Questions and answers on COVID-19: Basic facts

1. What is SARS-CoV-2?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus that first appeared in Wuhan, China in 2019. SARS-CoV-2 is a new strain of coronavirus that had not been identified in humans before.

2, What is COVID-19?

Coronavirus disease 2019 (COVID-19) is the respiratory disease caused by SARS-CoV-2.

3. Where do coronaviruses come from?

Coronaviruses can infect and circulate among different animal species, such as pigs, cats or dogs. Some groups of coronaviruses also circulate among humans and cause seasonal epidemics, mostly during the winter months. Coronaviruses that circulate among humans are thought to originate from animal reservoirs.

Bats are also considered natural hosts of these viruses and have been the source of coronaviruses that have transmitted to humans and caused severe disease. Sometimes this happens through an intermediate host. For example, the first severe acute respiratory syndrome coronavirus (SARS-CoV) originated in bats and was transmitted to humans via civet cats, causing severe acute respiratory syndrome (SARS) in humans in 2003. About 30% of people who get SARS die; however, no human cases have been reported since 2004.

Similarly, Middle East respiratory syndrome coronavirus (MERS-CoV) can transmit from camels to humans, causing MERS (Middle East respiratory syndrome). The first transmission was observed in 2012 and human infections are mostly limited to the Arabian Peninsula.

The precise way in which SARS-CoV-2 is transmitted from animals to humans is currently unknown.

4. Is SARS-CoV-2 comparable with SARS or with the seasonal flu?

The novel coronavirus detected in China in 2019, SARS-CoV-2, is closely related to the original SARS-CoV. These viruses cause respiratory diseases known as COVID-19 and SARS, respectively. Influenza, also known as the flu, is a respiratory illness that has similar symptoms but is caused by influenza viruses, not coronaviruses.

SARS, the respiratory illness caused by the original SARS-CoV, emerged in late 2002 in China and caused more than 8 000 cases in 33 countries over the course of eight months.

COVID-19, caused by SARS-CoV-2, emerged in late 2019 and spread very quickly across the globe. As SARS-CoV-2 is a new virus, most people did not have immunity against it, so the entire human population was potentially susceptible to SARS-CoV-2 infection at the start of the pandemic.

Within the first two years of the COVID-19 pandemic, more than 450 million cases were reported worldwide, with more than 100 million in the EU/EEA alone. Due to the nature of the disease, where some infected individuals may not have symptoms and not all individuals with symptoms will be tested, it is assumed that there are many undiagnosed cases.

The likelihood of death from COVID-19 depends on vaccination status, age and the presence of certain underlying conditions. Older age is the strongest contributing factor.

It is impossible to know when an influenza virus first infected humans, but convincing reports of influenza pandemics date back several hundred years. In a normal influenza year, about 1 in 1 000 people who develop seasonal flu die. The highest burden of disease for seasonal influenza is among children below the age of five years and in individuals older than 65 years of age.

The viruses that cause COVID-19 and seasonal flu can spread between people who are in close contact with one another. Both are mainly spread by virus-containing particles that are expelled when infected people cough, sneeze or talk. The virus that causes COVID-19 seems to spread more easily than influenza and much more easily than SARS.

Vaccines against COVID-19 have only recently become available, while influenza vaccines were developed as early as the 1930s. No vaccine is available for SARS.

5. How does SARS-CoV-2 spread?

SARS-CoV-2 is mainly spread via virus-containing particles from an infected person who sneezes, coughs, speaks, sings or breathes in close proximity to other people. Virus-containing particles can be inhaled or deposited in the nose and mouth or on the eyes.

More rarely, infection may be due to contact with surfaces contaminated with virus-infected particles.

The virus can survive on some surfaces for a few hours (copper, cardboard) and on others for up to a number of days (plastic and stainless steel). However, the amount of viable virus declines over time and it is rarely present on surfaces in sufficient amounts to cause infection.

Infection may occur when a person touches their nose, mouth or eyes with their hands if their hands have been contaminated by fluids containing the virus or by touching surfaces contaminated with the virus. 

An infected person can transmit the virus up to two days before they experience symptoms, as well as while they have symptoms.

6. When is a person infectious?

SARS-CoV-2 can be detected one to three days before symptoms begin. However, detection of the virus does not necessarily mean that a person is infectious and able to spread the virus to others.

Evidence indicates that people become infectious around 48 hours before symptoms start, but are most infectious when experiencing symptoms, even if the symptoms are mild and non-specific.

Available data indicate that adults with mild to moderate COVID-19 remain infectious no longer than 10 days after symptoms begin. This estimate has been the same for variants of concern, such as Delta and Omicron. Most adults with severe to critical illness or severe immune suppression may remain infectious for up to 20 days after symptoms begin.

Evidence shows that fully-vaccinated individuals who become sick with COVID-19 (referred to as ‘breakthrough infections’) can carry comparable amounts of virus as non-vaccinated people.

7. How severe is COVID-19 infection? 

Some people infected with SARS-CoV-2 will experience mild to moderate respiratory illness and most will recover without requiring special treatment. COVID-19 can sometimes be a severe disease with respiratory insufficiency requiring intensive care and potentially leading to death.

Older people and those with underlying medical conditions such as heart disease, diabetes, chronic respiratory disease and cancer are more likely to develop serious illness.

Vaccinated individuals are less likely to have severe disease or to be hospitalised. The severity of COVID-19 also varies according to variant and pre-existing immunity, developed after vaccination or recovery from a prior infection. 

Page last updated 12 Jun 2023