COVID-19. SARS-Cov-2 pandemic, transmission pathways, distribution features, and individual susceptibility
Authors: Tikhonov Dmitrii , Vladimirtsev Vsevolod 
Published: December 2020

In December 2019, an outbreak of pneumonia of unknown etiology was registered in Wuhan, Hubei province of the people's Republic of China. The virus was soon isolated and its genome sequenced. It is called the severe acute respiratory syndrome coronavirus‑2 (SARS-Cov-2, English SARS-Cov-2), and the disease caused by it is coronavirus infection – 19 (English COVID-19). Who recognized the COVID-19 outbreak as a pandemic on March 11. The entire world is currently affected by the pandemic. The first focus of coronavirus infection in Russia was detected on February 27, brought from Europe. The infection reached the most remote corners of Siberia by mid-April.

The aim of this study is to analyze the characteristics of SARS-Cov-2, its pathways into the body and individual susceptibility to the virus.

Methods and materials. The review of scientific articles on the research topic was based on the analysis of scientific articles on COVID-19. Articles were searched in the Web of Sciences, Scopus, PubMed, and eLIBRARY databases, as well as by article links.

Results. The SARS-Cov-2 virus is a single-stranded positive-chain RNA virus from the Coronavirus family (Coronaviridae). According to most researchers, the SARS-Cov-2 virus evolved from bat coronaviruses, with the approximate time of divergence from the nearest bat virus species RaTG13 occurring in 1963. It uses ACE-2 receptors, which are widely present throughout the body, to enter host cells. High virus contagiousness is provided by the acquisition of an additional furin site for cleavage of the spike protein in the form of the amino acid sequence Arg-Arg-Ala-Arg (682RRAR685). This site of the S1 domain of the spike protein can be cleaved by: transmembrane serine protease 2 (TMPRSS2), furin, but also many cellular and extracellular proteases, as well as plasmin(ogen) s. Many ways of cleavage of the spike protein significantly increase the ability of the virus to enter the cell and its contagiousness.

The main routes of transmission of SARS-Cov-2 are respiratory drops and close contact. The main entrance gate of the virus is the respiratory tract, may be conjunctiva, likely fecal-oral pathway. The article discusses the skin as an entrance gate. Some skin manifestations of the disease can be caused by this way. The incubation period of COVID-19 lasts on average 5-6 days, while the live infectious virus begins to be released 2-3 days before the first symptoms appear and stops on the 8th day after the symptoms appear, but only in severe patients the virus release can last up to 15 days. Asymptomatic patients may account for 40% of cases. Features of individual susceptibility to COVID-19 and the severity of clinical manifestations may be caused by: 1) the property of allelic variants of the virus and their virulence; 2) the infectious dose of the virus; 3) the use of protective equipment; 4) individual characteristics of the human body; 5) pathogenic mechanisms of infection development.

The hypothesis of the protective role of the mumps vaccine explains the phenomenon of extremely low morbidity, asymptomatic or mild infection in children more convincingly. Mass vaccination against mumps in our country began in 1981 (39 years ago), which is probably why children and people under 40 rarely get a severe form of infection in our country.

Conclusion. SARS-Cov-2 has pandemic potential and is estimated to be more severe than pandemic influenza viruses. Active isolation of the virus before the onset of symptoms, including by asymptomatic patients (including children), causes the rapid spread of infection and reduces the effectiveness of anti-epidemic measures. The presence of a significant segment of the population with cross-immunity to SARS-Cov-2, including and as a result of vaccination, it is the most likely cause of a high percentage of asymptomatic and mild forms of the disease among children and young people. Effective protection against coronavirus infection in 2019 can only be achieved by taking comprehensive measures to prevent the virus from entering the body through the respiratory tract, per os, conjunctiva and skin, although the latter pathway is not taken into account anywhere in the world. It should be noted that COVID-19 cannot be classified as a particularly dangerous infection, but its high contagiousness, the likelihood of multiple entry gates of the virus into the human body, multi-organ lesions and a high mortality rate of risk groups make it a special infection that requires significant efforts of humanity to eliminate it.