SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus that causes COVID-19, a respiratory illness that led to a major worldwide health crisis called the COVID-19 pandemic. This virus was first found in the city of Wuhan in China in late 2019. The World Health Organization declared it a serious global health concern from January 30, 2020, to May 5, 2023.

SARS-CoV-2 belongs to a group of viruses called coronaviruses, just like the virus that caused an earlier outbreak called SARS-CoV-1 in 2002–2004. Scientists believe this virus came from animals, most likely bats, and may have passed through another animal before affecting people. The virus spreads mainly through close contact and tiny particles released when an infected person talks, breathes, coughs, or sneezes. It can enter human cells by attaching to a specific protein called angiotensin-converting enzyme 2 (ACE2).

Without any protection or immunity, each infected person could spread the virus to about 2.4 to 3.4 other people on average during the early months of the pandemic. Later versions, or variants, of the virus were even better at spreading. Understanding SARS-CoV-2 helps us know how to stay safe and prevent the spread of COVID-19.

Terminology

When the virus was first found in Wuhan, China, people called it many things, like "the coronavirus" or "Wuhan coronavirus." In January 2020, the World Health Organization suggested using "2019 novel coronavirus" (2019-nCoV) as a temporary name. This followed their rules from 2015, which say not to use places, animals, or groups of people in virus names.

Later, on February 11, 2020, scientists officially named the virus "severe acute respiratory syndrome coronavirus 2" (SARS‑CoV‑2). To keep it clear from the sickness it causes, SARS, the WHO sometimes calls it "the COVID-19 virus." Some research papers used the name HCoV-19. The WHO says calling it the "Wuhan virus" can be harmful and has been seen as unfair by many.

Infection and transmission

Main article: Transmission of COVID-19

People can spread the virus that causes COVID-19 from one person to another. This was proven in January 2020 during the COVID-19 pandemic. The virus spreads mainly when someone coughs or sneezes, sending out tiny drops of liquid that can land on another person within about 1.8 metres, or 6 feet. Talking can also spread the virus far inside rooms with little air flow. Some studies think the virus can float in the air and be spread that way, too. When the virus spreads between people, it is thought that between 200 and 800 tiny pieces of the virus start a new infection. Touching surfaces that have the virus on them can also make people sick. Research shows the virus can live on plastic and stainless steel for up to three days but does not stay on cardboard for more than one day or on copper for more than four hours. Soap can kill the virus by breaking down its outer layer. Scientists have also found the virus in stool and semen from infected people.

It is not clear how infectious the virus is before someone feels sick, but studies show the most virus is in the nose about four days after infection. The virus can be spread even before someone feels sick, which may have helped the virus spread early in the pandemic. Some animals, like cats and ferrets, can also get infected with the virus.

There is still much we do not know about getting the virus again after recovering. The first known case of someone getting the virus again happened to a man in Hong Kong. He got the virus in March 2020, recovered, and then tested positive again in August 2020. Tests showed he had gotten a different version of the virus the second time. This shows that being infected once may not protect someone from getting the virus again. Another person in Nevada also tested positive twice, months apart, with different versions of the virus each time. Their second infection was worse than the first, but why this happened is not known.

Reservoir and origin

Further information: Origin of SARS-CoV-2

We do not know the natural home of the virus that causes COVID-19, called SARS-CoV-2. Before this virus, there were two other coronavirus diseases that came from animals: one caused by SARS-CoV-1 and another by MERS-CoV.

The first people known to get sick with SARS-CoV-2 were in Wuhan, China. We still do not fully understand where the virus came from or how it first spread to people. Many early patients worked at a seafood market in Wuhan, leading some to think the virus may have started there. But other research suggests people may have brought the virus to the market, where it then spread quickly. A report in 2021 suggested that the virus likely jumped to people from an animal, with bats being the next most likely source. It is also possible the virus came through our food supply, though this seems less likely.

Coronaviruses, like SARS-CoV-2, usually change quickly as they spread. However, SARS-CoV-2 changes more slowly than some other viruses because of how it copies itself. Scientists have found many similar viruses in bats, but none are exactly the same as SARS-CoV-2. Bats are thought to be the most likely natural home for this virus, but we do not know for sure how it first reached people. Early ideas that another animal, called a pangolin, may have helped spread the virus have not been supported by later research.

Phylogenetics and taxonomy

SARS-CoV-2 is a type of virus called a coronavirus. It belongs to a group of viruses that can infect humans and animals. Most coronaviruses cause mild illnesses like the common cold, but some can cause more serious diseases. SARS-CoV-2 is the seventh known coronavirus to infect people.

This virus has a long genetic code, which scientists study to understand how it changes and spreads. By looking at the genetic code, researchers can trace how the virus evolved and moved around the world. In early 2020, scientists began sharing the genetic codes of many versions of the virus, which helped them build family trees showing how the virus changed over time.

Main article: Variants of SARS-CoV-2

There are many different versions, or variants, of SARS-CoV-2. Some of these variants have changed in ways that make them spread more easily. The World Health Organization has identified several important variants that need special attention because they might spread faster or affect vaccines differently. These include variants first found in the United Kingdom, South Africa, Brazil, India, and Botswana. Each of these variants has unique changes in their genetic code that set them apart from earlier versions of the virus.

Virology

SARS-CoV-2, the virus that causes COVID-19, is very small, measuring between 60 and 140 nanometres across. Like other similar viruses, it has four main parts: the spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins. The spike protein helps the virus attach to and enter human cells.

The virus’s genetic code, or genome, is a single piece of RNA about 30,000 bases long. Scientists have studied millions of these genomes to learn more about how the virus changes. The genome contains specific patterns that help it copy itself inside cells and avoid some of the body’s natural defenses.

Virus infections begin when the virus attaches to a specific protein on human cells called ACE2, which acts like a keyhole the virus uses to get inside. Once attached, the virus uses other tools to break into the cell, release its RNA, and make the cell produce more copies of itself. These new virus particles then go on to infect other cells. The virus also makes certain proteins that help it spread and avoid the body’s immune system.

Treatment and drug development

Main article: Treatment and management of COVID-19

There are very few medicines that work well against the virus that causes COVID-19. One medicine called Masitinib was tested in animals and showed it could lower the amount of virus, but it is not approved for people with COVID-19. In December 2021, the United States allowed a medicine called Nirmatrelvir/ritonavir to be used in special cases to help treat the virus. Soon after, the European Union, the United Kingdom, and Canada also approved it. Studies showed this medicine could lower the chances of people needing hospital care or passing away by 88%.

The COVID Moonshot is a global effort that began in March 2020. It aims to create an easy-to-take medicine to treat the virus, and it is shared openly so that everyone can use it.

Epidemiology

Main article: COVID-19 pandemic

Tests done after the fact in Wuhan, China, showed that many people may have had the virus without knowing it in late 2019.

Studies from November 2020 looked at how easily the virus spreads. They found that each person with the virus could infect between 2.39 and 3.44 other people if no one was protected and no precautions were taken. In crowded places like cruise ships, the number could be even higher. Different countries and cultures saw different spread rates because of how people lived. For example, some places had lower spread rates, while others had much higher rates. Later versions of the virus spread even more easily. One version, called Omicron, spread very quickly, with some studies showing it could infect up to 20 people for each person who had it.

In the early weeks of the pandemic, there were about 96,000 confirmed cases in China. One model estimated that around 75,815 people in Wuhan may have been infected by January 25, 2020, even though only 2,015 cases were confirmed worldwide at that time. Most early deaths from the illness happened in the area around Wuhan. By March 10, 2023, the number of confirmed cases worldwide had reached 676,609,955, with 6,881,955 deaths linked to the virus.