Covid-19

COVID-19 ('coronavirus disease 20'19) causes coronavirus SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2, initially professionally referred to as 2019-nCoV''). Usually the disease manifests itself as an upper respiratory tract infection, in some patients develops pneumonia with a potentially serious, in some cases even fatal course. The infection may be associated with coagulopathy. Due to the global spread of the disease, who declared COVID-19 a pandemic on 11/03/2020.

Virology
The SARS-CoV-2 virus was first identified in China in early 2020 as the causative agent of an epidemic of pneumonia in the city of Wuhan. By sequencing epithelium from the respiratory tract of patients, it was possible to prove that the causative agent of the disease is the hitherto unknown β-coronavirus from the subgenus sarbecovirus subfamily Orthocoronaviridae. It is the seventh representative of the family coronaviruses, which causes human diseases. . Sekvence SARS-CoV-2 se ze 70 % shoduje s genetickou informací viru SARS-CoV. The first cases of this coronavirus at the end of 2019 were linked to a visit to a seafood and live animal market in the city of Wuhan. The probable source is a bat, e.g. Rhinolopus affinis, sinicus or ferrumequinum. Some authors believe that transmission to humans may have occurred directly, as excrement and dried parts of bat bodies are used in Chinese folk medicine. However, the virus isolated from bats differs from viruses that are transmitted interhumanly in several amino acids crucial for binding to human cells. It is therefore more likely that the transmission to humans occurred through intermediate hosts, which could be, for example, some snakes, turtles or minks. Especially discussed are pangolins, whose meat is consumed in China and whose body parts are also used in folk medicine. The sequence RNA isolated from pangolin coronaviruses differed more from SARS-CoV-2 than from bats coronaviruses, but it was identical in the domain responsible for binding to human cells. . Uvažuje se proto, že SARS-CoV-2 vznikl rekombinací velmi podobného netopýřího koronaviru s koronavirem luskounů.

náhled | Model virionu koronaviru SARS-CoV-2. Na stavbě se podílejí čtyři strukturní bílkoviny. Šedě je znázorněná obálka, kterou tvoří fosfolipidová dvojvrstva. Pod ní je nukleokapsidový protein N s navázanou ribonukleovou kyselinou viru. Do obálky virionu jsou zavzaty proteiny S, E a M. Červeně znázorněný peplomerový glykoprotein S (spike) je odpovědný za vazbu na hostitelskou buňku. Dále jsou vyznačeny proteiny E (envelope) a M (membránový protein).

Pro vstup viru SARS-CoV-2 do hostitelské buňky je klíčový jeden z glykoproteinů virionového obalu („korony“), S-protein (spike-protein). Ten se váže na angiotenzin konvertující enzym 2 (ACE2) exprimovaný na povrchu vnímavých buněk a využívá jej jako receptor. Analysis of the spike protein coronavirus identified two variants, caused by the substitution of glycineu (G) for aspartate (D) at position 614. The G614 variant was associated with higher viral loads in vitro and may therefore indicate higher infectivity. The effect on the course of the disease and the risk of hospitalization has not been proven.

Incubation period and transfer
The average incubation period is given as 4–5 days, with the range of the incubation period usually being 2–14 days. . Transfer path data is not yet complete. The basic mode of transmission is direct interpersonal contact, it is assumed that it occurs mainly by droplet infection. Droplets usually do not spread further than 2 meters and do not remain in the air. The risk of direct transmission through the air is also discussed, especially in aerosol-producing procedures, but its significance is debatable and the impact on the spread of the pandemic rather questionable. The possibilities of transmission over longer distances are likely to increase in enclosed, unventilated areas (restaurants, bus,...). .

Viral RNA has also been demonstrated in the blood, faeces and urine of patients, although these routes of transmission are probably not very epidemiologically significant. The epidemiological significance of transmission by touching contaminated areas and subsequent contact with eyes, mouth or nose is still unclear, but extensive contamination of areas near patients with viral particles that can be a source of infection has been repeatedly described. The risk of infection from surfaces is higher in the case of massive contamination, for example, in the house of a infected person.

Risk of infection depends on the course of the disease – an infected patient with a mild course of the disease usually becomes infectious approximately 2.5 days before the onset of symptoms, the highest risk is around the onset of symptoms and gradually decreases approximately by the 7th–10th day after the onset of symptoms. The risk of transmission after day 10 is small in immunocompetent patients with a mild course of the disease. Při závažném či kritickém průběhu onemocnění (dušnost, pneumonie) je obvykle pacient infekční ne déle než 20 dní. PCR pozitivita však může přetrvávat výrazně déle i při neinfekčnosti vzhledem k přítomnosti neviabilních virových částic na sliznicích. Medián doby do negavního výsledku PCR testů ze sliznic je 18,4 dne, avšak pozitivita PCR detekce virové RNA může přetrvávat až po 3 měsíce.

The degree of risk of transmission of infection also depends on the time of contact, the protective equipment used and epidemiological measures, or the amount of viral particles in the secretion of the upper respiratory tract. The most common secondary transmission has been described between members of the same household or in healthcare facilities where personal protective equipment was not used. Infection is also possible from asymptomatic carriers of the virus, who are likely to be infectious for a similar length of time as symptomatic patients, but the importance of this type of transmission for the spread of the epidemic is still unknown.

Animal infections have been described in a case-by-case manner, but there is no evidence that transmission from animals to humans is present in a significant percentage. It is assumed only at the beginning of the epidemic, the further meaning is not confirmed.

Basic reproductive constant R 0 je kolem 3.

Clinical course
The clinical course of the disease can be diverse, the manifestations range from asymptomatic or very mild to a critical course ending in the death of the patient.

The frequency of asymptomatic infections is questionable. Some studies estimate their incidence to be around 40%, but for now there are no analyses with a long enough follow-up to assess whether the symptoms did not manifest themselves later. Thus, the number of purely asymptomatic cases is probably lower.

According to the current concept, no symptom is downright pathognomonic and diagnosis solely on the basis of clinical signs can be difficult.

Symptoms of ongoing COVID-19 may include :
 * fever – the estimated incidence and height of which fluctuates significantly between studies (often only subfebrile is reported),
 * fatigue,
 * dry cough,
 * muscle pain, headache,
 * sore throat,
 * nausea, vomiting, diarrhea,
 * loss or disturbance of smell or taste – various studies report the incidence of these disorders in the range of 5–98%, but appears to be more common in the early stages of the disease than in other respiratory tract diseases,
 * shortness of breath – occurs in approximately one third of patients, typical is its onset approximately 5 days after the onset of the disease,
 * stuffy nose or runny nose.

Complications
In the course of the disease, even with an initially mild course, a spectrum of complications may develop. Originally mild pneumonia can progress, as mentioned in the previous chapter, to a more severe state with shortness of breath approximately 5 days after the onset of symptoms. Other complications include :
 *  'respiratory insufficiency'  to image failure  'ARDS' ;
 *  'cardiac and cardiovascular complications' , including arrhythmias, acute coronary syndrome or shock;
 *  'thromboembolic complications' , such as CMP or pulmonary embolism, may occur in younger patients without risk factors for complicated disease;
 * dysregulation inflammatory responses;
 * secondary infections.