The world is currently facing the greatest epidemic battle of this century. For years, the World Health Organization (WHO) has maintained a list of the pathogens that pose a real and present threat to humanity, and experts have long warned that the next great pandemic is not merely a science fiction plot. And yet, it now seems clear that we were not prepared. Even though it is not the most lethal virus we have encountered in recent times, the dizzying spread of the new coronavirus SARS-CoV-2 and the disease COVID-19 has provoked a major disruption of society and human activity on a planetary scale. Here we review what science knows about the virus and its disease, and what has already been achieved in the fight against this new scourge.
Interactive timeline: Science to Fight Coronavirus
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The origin of the virus
After a cluster of cases of pneumonia of unknown origin was detected in December 2019 and January 2020 in the Chinese city of Wuhan, in February three teams of Chinese researchers finally published all the details of the cause, a novel human coronavirus which the WHO provisionally called 2019-nCoV. The coronavirus, subsequently designated SARS-CoV-2 by the International Committee on Taxonomy of Viruses, is the seventh known human pathogen in this class. Four of them cause common colds, while the remaining two induce, respectively, Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS).
The analysis of its genome revealed that it is 79.6% identical to the SARS virus and 96% identical to bat coronaviruses, so it is assumed that it was transmitted from these animals to humans through another, as yet unconfirmed, species—pangolin was suggested as a suspect, but this has been questioned. The comparison of 93 genetic sequences of the virus indicates that it jumped to humans in November 2019. Chinese government research published in the South China Morning Post indicated that the first patient was a 55-year-old person in Hubei province who contracted the virus on November 17. Although the original outbreak was initially located in the Wuhan seafood market, there is no evidence of this place being the source.
Coronaviruses have been named as such since 1968 because under the electron microscope they appear surrounded by a barrier of rounded, crown-like projections, different from the sharp spikes of the flu virus. The envelope of the virus is formed by the Spike (S), Envelope (E) and Membrane (M) proteins, while in its interior is found the Nucleocapsid (N) protein together with its genome, a long single-stranded RNA with almost 30,000 bases.
The S protein is the key that the virus uses to invade human cells, binding to a cell-surface protein called Angiotensin-Converting Enzyme 2 (ACE2), which is found mostly in the blood vessel cells of the heart and kidney, as well as in the epithelial cells of the lungs and intestine. The cell receptor is the same as that used by the SARS virus, but the S protein of the SARS-CoV-2 has been found to have 10-20 times more affinity for the ACE2 receptor than its SARS equivalent, which may partly explain its ease of spread. The high infectivity of the virus is facilitated by a peculiarity of its S protein, which is activated when a cellular enzyme called furin cleaves it in two during the process of invasion of the cell. This furin-cleaving sequence is not found in other human coronaviruses such as SARS, but is seen in other viruses such as influenza, HIV or Ebola; the presence of this trait is often associated with high infectious potential.
In February, the WHO gave the name COVID-19 (COronaVIrus Disease 2019) to the illness caused by SARS-CoV-2, which had been previously announced in January as an outbreak of pneumonia of unknown cause in the Chinese city of Wuhan that initially affected 44 people. The patients exhibited fever, cough and breathing difficulties, and X-rays showed signs of pneumonia in some of them.
Today it is known that the virus is easily transmitted between humans by the tiny droplets expelled with a cough or sneeze. Its basic reproduction number R0 (the number of people that each patient usually infects) has been estimated at around 2.2, between 2 and 2.68. In terms of prognosis, a large study in China showed that 80.9% of clinical cases were mild, 13.8% had severe symptoms, and 4.7% were critical. The disease is aggravated in older people and those with previous pathologies, while it does not seem to affect children. Although the WHO has declared a lethality rate of 3.4% among the sick, it is known that there is a large proportion of those infected who are asymptomatic: one study suggests that 86% of those infected in Wuhan were not reflected in the data, while another estimates the total number of those infected in that city at almost two million people, 19.1% of the total population. Assuming a number of asymptomatic people 40 times higher than the clinical cases, the lethality among all infected people would be 0.04-0.12%.
A clinical diagnosis of COVID-19 by chest imaging tomography can be confirmed by genetic or serological (presence of antibodies) tests. In January, the WHO published the first protocols for genetic detection by Polymerase Chain Reaction (PCR) in respiratory mucosa or blood samples, followed by a test developed at the hospital Charité – Berlin University of Medicine, of which the WHO distributed 250,000 units to laboratories worldwide. Subsequently, new diagnostic kits have been created, including those produced by several South Korean companies that have enabled more than 270,000 people to be tested in that country. In February, the first antibody detection test was applied in Singapore, and several serological tests are currently in the development and approval phase.
Prevention and treatment
Although hundreds of clinical trials against COVID-19 are underway, there is still no specific approved therapy, so patients receive mostly symptomatic treatments. Vaccines are already being tested, but approval will take more than a year, perhaps two. Currently, control of the virus is focused on prevention through isolation of confirmed cases, quarantine of suspects, social distancing and hygiene, especially hand washing, which health authorities regard as one of the most powerful measures to contain the contagion.