability): If the two meet, the probability of another person's virus spread from one person be? Kuhalsky and his team estimate that this spread occurs once in every three meetings.

Susceptibility (Susceptibility): When this virus to spread to another person, then the possibility of the sick individual and how much? Because there is currently no protection mechanism and vaccine against the new coronavirus, its susceptibility is almost 100%.

The rest is arithmetic: multiply D, O, T and S to get the basic infection number. Each reduction in parameters will slow down the spread of the virus accordingly. In general, the role of vaccines is particularly prominent. Before effective vaccines were introduced, people could only work on D, O and T: isolate patients, avoid social contact, cough at elbow fossa and wash hands frequently.

The current goal of these measures is to "flatten the curve". The number of patients cannot exceed the range that the medical system can withstand, thus avoiding the situation where doctors must choose who to treat.

Why are the fatality rates in individual countries so different? For example, Italy and Germany have the same number of diagnoses, but the case fatality rate is very different.

The University of Bonn economists Moritz Kuhn and Christian Bayer tried to find answers to the questions. Clinical figures show that the older the patient, the higher the mortality rate. Kuhn and Bayer estimate that people are particularly susceptible to infections in their daily work, so office workers are particularly likely to be infected.

Regarding the social composition of the population, there are currently different models. Generations can live separately (pictured country A) or live under the same roof (pictured country B).

Two economists at the University of Bonn found that the more office workers and their parents live together, the more frequent the intergenerational exchanges, the higher the fatality rate. If their theory is established, then India, Taiwan, Thailand, and Serbia and Poland in Europe are all countries that are more threatened by the epidemic.

However, the examples of Asian countries do not fit this trend. Bayer speculates that this may be related to different cleaning standards and physical contact methods in various countries.

Kuhn suggested: First of all, we should reduce the contact between the elderly and young people. Not only that, if we want to control the spread of the virus, we must fundamentally reconsider our social networks, the elderly should also avoid contact with their peers, and dual-worker families return to the single-worker model. "We are crashing into the tail of a traffic jam at a speed of 180 kilometers per hour. The only thing we can do now is to brake with full force. Then see if we can stop in time, or even if it hits, the damage is not too great."