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Severe Acute Respiratory Syndrome Coronavirus 2
Virus that causes coronavirus disease 2019 (COVID-19)
The name "2019-nCoV" in use in a trilingual sign at a Lisbon health facility in February 2020.
During the initial outbreak in Wuhan, China, various names were used for the virus; some names used by different sources included the "coronavirus" or "Wuhan coronavirus". In January 2020, the World Health Organisation recommended "2019 novel coronavirus" (2019-nCov) as the provisional name for the virus. This was in accordance with WHO's 2015 guidance against using geographical locations, animal species, or groups of people in disease and virus names.
On 11 February 2020, the International Committee on Taxonomy of Viruses adopted the official name "severe acute respiratory syndrome coronavirus 2" (SARS-CoV-2). To avoid confusion with the disease SARS, the WHO sometimes refers to SARS-CoV-2 as "the COVID-19 virus" in public health communications and the name HCoV-19 was included in some research articles.
The general public often calls both the virus and the disease it causes, "coronavirus". U.S. President Donald Trump repeatedly referred to the virus as the "Chinese virus" in tweets, interviews, and White House press briefings, which drew some criticism that he was stigmatizing the disease with racial or nationalistic overtones.
The degree to which the virus is infectious during the incubation period is uncertain, but research has indicated that the pharynx reaches peak viral load approximately four days after infection or the first week of symptoms, and declines after.
A study by a team of researchers from the University of North Carolina found that the nasal cavity is seemingly the dominant initial site for infection with subsequent aspiration-mediated virus seeding into the lungs in SARS-CoV-2 pathogenesis. They found that there was an infection gradient from high in proximal towards low in distal pulmonary epithelial cultures, with a focal infection in ciliated cells and type 2 pneumocytes in the airway and alveolar regions respectively.
There is some evidence of human-to-animal transmission of SARS-CoV-2, including examples in felids. Some institutions have advised those infected with SARS-CoV-2 to restrict contact with animals.
On 1February 2020, the World Health Organization (WHO) indicated that "transmission from asymptomatic cases is likely not a major driver of transmission". One meta-analysis found that 17% of infections are asymptomatic, and asymptomatic individuals were 42% less likely to transmit the virus.
However, an epidemiological model of the beginning of the outbreak in China suggested that "pre-symptomatic shedding may be typical among documented infections" and that subclinical infections may have been the source of a majority of infections. That may explain how out of 217 onboard a cruise liner that docked at Montevideo, only 24 of 128 who tested positive for viral RNA showed symptoms. Similarly, a study of ninety-four patients hospitalized in January and February 2020 estimated patients shed the greatest amount of virus two to three days before symptoms appear and that "a substantial proportion of transmission probably occurred before first symptoms in the index case".
There are still a lot of questions about reinfection and long-term immunity. It is not known how common reinfection is, but reports have indicated that it is occurring with variable severity.
The first reported case of reinfection was a 33-year-old man from Hong Kong who first tested positive on 26 March 2020, was discharged on 15 April 2020 after two negative tests, and tested positive again on 15 August 2020 (142 days later), which was confirmed by whole-genome sequencing showing that the viral genomes between the episodes belong to different clades. The findings had the implications that herd immunity may not eliminate the virus if reinfection is not an uncommon occurrence and that vaccines may not be able to provide lifelong protection against the virus.
Another case study described a 25-year-old man from Nevada who tested positive for SARS-CoV-2 on 18 April 2020 and on 5 June 2020 (separated by two negative tests). Since genomic analyses showed significant genetic differences between the SARS-CoV-2 variant sampled on those two dates, the case study authors determined this was a reinfection. The man's second infection was symptomatically more severe than the first infection, but the mechanisms that could account for this is not known.
Reservoir and zoonotic origin
Transmission of SARS-CoV-1 and SARS-CoV-2 from mammals as biological carriers to humans
The first known infections from SARS-CoV-2 were discovered in Wuhan, China. The original source of viral transmission to humans remains unclear, as does whether the virus became pathogenic before or after the spillover event. Because many of the early infectees were workers at the Huanan Seafood Market, it has been suggested that the virus might have originated from the market. However, other research indicates that visitors may have introduced the virus to the market, which then facilitated rapid expansion of the infections. A phylogenetic network analysis of 160 early coronavirus genomes sampled from December 2019 to February 2020 showed that the virus type most closely related to the bat coronavirus was most abundant in Guangdong, China, and designated type "A". The predominant type among samples from Wuhan, "B", is more distantly related to the bat coronavirus than the ancestral type "A".
Samples taken from Rhinolophus sinicus, a species of horseshoe bats, show an 80% resemblance to SARS-CoV-2.
Bats are considered the most likely natural reservoir of SARS-CoV-2, but differences between the bat coronavirus and SARS-CoV-2 suggest that humans were infected via an intermediate host. Although studies have suggested some likely candidates, the number and identities of intermediate hosts remain uncertain. Nearly half of the virus's genome has a phylogenetic lineage distinct from known relatives.
A study published in July 2020 suggested that pangolins are an intermediate host of SARS-CoV-2-like coronaviruses. However, additional studies indicate that pangolins are unlikely to be reservoirs or intermediate hosts for SARS-CoV-2. Isolates obtained from pangolins seized in Guangdong were only 92% identical in sequence to the SARS-CoV-2 genome, a number that is too low for the pangolin virus to be an intermediate host. In addition, pangolins are unlikely to be reservoirs for SARS-CoV-2-like viruses because they get sick from the infection, unlike true reservoirs like bats. The receptor-binding domain of the pangolin virus spike protein is highly similar to that of SARS-CoV-2, with the five critical amino acid residues in the receptor-binding motif being identical in both viruses. However, the pangolin virus turns out to bind poorly to the human ACE2 receptor.
Like the SARS-related coronavirus implicated in the 2003 SARS outbreak, SARS-CoV-2 is a member of the subgenus Sarbecovirus (beta-CoV lineage B). Its RNA sequence is approximately 30,000 bases in length, relatively long for a coronavirus. SARS-CoV-2 is unique among known betacoronaviruses in its incorporation of a polybasic cleavage site, a characteristic known to increase pathogenicity and transmissibility in other viruses.
With a sufficient number of sequenced genomes, it is possible to reconstruct a phylogenetic tree of the mutation history of a family of viruses. By 12 January 2020, five genomes of SARS-CoV-2 had been isolated from Wuhan and reported by the Chinese Center for Disease Control and Prevention (CCDC) and other institutions; the number of genomes increased to 42 by 30 January 2020. A phylogenetic analysis of those samples showed they were "highly related with at most seven mutations relative to a common ancestor", implying that the first human infection occurred in November or December 2019. As of 7 May 2020,[update] 4,690 SARS-CoV-2 genomes sampled on six continents were publicly available.[clarification needed]
In July 2020, scientists reported that a more infectious SARS-CoV-2 variant with spike protein variant G614 has replaced D614 as the dominant form in the pandemic. In October 2020 scientists reported in a preprint that a variant, 20A.EU1, was first observed in Spain in early summer and has become the most frequent variant in multiple European countries. They also illustrate the emergence and spread of other frequent clusters of sequences using Nextstrain.
In October 2020, researchers discovered a possible overlapping gene named ORF3d, in the Covid-19 virus genome. It is unknown if the protein produced by ORF3d has any function, but it provokes a strong immune response. ORF3d has been identified before, in a variant of coronavirus that infects pangolins.
There are many thousands of variants of SARS-CoV-2, which can be grouped into the much larger clades. Several different clade nomenclatures have been proposed. Nextstrain divides the variants into five clades (19A, 19B, 20A, 20B, and 20C), while GISAID divdes them into seven (L, O, V, S, G, GH, and GR).
Several notable variants of SARS-CoV-2 emerged in the fall of 2020.
The Variant of Concern 202012/01 (VOC 202012/01) is believed to have emerged in the United Kingdom in September. Preliminary epidemiological markers suggest that the variant is more highly transmissible, but there is no evidence that it affects disease severity or vaccine efficacy. Among the variant's several mutations is one in the receptor-binding domain of the spike protein that changes the asparagine at position 501 to tyrosine (N501Y). This mutation may cause the virus to bind more tightly to the ACE2 receptor.
The 501Y.V2 Variant, which has the same N501Y mutation, arose independently in South Africa. It was detected in patient specimens collected at the beginning of October 2020. There is no evidence that the mutations increase transmissibility of the variant.
The B.1.207 variant appeared in Nigeria. It has a mutation in the spike protein (P681H) that is also found in the VOC 202012/01 variant. P681H is located near the S1/S2 furin cleavage site. There is no evidence that the mutations increase transmissibility of the variant.
The Cluster 5 variant emerged among minks and mink farmers in Denmark. It has a set of mutations that have not been observed in other variants, including four amino acid changes in the spike protein. The variant moderately resists neutralizing antibodies. After strict quarantines, a ban on mink farming, and a mink euthanasia campaign, it is believed to have been eradicated.
There is no evidence that these variants increase disease severity.
Each SARS-CoV-2 virion is 50-200 nanometres in diameter. Like other coronaviruses, SARS-CoV-2 has four structural proteins, known as the S (spike), E (envelope), M (membrane), and N (nucleocapsid) proteins; the N protein holds the RNA genome, and the S, E, and M proteins together create the viral envelope. The spike protein, which has been imaged at the atomic level using cryogenic electron microscopy, is the protein responsible for allowing the virus to attach to and fuse with the membrane of a host cell; specifically, its S1 subunit catalyzes attachment, the S2 subunit fusion.
Protein modeling experiments on the spike protein of the virus soon suggested that SARS-CoV-2 has sufficient affinity to the receptor angiotensin converting enzyme 2 (ACE2) on human cells to use them as a mechanism of cell entry. By 22 January 2020, a group in China working with the full virus genome and a group in the United States using reverse genetics methods independently and experimentally demonstrated that ACE2 could act as the receptor for SARS-CoV-2. Studies have shown that SARS-CoV-2 has a higher affinity to human ACE2 than the original SARS virus. SARS-CoV-2 may also use basigin to assist in cell entry.
Initial spike protein priming by transmembrane protease, serine 2 (TMPRSS2) is essential for entry of SARS-CoV-2. The host protein neuropilin 1 (NRP1) may aid the virus in host cell entry using ACE2. After a SARS-CoV-2 virion attaches to a target cell, the cell's protease TMPRSS2 cuts open the spike protein of the virus, exposing a fusion peptide in the S2 subunit, and the host receptor ACE2. After fusion, an endosome forms around the virion, separating it from the rest of the host cell. The virion escapes when the pH of the endosome drops or when cathepsin, a host cysteine protease, cleaves it. The virion then releases RNA into the cell and forces the cell to produce and disseminate copies of the virus, which infect more cells.
Based on the low variability exhibited among known SARS-CoV-2 genomic sequences, health authorities likely detected the virus within weeks of its emergence among the human population in late 2019. The earliest case of infection currently known is dated to 1 December 2019, although an earlier case could have occurred on 17 November 2019. The virus subsequently spread to all provinces of China and to more than 150 other countries across the world. Human-to-human transmission of the virus has been confirmed in all these regions. On 30 January 2020, SARS-CoV-2 was designated a Public Health Emergency of International Concern by the WHO, and on 11 March 2020 the WHO declared it a pandemic.
The basic reproduction number () of the virus has been estimated to be around 5.7. This means each infection from the virus is expected to result in 5.7 new infections when no members of the community are immune and no preventive measures are taken. The reproduction number may be higher in densely populated conditions such as those found on cruise ships. Many forms of preventive efforts may be employed in specific circumstances to reduce the propagation of the virus.
There have been about 96,000 confirmed cases of infection in mainland China. While the proportion of infections that result in confirmed cases or progress to diagnosable disease remains unclear, one mathematical model estimated that 75,815 people were infected on 25 January 2020 in Wuhan alone, at a time when the number of confirmed cases worldwide was only 2,015. Before 24 February 2020, over 95% of all deaths from COVID-19 worldwide had occurred in Hubei province, where Wuhan is located. As of 18 January 2021, the percentage had decreased to 0.16%.
As of 18 January 2021, there have been 95,179,173 total confirmed cases of SARS-CoV-2 infection in the ongoing pandemic. The total number of deaths attributed to the virus is 2,033,641. Many recoveries from both confirmed and untested infections go unreported, since some countries do not collect this data, but at least 52,456,617 people have recovered from confirmed infections.
^Surveillance case definitions for human infection with novel coronavirus (nCoV): interim guidance v1, January 2020 (Report). World Health Organization. January 2020. hdl:10665/330376. WHO/2019-nCoV/Surveillance/v2020.1.
^"Naming the coronavirus disease (COVID-2019) and the virus that causes it". World Health Organization. Archived from the original on 28 February 2020. Retrieved 2020. ICTV announced "severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)" as the name of the new virus on 11 February 2020. This name was chosen because the virus is genetically related to the coronavirus responsible for the SARS outbreak of 2003. While related, the two viruses are different.
^"Naming the coronavirus disease (COVID-2019) and the virus that causes it". World Health Organization. Archived from the original on 28 February 2020. Retrieved 2020. From a risk communications perspective, using the name SARS can have unintended consequences in terms of creating unnecessary fear for some populations.... For that reason and others, WHO has begun referring to the virus as "the virus responsible for COVID-19" or "the COVID-19 virus" when communicating with the public. Neither of these designations are [sic] intended as replacements for the official name of the virus as agreed by the ICTV.
^Giovanetti M, Benedetti F, Campisi G, Ciccozzi A, Fabris S, Ceccarelli G, Tambone V, Caruso A, Angeletti S, Zella D, Ciccozzi M (November 2020). "Evolution patterns of SARS-CoV-2: Snapshot on its genome variants". Biochemical and Biophysical Research Communications. doi:10.1016/j.bbrc.2020.10.102. PMID33199021. S2CID226988090.