The COVID-19 pandemic (colloquially known as coronavirus pandemic, corona pandemic, coronavirus crisis or corona crisis) is an outbreak of the novel respiratory disease COVID-19 (or "Covid-19", for corona virus disease 2019). This disease first became noticeable at the end of December 2019 in the megacity of Wuhan in the Chinese province of Hubei, developed into an epidemic in China in January 2020 and finally spread worldwide. The outbreak was triggered by the previously unknown corona virus SARS-CoV-2. This virus is also known as the novel corona virus.
In order to counteract its spread to countries without efficient health systems, the World Health Organization (WHO) declared an international health emergency on 30 January 2020. From 28 February 2020, the WHO assessed the risk at a global level as "very high" in its reports (WHO risk assessment, global level: very high), previously as "high". On 11 March 2020, the WHO officially declared the previous epidemic a pandemic, the first since pandemic H1N1 2009/10.
On 13 January 2020, the first laboratory-confirmed infection with SARS-CoV-2 was reported from Thailand - and thus outside the People's Republic of China - and on 23 January 2020, the first case of infection outside Asia was reported in the United States of America. In each case there was a connection to travel to or from Wuhan. On 2 February 2020 the first case of death outside China occurred in the Philippines. It was a Chinese person from Wuhan. On 9 February 2020, the number of registered deaths exceeded the total number of deaths from the 2002/2003 SARS pandemic, at over 800. On 15 February 2020, France reported the first death outside Asia, a person who had travelled in from China. On 23 February 2020, Italy reported the first two European deaths from COVID-19. The WHO report of 26 February 2020 reported for the first time more new infections outside China than inside; with the WHO report of 16 March 2020, the number of cumulative infections outside China (86,434) then exceeded the number inside China (81,077).
On 7 March 2020, the WHO reported over 100,000 infected people worldwide for the first time, with 3,486 deaths. On 19 March, the WHO reported over 200,000 infected people for the first time, with 8,778 deaths. Only four days later, on 23 March 2020, the WHO reported over 300,000 infected persons, with 14,510 deaths, and after only two more days, on 25 March 2020, over 400,000 infected persons, with 18,440 deaths. Italy was increasingly becoming the new centre of the pandemic; on 19 March, for the first time, more deaths were reported here than in China. On 25 March, Spain also overtook China in the number of victims. On 23 March 2020, apart from China, Italy and Spain, most cases of infection occurred in the United States, Germany, Iran and France; only a few new infections were reported from China. In the evening of 26 March, the number of infections in the USA alone, with 81,578 cases, exceeded the number of cumulative infections in China (81,340) for the first time. Within one day an increase of more than 13,000 cases had been reported there. Only one day later, over 100,000 infected persons were reported in the USA for the first time. North America is thus developing into the new centre of the pandemic alongside Europe and after China. As of 26 March 2020, 195 countries or territories are affected: 21 countries in Asia, 22 in the Near and Middle East, 41 in Africa, 57 in Europe, 47 in America and 7 in Australia and Oceania. In a large number of countries, the pandemic is now characterised by massive cuts in the public life of society and in the private lives of its citizens, as well as by an economic crisis.
The disease COVID-19 and the pathogen SARS-CoV-2, which triggered the epidemic in the People's Republic of China at the end of 2019, were not known before. In the course of the event, which was initially classified as a local outbreak and relatively quickly became an epidemic, it was therefore first necessary to gain substantial knowledge about the disease. This concerned the disease-causing trigger as well as basic parameters that determine the danger of a disease (see virulence and pathogenesis of the triggering virus SARS-CoV-2), and the facts concerning its spread and possible measures against it.
At the same time as the fundamental problem of first finding out what it is (see History of the discovery of SARS-CoV-2), the rapid spatial spread and other characteristics of the disease made it difficult to collect sufficiently accurate data to determine its spread, extent and dangerousness with scientific precision (see Model calculations below). This refers to the similar symptoms of the new disease COVID-19 and the various forms of viral influenza: Coughing, breathing difficulties and rising fever (see Clinical manifestations of COVID-19). A particular problem is the apparently high infectivity already in the incubation period before these or other symptoms appear.
Initially, local authorities wrongly did not assume human-to-human transmission, direct infection, as this is the exception rather than the rule in the case of a pathogen that passes from animals to humans, a zoonosis, and it was thought that no such cases were initially identified. After the pathogen was identified as a novel subspecies of the virus species SARS-associated coronavirus (short SARS), the disease control authorities assumed conditions similar to those of the previously known disease Severe Acute Respiratory Syndrome, caused by the virus previously known briefly as SARS and later referred to as SARS-CoV-1 in distinction to the case described here and the SARS pandemic 2002/2003 triggered by it. As it later turned out, these assumptions were wrong, especially with regard to human-to-human transmission and spread of the disease by carriers with no or only very mild symptoms. In the course of February 2020, it became clear that COVID-19 is transmitted much more easily than SARS, that many infected persons show no symptoms and that, on the other hand, carriers of the virus without clear symptoms can also be contagious. Cases have even been reported in which people have infected other family members without showing symptoms themselves. In addition, the authorities in Wuhan were overwhelmed, which led to such a high number of unreported cases in this region "that the cases counted there are not very meaningful", according to epidemiologist René Niehus.
Various factors are considered in assessing the risk of an epi or pandemic: A distinction must be made as to whether a disease is dangerous for a specific individual or for a specific population group, for the population of a state, a region or the whole world.
In its daily reports, the WHO therefore lists the risk situation separately for China, regional level and global level (as of 26 February 2020). The most important characteristics of an epidemic are the transmission rate, the dynamics of spread over time and space, the proportion of severe courses, lethality and the incubation period. Some of these data can only be determined retrospectively, see the section Problems of interpretation in the article Letality. Others are largely determined by human behaviour and are therefore highly variable. This is particularly true for spatial distribution and baseline reproductive numbers, which are strongly influenced by individual factors such as travel behaviour, communication and contact types and hygiene behaviour.
On 19 February 2020, the WHO summarised the scientifically substantiated findings (partly based on estimations by model calculations) up to that date:
Time between infection and appearance of symptoms (incubation period): 5-6 days (in individual cases up to 14 days)
Time between "Case 1" and the "Case 2" connected to it in a transmission chain (serial interval): about 5-7 days
Letality: not reliably known. The WHO, like science in general, distinguishes between two forms of calculation. One is the ratio of known deaths to confirmed cases of disease, the cCFR (WHO) or only CFR for (comfirmed) case fatality rate. The second is the statistical probability of dying from the consequences of an infection, the IFR for infection fatality rate. For both variants, only limited reliable statements are possible during a dynamically spreading disease wave and presumably high numbers of unreported cases. The statistical probability of an infected person dying regardless of individual characteristics, the IFR, is given by the WHO as 0.3-1 %, a study cited by the WHO cites a provisional estimate of 0.4-2.6 %, with the most probable value given as 0.94 %.
A Chinese study published in mid-February, in which 44,000 confirmed COVID-19 cases were examined, concluded that the lethality of COVID-19 at the time of publication is not as high as that of other corona viruses, such as SARS or MERS. More than 80% of patients have mild forms of COVID-19 and in about 14% of cases the virus causes disease progression with dyspnea. About five percent of patients experienced respiratory arrest, septic shock or multiple organ failure. In two percent of cases COVID-19 is fatal According to the study, the risk of dying increases with age. According to the WHO, there are relatively few cases of sick children. In a mild course of the disease (the most common case), the signs of the disease, if any, usually subside within two weeks, according to the WHO. People with a severe course of the disease take between three and six weeks to recover from the disease.
The study by Mike Famulare, Institute for Disease Modeling, cited by the WHO, estimated, using a framework for assessing influenza epidemics and pandemics and its scales for transmissibility and clinical severity, that COVID-19 was equally transmissible as Spanish flu, but one level lower in clinical severity, and thus one level less severe than the latter, but two levels more severe than Asian flu and Hong Kong flu. A general comparison between the COVID-19 epidemic discussed in this article and influenza is not possible, because the influenza virus is extremely mutable and each flu epidemic, each flu season, differs significantly in its basic values. Since corona viruses are also mutable - although not as much as influenza viruses - this is to be assumed for COVID-19, the virus should become endemic. This is the case if it cannot be eradicated as in SARS or strongly contained as MERS can
Virologist Alexander S. Kekulé, who has been quoted several times in the German media with the statement "five times more dangerous than influenza", therefore explicitly referred his comparison to the seasonal, currently mild flu Lothar H. Wieler, head of the Robert Koch Institute (RKI), gave a similar assessment in a press conference on 27 February 2020. He compared the percentage of deaths among infected persons for a light flu season with a lethality of 0.1-0.2% with an assumed 1-2% for the SARS-CoV-2 epidemic. The virologist Jonas Schmidt-Chanasit classified the situation regarding COVID-19 at the end of February 2020 in principle as rather worrying, but concluded at that time that COVID-19 was still far from the level of seasonal influenza with about 650,000 deaths per year worldwide, even though this could soon change. He referred to the lack of immunity of the population to COVID-19 in contrast to normal, seasonal influenza, as well as to the lack of vaccination possibilities. In the influenza season 2019/20, from 40th week 2019 to 11th week 2020, a total of about 165,000 laboratory-confirmed influenza cases, 23,646 of which were hospitalized, and a total of 265 deaths with influenza virus infection were reported to the Robert Koch Institute. (data status: 13 March 2020)
In its Situation Reports, the WHO does not provide figures for those who have recovered or for those currently reported as infected (i.e. the number of infected persons confirmed to date minus those who have died or recovered). Nevertheless, in order to understand an epidemic, in addition to the development of the total number of infected persons, the number of infected persons at the current time, i.e. those infected who have neither died nor recovered, is relevant, as is the number of those recovered and thus probably future immune cells. The numbers of those recovered and dead are delayed in comparison to the number of infected persons - by the time between confirmation of the infection and the end of the disease through recovery or death. In particular, they do not allow simple conclusions to be drawn about the dangerousness or lethality. The Center for Systems Science and Engineering (CSSE) at Johns Hopkins University developed an alternative collection of cases reported by local authorities and presentation of the data via an online tool, see web links below. The figures given here are subject to greater caveat than the WHO data shown elsewhere because there is no standard definition of recovery. Time wrote on 4 March that the figures from China were strictly speaking 'hospitalised', as the figures only covered those who had been discharged from hospital. In China, anyone who has been free of fever for three days and has twice tested negative for the virus is considered to have recovered, but it is doubtful whether, under the strong pressure on the health care system, all those discharged were really no longer contagious. She further wrote that there are no official figures in Germany because there is no obligation to report convalescence, and quoted a health office director who said that they are not allowed to give the Robert Koch Institute (RKI) any information about convalescence at all for reasons of data protection. In Germany, infected persons are not considered to have recovered until ten days after the onset of their symptoms at the earliest; in addition, they must not have had a fever for 48 hours, not have had any further symptoms for more than 24 hours, and two swabs taken from the nasopharynx at least 24 hours apart must be free of the virus. Early discharge practice in China and in other individual cases is particularly problematic for the question of the risk of relapse, since some cases were tested positive for the virus again later, thus raising the question of whether multiple infection is possible. However, those who were supposedly doubly ill had probably only been discharged from hospital too early.
Since the virus first spread to humans as a zoonosis, the population had no adaptive immunity, i.e. acquired immunity through previous contact with the pathogen or vaccination Cross-immunity due to very similar, already circulating pathogens is also not known or is unlikely to exist Herd immunity is mainly important for the development of an epidemic in the total population. This increases with the proportion of the immunized population and slows down the spread until the net reproduction rate is below 1 and further spread is reduced. For COVID-19, the estimate of the proportion of infected persons from which the net reproduction rate falls below 1 is around 60% to 70% of the population. A policy that relies on the rapid effectiveness of herd immunity in its local authority would therefore have to accept that 60% to 70% of the population would become infected within a relatively short time. To prevent risk groups from being "infected" just as quickly, they would have to be isolated from the rest of the population.
Comparisons with other corona viruses lead most researchers to assume that people acquire immunity after living through COVID-19 or being infected with SARS-CoV-2, but these assumptions still need to be scientifically proven. According to studies of the four already known endemic corona viruses that cause colds, immunity is short-lived and even people with high levels of antibodies in their blood can still be infected with viruses from this family. However, for the two corona viruses SARS-CoV-1 and MERS-CoV, which triggered the previous epidemics, the evidence is unclear due to lack of data. According to previously unpublished data, antibodies could still be detected in the blood for the coronavirus SARS-CoV-1, which was already known before COVID-19, 15 years after infection .
Whether this can also be assumed for the current SARS-CoV-2 is open. There were also doubts about the acquired immunisation in SARS-CoV-2 due to people who were supposedly reinfected (see section on recovery). However, after a preliminary publication in March 2020 with an outstanding peer review, researchers were able to prove in animal experiments that rhesus monkeys cannot be reinfected after surviving infection. The researchers interpreted their results in such a way that primary infection with SARS-CoV-2 could certainly protect against later infections.
In mid-March 2020, Christian Drosten's summary assessment of immunity to SARS-CoV-2 (COVID-19) is as follows:
"Once infected, a person is probably immune for the rest of the pandemic, I would think for a few years. And even if you get infected again, you will get it as a harmless cold. The next infection is then no longer difficult. At least that's what I'd appreciate right now."
- Christian Drosten in an interview in the time of 20 March 2020.
Investigations into which parts of the population have particular risks of infection, disease or serious consequences have not been completed and must be carried out separately for different populations. At the end of March 2020, the Süddeutsche Zeitung summarised for the explanation of the differences in the figures for different population shares that until then there were too few studies on sufficiently large patient groups to support the hypotheses Nevertheless, these differences in the statistics indicate that pre-existing conditions such as cardiovascular disease, diabetes, chronic respiratory diseases, high blood pressure or cancer increase the risk of severe courses. According to the RKI, not only the elderly and those with previous illnesses, but also smokers are risk groups for a severe course. Furthermore, young people can also be among the risk groups, e.g. with a high body mass index or coronary heart disease.
An investigation of deceased persons with positive COVID-19 findings in Italy (as of 17 March 2020) showed that only 0.8% of these persons had no previous illness, while 25.1% of deaths had one and 25.6% had two previous illnesses. Three or more pre-existing conditions were found in 48.5% of cases.
In prevention, a distinction must be made between social prevention and individual prevention.
The aim of social prevention is to stop or slow down the spread of an epidemic as far as possible. In addition to the characteristics of the virus (see section #Dangerousness of the disease), the spread of an epidemic is largely determined by the social behaviour of people. Therefore, hygiene measures, early detection and the management of contact persons (identification and, for example, domestic isolation or quarantine) are the first measures to prevent or slow down the further spread of the virus. In the case of a novel pathogen, an attempt is made to stifle the spread in the early stages and to eliminate the pathogen by means of a containment strategy. The measures are:
In the case of a further spread, various measures are taken, all of which aim at the so-called Social Distancing, German Spatial Distancing, i.e. the increase of the social distance, the reduction of contact. For in the case of pathogens that are transmitted from person to person, social proximity determines to a large extent the basic reproduction rate, i.e. the number of people an infected person infects. These measures include:
When a pathogen becomes endemic and is no longer considered containable and eradicable, the precautionary objectives change from controlling the spread to slowing the spread and health care. In this phase, the overriding goal is "to avoid the simultaneous sickness of an extremely large number of people in need of treatment and the resulting threat of overloading the medical infrastructure" The National Pandemic Plan for Germany defines the following goals:
Experts call for special protection for risk groups such as the elderly and people with pre-existing conditions Virologist Christian Drosten of the Berlin Charité Hospital therefore recommended that families should no longer send their children to grandparents for care in the near future. He also suggested that older people should do their shopping so that they would no longer have to go to the supermarket. Employers should, if possible, enable chronically ill people to work from home The entire population is called upon to avoid unnecessary social contacts.
Prevention by means of immunisation through vaccination is not yet possible, work is being done on the development of coronavirus vaccines. Individuals can minimize the risk of infection by reducing contact (see above) and hygiene.
Individual measures in the COVID 19 pandemic
Specifically, the World Health Organization (WHO) and others (shown separately) recommend the following:
So far, there is no specific treatment for COVID-19 disease, a therapy aims to alleviate the symptoms. However, it is being investigated whether already known antiviral drugs are also effective against an infection with SARS-CoV-2.
The triggering virus is almost certainly of animal origin (zoonosis), although the reservoir of the pathogen is not yet completely clear. Initially, it was suspected that the virus originated in bats, from which the virus was transmitted to humans via an intermediate host, probably another mammal As of the end of March 2020, it is assumed that the virus is equally likely to have originated in pangolin animals. Finally, it is suspected that recombination occurred in a double infection and that the virus SARS-CoV-2 is a new chimera from these two earlier viruses .
The first known cases of human disease occurred in December 2019. Since several of the first infected persons worked as sellers or traders on the "Wuhan Huanan Wholesale Market for Fish and Seafood" (Chinese 武汉华南海鲜批发市场, Pinyin Wǔhàn huánán hǎixiān pīfā shìchǎng), the primary site of infection was suspected to be there. On the wet market, not only marine animals but also many other wild animals used in Chinese cuisine and traditional Chinese medicine were traded. Later, after a study by Wuhan Hospital, it became known that the first identified patient had not visited this market. On 31 December 2019, the Chinese Epidemic Control Agency sent a team to Wuhan to investigate the unclear cases. Initially, the authorities and the WHO assumed that the pathogen of the infection was not transmissible from person to person or only with great difficulty.
Government documents show that the first patient whose samples are subsequently attributed to COVID-19 may have become infected on November 17, 2019, according to a March 2020 report in the South China Morning Post. From then on, between one and five cases were registered daily, nine in November. None of the early cases is considered to be patient zero. According to the report, by 15 December 27 would have been infected, on 17 December two-digit new infections would have occurred for the first time, on 27 December more than 180 people would have been infected, by the end of the year at least 266 people, and at the beginning of the year 381 infections would have occurred without these cases, which were unclear at that time, being made public. In an article in Science, the origin of the outbreak was dated 18 September 2019.
The local health authority was informed of the finding of a SARS-like pathogen by 27 December at the latest. Following the experience of the SARS pandemic, China had introduced an early warning system to ensure that information on the disease was immediately forwarded to the Centre for Disease Control, regardless of political considerations. However, this did not happen in the case of the novel coronavirus .
On 30 December, Chinese doctor Li Wenliang, in a WeChat group with colleagues, warned of a series of pneumonia (pneumonia) in the local hospital in Wuhan of a virus that he thought at the time was causing severe acute respiratory syndrome (SARS) After their warning spread on the Internet, Li and at least seven other colleagues were summoned by the police. They were accused of having made "untrue allegations" that "seriously disrupted the social order" and were forced to sign confidentiality statements under threat of harsh punishments, which Li Wenliang later violated. On 1 January 2020, the Xinhua state news agency reported on the alleged "false reports" by doctors and confirmed that there were no signs of human-to-human transmission of the new disease.  Li Wenliang died on February 7, 2020 at the age of 33, presumably from the disease he had warned of.  The doctors had continued to send patient samples to analysis laboratories on their own initiative, past the official channels, in order to investigate the cause of the disease on their own initiative.  They were only rehabilitated by the Supreme People's Court at the end of January. 
On December 31, 2019, the Chinese authorities officially informed the World Health Organization (WHO) that several cases of severe pneumonia had occurred in the city of Wuhan since the beginning of December 2019, the pathogen of which could not yet be identified and for which a previously uncharacterized pathogen was assumed to be the trigger. The report was spread by the news agencies on the same day  The American CDC then also learned of the pneumonia accumulation in Wuhan. An epidemiologist of the CDC, who had previously worked embedded at the Chinese disease control authority and whose job it would also have been to pass on information about potentially dangerous outbreaks at an early stage, had been recalled by the US government in July and the position had not been filled again. 
Between 31 December 2019 and 3 January 2020, a total of 44 cases of pneumonia with unknown cause were reported to the WHO from Wuhan.
The fish market in Wuhan was closed and disinfected by the local authorities on 1 January 2020.  On 5 January 2020, the Chinese Center for Disease Control ruled out MERS-CoV and SARS-CoV as pathogens on the basis of test results.  On 7 January 2020, the Chinese side announced the identification of a novel coronavirus in several patients. The virus was given the provisional designation 2019-nCoV (2019 novel coronavirus).  On 13 January 2020, the Thai Ministry of Health announced that the new virus had been detected in a traveller from Wuhan in Thailand. On 15 January an analogous case from Japan was reported and on 20 January a case from South Korea .
Epidemiologists concluded from the fact that several cases of illness had occurred in several long-distance travellers that the number of cases must be much higher than the 44 cases initially reported. Experts estimated the number of people infected in Wuhan to be about 1,700 on 17 January 2020.  From the fact that several members of the hospital staff involved in treating the sick also fell ill, it became clear that the virus was in principle transmissible from person to person. This was confirmed by the Chinese authorities on 20 January 2020 .
On January 18, almost 40,000 families gathered in Wuhan unhindered for Chinese New Year celebrations (potluck). It is suspected that a particularly large number of infections occurred during this event .
Meanwhile, the disease had also reached Beijing, where infected persons were concentrated in certain hospitals since 20 January .
According to official Chinese data, 2,744 infected persons had been registered in China up to and including 26 January 2020. The number of confirmed fatalities rose to 80 (all in China).  On 26 January 2020, the German Robert Koch Institute classified Hubei province including the city of Wuhan as the first risk area in China.
On 27 January 2020, Chinese state television reported an increase of 1,200 cases, bringing the total number of cases reported in China to about 4,000; a deviation of 500 from WHO figures .
At the beginning of February, the number of new infections in China initially seemed to be falling. The German Robert Koch Institute declared on 7th February 2020 that the Chinese risk areas would be expanded and that four large cities (Wenzhou, Hangzhou, Ningbo, Taizhou) in the province of Zhejiang near the coast in eastern China would be included in the previous risk area. On 13 February, the Chinese authorities changed the notification criteria and, in contrast to the WHO guidelines, clinically diagnosed cases without virus detection were also recorded. This resulted in the highest number of reported new infections for Hubei alone, with 14,000 new cases  This change was withdrawn on February 20 and the notification criteria have since been in line with WHO guidelines again. On February 14, 2020, the National Health Commission announced that 1,716 health workers were infected on the reporting date. Six of them were reported as deceased.  By 20 February 2020, there were 2,055 laboratory-confirmed infections among health care workers, 88% of them in Hubei province, in a total of 476 Chinese clinics. 
On 18 February 2020, the number of new infections per day in China was below 2,000, on 20 February it fell below 1,000 and, according to the WHO report of 26 February 2020, for the first time there were more new infections outside (460) China than inside (410)  The number of new infections per day in China, which were recorded by the WHO from 16 to 24 February 2020, was below 1,000. 3] The WHO-China joint mission to China from 16 to 24 February 2020 concluded that the radical measures taken in China were appropriate to contain the virus and to significantly reduce the number of newly infected persons since the end of February 2020. This was achieved by intensive tracking of infected persons and their contacts. Restrictions on human-to-human transmission by the cancellation of numerous major events and the extension of holidays on the occasion of the Chinese New Year also served this purpose. An extreme measure was the sealing off of the city of Wuhan, which placed the entire population in domestic quarantine  (see section Measures in China).
In his speech on 9 March 2020, WHO Director-General Tedros Adhanom Ghebreyesus stated that more than 70% of the approximately 80,000 cases of infection had now recovered and left the clinic.  The number of new infections in China on that day was 45.
The report of the WHO mission, for which scientists from eight countries spent nine days in the country in February, states that China has made "possibly the most ambitious, agile and aggressive disease control effort in human history"   .
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