SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2, is a highly contagious virus.

It is the biological cause of the disease known as COVID-19. This virus belongs to the species Betacoronavirus pandemicum. This species also includes SARS-CoV-1, the virus behind the 2002–2004 SARS outbreak. SARS-CoV-2 is classified as a positive-sense single-stranded RNA virus. This means it carries its genetic instructions in a single strand of RNA. The virus triggered a global pandemic that began in late 2019.

To begin an infection, the virus must enter human cells. It achieves this by binding to a specific membrane protein called ACE2. This protein, known as angiotensin-converting enzyme 2, regulates the renin–angiotensin system. Once the virus attaches to ACE2, it can enter the body. Research suggests the nasal cavity is the primary initial site of infection. From there, the virus can move into the lungs through aspiration. This process creates an infection gradient in the pulmonary epithelial cells. The virus often targets ciliated cells and type 2 pneumocytes in the airways.

SARS-CoV-2 spreads primarily through the air between people. It travels via aerosols and respiratory droplets exhaled during breathing, talking, or coughing. Some studies suggest that simply speaking can be a far-reaching mode of transmission. It is thought that between 200 and 800 infectious virions can start a new infection. The virus can also spread through indirect contact with contaminated surfaces. It can remain viable on plastic or stainless steel for up to three days. However, it survives on cardboard for only one day. On copper surfaces, it stays active for only four hours.

The virus was first identified in Wuhan, Hubei, China. In January 2020, the World Health Organization (WHO) used the provisional name 2019 novel coronavirus, or 2019-nCoV. This followed WHO guidance to avoid using geographic locations or animal species in names. On February 11, 2020, the International Committee on Taxonomy of Viruses adopted the official name. The WHO designated the outbreak a public health emergency of international concern from January 30, 2020, to May 5, 2023. Scientists use the term "spillover" to describe how the virus moved from animals to humans. This event likely occurred in late 2019 due to low genetic diversity.

Scientists believe SARS-CoV-2 has a zoonotic origin. This means it emerged from an animal source. The closest known relative is the BANAL-52 bat coronavirus found in Laos. This virus shows a 96.8% resemblance to SARS-CoV-2. Another closely related virus is RaTG13, which has a 96.1% resemblance. While bats are likely the natural reservoir, the exact path is still being studied. It is unclear if the virus moved directly from bats or through an intermediate host. Some researchers investigated pangolins, but their virus samples were too genetically distant.

Understanding the timing of infection is vital for public health. People can spread the virus even before they feel sick. This is known as pre-symptomatic shedding. One study found patients began shedding the virus four to five days before symptoms appeared. Some infections are also asymptomatic, meaning the person shows no symptoms at all. A meta-analysis found that 17% of infections are asymptomatic. These individuals were 42% less likely to transmit the virus compared to others. However, the total amount of virus a person sheds can vary greatly.

There is ongoing research regarding reinfection and long-term immunity. Some reports show that people can be infected more than once. For example, a man in Hong Kong tested positive again 142 days after his first infection. In another case, a man in Nevada experienced a more severe second infection. Genomic analysis showed these were different clades, or groups, of the virus. These findings suggest that herd immunity may not stop the virus entirely. It also means that vaccines may not provide lifelong protection. This complexity makes the study of SARS-CoV-2 a major focus of modern science.