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  • Facts about the virus and COVID-19 disease

Facts about the SARS-CoV-2 virus and COVID-19 disease

The SARS-CoV-2 coronavirus was discovered in January 2020. New knowledge about the outbreak, the disease and risks will be regularly updated.

Foto: CDC, Alissa Eckert
Foto: CDC, Alissa Eckert

The SARS-CoV-2 coronavirus was discovered in January 2020. New knowledge about the outbreak, the disease and risks will be regularly updated.

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About the virus

The coronavirus family includes many different viruses that can cause respiratory infection. Many coronaviruses only cause colds, while others can cause more serious illness and in some cases, death.

The novel coronavirus SARS-CoV-2 was first discovered in January 2020. It has some genetic similarities to the SARS virus (Severe Acute Respiratory Syndrome) which also belongs to the coronavirus family. The virus that causes MERS (Middle East Respiratory Syndrome) is another coronavirus. 

Coronaviruses are also detected in animals. In rare cases, these coronaviruses can develop so they can transmit from animals to humans and between humans, as seen during the SARS epidemic in 2002. The SARS virus infection probably came from bats via civet cats and other animals.  Dromedaries and camels were the source of infection for the MERS virus discovered in 2012.

SARS-CoV-2 is believed to come from bats and was transmitted to humans in the end of 2019, either directly or via other animals. 

Different variants of SARS-CoV-2

As with other viruses, there are small changes in the genetic material (RNA) of SARS-CoV-2 as it multiplies. These are called mutations. Most mutations have little or no effect on the properties of the virus, but occasionally mutations occur that lead to changes that may affect the virus' infectivity, ability to cause severe disease in the host and ability to escape the immune system after vaccination or having had the disease (immune escape).

These variants are closely monitored as they may have an impact on the development of the pandemic. So far, several variants of SARS-CoV-2 have been identified that are monitored internationally. These are called VOC (Variants of Concern) and VOI (Variants of Interest). See an overview of these on the pages of the European Centre for Disease Control, ECDC. NIPH closely monitors the development of these variants and updated information can be found in our risk assessments and in our advice on the detection and monitoring of these variants.

All vaccines that are approved in Norway provide good protection against severe disease progression in all the currently known virus variants, including those identified as VOC and VOI.

Modes of transmission

Transmission of SARS-CoV-2 mainly occurs during close contact with an infected person by exposure to small and large droplets containing viruses from the respiratory tract. People with COVID-19 are most contagious for 1-2 days before the onset of symptoms (pre-symptomatic period) and in the first days after the onset of symptoms. Someone can be infected with SARS-CoV-2 without developing the disease (asymptomatic), yet still be contagious. People who do not develop symptoms are likely to be contagious to a lesser extent than those who develop symptoms.

Traditionally, modes of transmission of respiratory tract diseases have been divided into three categories: contact transmission, droplet transmission and airborne transmission. In recent years and during the ongoing pandemic, a lot of research has been carried out, not least the spatial transmission of droplets of various sizes from the respiratory tract. Much has also been learnt about how SARS-CoV-2 is transmitted. What has been defined as droplet transmission and airborne transmission overlap more than previously described. Two important factors that are important for transmission are droplet size and distance to the source of infection. Instead of defining something as droplet transmission or airborne transmission, the European and American centres for disease control (ECDC and US CDC) have begun to describe the modes of transmission of respiratory tract diseases in three categories:

  1. Inhalation of small and medium-sized droplets containing infectious virus. The risk of transmission is greatest near a carrier, where the concentration of the droplets is greatest.
  2. Deposition of large and medium-sized droplets of virus onto exposed mucous membranes, such as droplets from coughing and sneezing that hit the eyes or mouth. The risk of transmission is greatest near an infected person.
  3. Contact transmission: touching mucous membranes (eyes, mouth, nose) with virus particles from unclean hands, for example after contact with surfaces contaminated with viruses, direct contact with the carrier, or deposition of viruses on hands.

People with SARS-CoV-2 infection can emit droplets of virus from the mouth and nose. The degree of droplet formation from a person depends on both the individual and the activity. Sneezing, coughing, shouting, singing and exercising increase droplet formation. In addition, there are individuals who, for unknown reasons, produce more droplets than others (10-100 times more). The amount of virus in each droplet can also vary throughout the disease course and between individuals.

The concentration of droplets decreases with increasing distance from the source as large droplets quickly fall to the ground, and the concentration of the smaller droplets is diluted in the air. Current knowledge suggests that SARS-CoV-2 is mainly transmitted by inhalation and deposition of droplets with close contact. Small droplets can remain suspended in the air for a long time (minutes to hours) and move further than larger droplets. Although the general risk of transmission decreases with increasing distance, transmission over longer distances can occur by the spread of virus-containing droplets from the nose and mouth of an infected person. The risk of transmission with small droplets over longer distances increases with increasing time spent in rooms with a low air volume or insufficient ventilation, and in connection with activities that increase droplet formation. Mass virus transmission episodes have occurred after indoor gatherings with a high person density and activities that increase droplet formation, such as singing. The risk of transmission in general, and of mass virus transmission episodes, seems to be much lower outdoors.

Contact transmission can occur either through direct contact with a contagious person (e.g., hugging and shaking hands) or indirectly via contact with other surfaces that have been contaminated with viruses (e.g., door handles, light switches or payment terminals). Drops of infectious virus can therefore be transmitted from the respiratory tract to surfaces that become contaminated and onto the mucous membranes of the nose, mouth and eyes of a susceptible person.

Laboratory studies suggest that SARS-CoV-2 can survive on surfaces from a few hours to several days, depending on the amount of virus, type of surface, temperature, sunlight and humidity. However, the presence of live viruses on different surfaces is not the same as their ability to cause infection in humans.

It is unknown what proportion of COVID-19 patients are infected via the various modes of transmission, and in many of the studies performed, it is also difficult to distinguish between the various modes of transmission with certainty. Recent knowledge indicates that inhalation from a short distance is the most important mode of transmission. It has also been shown that inhaled transmission over distances of more than two metres can occur under certain conditions.

Although contact transmission is probable in some cases, it is still unclear how important this mode of transmission is for SARS-CoV-2. The virus has been detected in faeces, blood and urine, but so far it is not known that anyone has been infected by contact with these body fluids.

Even with a different division of modes of transmission and increased knowledge that inhalation transmission can occur over distances of more than two metres, we do not currently recommend a change in current routines and infection control measures in the population or for the health service, including the laboratory service. We will continue to recommend stricter measures for indoor than outdoor activities. Experience from Norway shows that the infection control advice has worked well to limit transmission of SARS-CoV-2 when routines have been followed (including the use of personal protective equipment) in hospitals and other healthcare institutions.

Infection from food, water and animals

Currently, there are no known cases of infection via food produced in Norway or imported, or from water and animals. Several systematic reviews have concluded that this is an unlikely mode of transmission.

There have been some cases of transmission between animals and humans. Mink and other species in the marten family appear to be highly susceptible to infection, and both human-to-mink and mink-to-human infections have occurred in the Netherlands and Denmark. It is important that people with COVID-19 or people in quarantine do not go to work as keepers and have close contact with mink.

Infection from animals does not appear to play a role in transmission of the virus.

In the case of swimming pools, the chlorine content of the pool water will be sufficient to inactivate coronaviruses and other viruses. However, physical contact in changing rooms and by the pool could lead to transmission as with any other close contact. 

How contagious is it?

Calculations estimate that a person infected with coronavirus infects 2-3 others whereas a person with influenza will infect 1-2 people. Probably fewer than 20 per cent of those infected with SARS-CoV-2 virus account for 80 per cent of the transmission. This indicates that the majority of confirmed cases will not transmit further, while a minority will infect many.

The number will probably be lower than 2-3 in Norway because we have a lower population density and have implemented infection control measures. 


The incubation time (from infection until symptoms appear) is usually 4-5 days. Based on current figures, 98 - 99.9% of infected people develop symptoms within 10 days, but a few will develop symptoms later.

Symptoms and disease

The new coronavirus causes respiratory tract infections, ranging from mild symptoms to severe disease and, in rare cases, death.

Some people may have COVID-19 without developing symptoms. This is especially true for children and younger adults. 

Usually, people who become ill first develop upper respiratory tract symptoms (sore throat, cold symptoms, mild cough), as well as feeling generally unwell and having muscle pain. Stomach pains may be present and diarrhoea may occur in some cases. Loss of the senses of smell and taste is characteristic of the disease, but probably occurs among a minority of those infected. Some develop more severe symptoms such as wheezing, chest pain and confusion.

The course of the disease varies widely between individuals. Currently, the typical courses appear to be:

  • Mild to moderate course: This applies to the majority of people who become sick (over 80 %). Symptoms pass within one to two weeks. These people rarely need treatment from the healthcare service, but people in risk groups should be monitored closely in case they deteriorate.
  • Severe course: Some will need to be admitted to hospital. Breathing difficulties and / or decreased oxygen saturation in the blood are often early signs of severe disease course. Other symptoms can include chest pain, confusion and greatly impaired general condition. This often occurs 4-7 days after the onset of symptoms. X-ray examination of the lungs may show changes consistent with viral pneumonia (pulmonary infiltrates)
  • Critical course: 15-35% of hospitalied COVID-19 patients need intensive care. The most common cause is primarily respiratory failure, while other organ failure (usually heart, kidney, liver), coagulation disorders and septic shock can be causes of critical disease. Critical disease is most often characterised by signs of a strong inflammatory activation with elevated levels of inflammatory markers (for example, CRP, ferritin, IL-6). As with other severe infections, various complications of severe COVID-19 can be seen, including lung damage, cardiovascular disease and coagulation disorders.

Admission to hospital, intensive care treatment and death are more common among the elderly and people with underlying diseases, particularly among the elderly with underlying diseases, but can also occur among people without known risk factors.

More about the symptoms of COVID-19

It appears that SARS-CoV-2 mainly infects upper and lower respiratory tract cells and causes respiratory tract infection with symptoms such as cough, fever and sore throat. Most people who get COVID-19 only develop these symptoms.

A minority enter a more severe phase with breathing difficulties and symptoms of systemic disease that are probably caused by the body's immune response to the infection. There is a good basis to assume that SARS-CoV-2 also infects the gastrointestinal tract, and related symptoms are frequently reported.

We are learning more about the symptoms of COVID-19. Much of the knowledge is based on patients admitted with severe disease. These account for about 20 per cent of all people who develop symptoms and probably do not reflect the overall clinical presentation.

Among the vast majority with a mild disease course, these are probably the most common symptoms: respiratory tract symptoms (cough, runny nose and sore throat), headache, and mildly reduced general condition (lethargy, mild muscle aches). Fever is often seen in hospitalised patients (over 80%), but it otherwise seems to affect less than 20% of all COVID-19 patients.

Altered taste and sense of smell appear to be a fairly specific symptom of COVID-19. Nevertheless, the occurrence of this is far from certain. Studies have reported this in 5 to 98% of cases. The real figure is probably around 30 to 50 %.

Shortness of breath (dyspnoea) is frequently reported (15-40 %) and is an indication of severe disease that often requires hospitalisation. The proportion that develops shortness of breath appears to be small if mild cases are included in the calculation.

Gastrointestinal symptoms such as pain, nausea / vomiting and diarrhoea have been reported in 2-40 % of cases. In one meta-study of 4243 patients, 17.6 % of patients had gastrointestinal symptoms, which may be the first and, in some cases, the only obvious symptom of the disease.

Neurological symptoms other than headache and changes in senses of taste and smell. These may include dizziness, altered mental state, ataxia and tremor. Cases of diarrhoea and haemorrhagic strokes, encephalopathy and Guillain-Barré are also linked to SARS-CoV-2 infection.

In addition, symptoms have been reported for most organ systems such as the eye (e.g., conjunctivitis), skin (rash), heart (myocarditis, arrhythmias, infarction) and kidneys (acute kidney damage) associated with the disease.

Coagulation disorders

Coagulation disorders appear to occur frequently in seriously ill COVID-19 patients, but are also described among patients with mild disease. This may explain symptoms such as blood clots in the lungs (pulmonary embolism) and deep vein thrombosis, and also some of the neurological and cardiovascular symptoms described above. 

Silent hypoxia

There is an increase in the number of reports of COVID-19 patients with hypoxia without dyspnoea or other symptoms of hypoxia. The clinical significance of this is currently unknown, but frequent measurement with an oximeter may be considered in COVID-19 patients at risk of a severe course (for example, in a hospital). Patients with silent hypoxemia should be assessed by a doctor for possible causes, such as pulmonary embolism.

Symptoms of COVID-19 among the elderly

High age is the most certain risk factor for severe disease and death from COVID-19.

Elderly patients often have an atypical clinical presentation with many diseases and this also applies to COVID-19. In addition to the symptoms mentioned above, there are a number of atypical symptoms that occur among the elderly with COVID-19, such as delirium (acute onset of cognitive failure and altering emotions and behaviour), confusion (new-onset or deterioration), tendency to fall (new-onset or deterioration), lethargy and weakness (new-onset or deterioration), generally poorer function level, weight loss, decreased appetite. Elderly people with dementia have a significantly increased risk of delirium with infections. These symptoms may overshadow other and more common symptoms of infection, such as respiratory problems.

The reason why older people more often present with atypical symptoms may be age-related changes in the immune system, temperature regulation may be poorer, and the ability to remember and recall symptoms may be impaired. Elderly people also more often have underlying chronic conditions that can affect the clinical presentation, e.g., strokes and other neurological disorders can impair the cough reflex.

Therefore, COVID-19 should be considered as a differential diagnosis among the elderly, even with atypical and / or mild symptoms.

Symptoms that are particularly important to be alert for in this group are tachypnea (rapid breathing), tachycardia (rapid heartbeat), hypotension (low blood pressure) and low-grade fever (above 37.5 °C).

The assessment of elderly patients should be made by people who are familiar with the patient’s normal functional level so that changes can be detected quickly; Here, both health professionals and relatives can play a role. An early diagnosis will ensure the best possible treatment and prevent the spread of infection, especially in hospital and nursing homes.

Symptoms among children

Children appear to have a similar clinical presentation to adults, but the proportion with severe disease is much lower. Most have symptoms of colds, such as cough, fever and sore throat, which last from several days to a week.

Fever (41-48 %) and cough (39-48 %) are the two most common symptoms. A sore throat was observed among 46 % in one study. Other relatively frequently reported symptoms are diarrhoea (7-9 %), nausea / vomiting (6 %), lethargy (7.6 %), mucus and runny nose (7.6 %). Other rarer symptoms include stuffy nose, headache, abdominal pain and shortness of breath.

Multisystemic Inflammatory Disease in Children (MIS-C)

At the end of April 2020, British health authorities reported that an increasing number of children were admitted to intensive care with symptoms similar to Kawasaki disease. 4 out of 8 children had previously known exposure to SARS-CoV-2.

The condition has now been reported worldwide and is called MIS-C. The condition is similar, and sometimes overlaps with Kawasaki syndrome, which is an acute condition with inflammation of blood vessels in various parts of the body. Typical of Kawasaki is fever for more than 5 days, rash, mucous membrane changes on the lips and in the oral cavity, inflammation of the eye without pus, swollen lymph nodes on the neck and swollen / red hands and feet.

In contrast to Kawasaki syndrome, which is most commonly seen in children under 5 years of age, MIS-C usually affects older children and adolescents.

Typical for MIS-C are fever, abdominal pain, diarrhoea and various types of organ failure (heart, kidney, liver). The condition follows 2-6 weeks after undergoing SARS-CoV-2 infection, and is the cause of a severe inflammatory picture and coagulation disorders. Treatment is similar to that for Kawasaki syndrome, with immunomodulatory drug (intravenous immunoglobulin, steroids). The prognosis is usually very good.

It has not been possible to define which children are exposed to developing MIS-C, but the condition is still very rare.

ECDC and WHO stress that COVID-19 mainly causes mild illness among children, and that it is extremely rare for children to become seriously ill. The most important advice for parents is the same as always: they should seek medical attention when a child with a fever has a poor general condition and they are concerned for their child, for whatever reason.

Information about om MIS-C:

ECDC and WHO's assessments can be found here:


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23.06.2021: Removed text about long term health consequences, which has been moved to its own article in this guidance and updated.

16.06.2021: Removed text about VOI / VOC, but added links to updated info about VOI / VOC Removed section on late effects after COVID-19 disease, this has been given a separate chapter. Other minor updates according to Norwegian version.

02.06.2021: Correction in wording to clarify the difference between inhalation and deposition

01.06.2021: We have updated the section on the modes of transmission with a revised description from the European and American centres of disease control (ECDC and CDC)

12.02.2020 Updated according to Norwegian version

11.12.2020 updated information about incubation time

23.11.2020 general updates as per Norwegian version

20.11.2020 Added paragraph about long-term health consequences

17.07.2020 Changed value from 10 to 5 in "Aerosols smaller than 5 µm can remain suspended in the air for a long time" under "More about transmission"

14.07.2020 Updated text in section about Transmission route

07.07.2020 Removed sentence about coronavirus transmission being traced back to direct or indirect physical contact, as well as the number of summaries and studies.

06.07.2020 Added paragraph about mink.

01.07.2020 Added paragraph about wastewater surveillance.

19.06.2020 Updated text as per Norwegian version. Added references and factbox about symptoms.

27.04.2020  Updated sentence about closure of swimming baths etc. 

Added symptoms, stomach ache, loss of senses of smell and taste.

Added paragraph about water. Changed title. Removed paragraph about how infection is handled in Norway to article about Infection Control Measures. Added paragraph about airborne transmission with link to systematic review fra FHI. Added symptoms.

Moved section about infection from animals to this article to reflect the Norwegian version


Changes according to Norwegian text. WHO declares a pandemic. Update about risk assessment

Changes according to Norwegian text. Added information about virus properties on surfaces.


Changes according to Norwegian text. Moved section about pregnancy to "advice to public" article. Updated "what is NIPH role?" and "About the outbreak"

Changes according to updates in the Norwegian text - advice to pregnant women and risk groups

Paragraph about risk assessment added and what happens in Norway updated according to the Norwegian version

Paragraph about affected countries removed and published as its own page.

Changed name of virus from COVID-19 to SARS-CoV-2

Removed sentence about which countries are in mainland China.

Changed the name of the virus from "2019-nCoV" to "COVID-19".