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31st March, 20208 min read

Coronavirus: What’s the difference between mild, moderate and severe illness?

Coronavirus: What’s the difference between mild, moderate and severe illness?
Medical reviewer: Healthily's medical team
Last reviewed: 01/04/2020
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By Dr Mary Lowth

More than 690,000 people have been confirmed to have COVID-19 globally, but the severity of the illness can vary significantly.

Despite the scale and global efforts to understand the infection, there are, as yet, no official definitions to help people understand what doctors mean by mild, moderate or severe COVID-19.

Some people confirmed to have the illness have also reported no symptoms at all (asymptomatic).

Guidance on classifying the severity of illness is appearing for doctors, and while there is no substitute for a doctor’s direct advice, the public would benefit from knowing the difference between these severity levels, writes Dr Mary Lowth, a London-based GP and GP trainer.

Coronavirus attacking the lungs

Degrees of illness

It’s important to note that the more severe forms of illness are far less common than the less severe forms. Most people who are young and healthy are likely to be in group 2 (mild) or 3 (moderate).

The vast majority of healthy younger people don't develop a critical form of COVID-19. However, some young people have been seriously affected, and even died.

But the risk appears to rise with age and ill health.

If you’re over 60 or have heart disease, lung disease, cancer, kidney disease or immune deficiency, or you’ve had a solid organ transplant, then you’re in a vulnerable group and have a greater chance of severe illness.

Currently, doctors describe the way COVID-19 presents in terms of its severity. The terms commonly used are:

  1. Asymptomatic
  2. Mild
  3. Moderate
  4. Severe
  5. Critical

Asymptomatic COVID-19

Being asymptomatic means that you have no symptoms.

However, there are reports of loss of sense of smell in people who otherwise have no symptoms. This is now recognised as an official symptom of the virus and people experiencing it should self-isolate.

It may be that a large proportion of the population are infected or will become infected yet have no symptoms, but we don’t know that yet. It seems likely that this will happen most often in the healthiest and younger age groups, including most children.

The UK government is hoping to roll out testing for immunity against the virus. This involves testing a person’s blood for antibodies against the virus to see if people have already had it.

From this we can learn how many people have had the virus without knowing it, and where the virus has spread.

Early research on small numbers suggests as many as 50 to 75% of cases could be asymptomatic.

If you live in a house with people who think they’re infected because they have symptoms, but you have no symptoms, you may be an asymptomatic case. However, you won’t know for sure until an antibody test is available.

Mild COVID-19

Most healthy people under 60 who have symptoms will have this form of the illness. The symptoms typically last about 7 to 10 days.

As many as 80% of symptomatic cases are mild and remain mild in severity. However, people with mild illness can take a turn for the worse, sometimes quickly, and this is more likely in vulnerable groups.

The virus mainly affects the large airways in your upper respiratory tract. The lungs are made up of large airways (bronchi that lead into smaller airways (bronchioles) with tiny air sacs on the end (alveoli).

Your lungs contain a fluid called surfactant, which keeps them stretchy and helps keep the air sacs open.

When you have coronavirus, the virus inflames these airways and the severity of the illness appears to depend on the intensity of the inflamamtion and how far into the lungs it extends.

The main symptoms of mild illness (and how commonly they are seen) are:

  • temperature (in 90% of cases)
  • a dry cough (70%)
  • tiredness (40%)
  • feeling slightly breathless (20%)
  • muscle pain (15%)
  • headache (15%)
  • sore throat (14%)
  • diarrhoea (4%)

There are a few symptoms that are less common, including runny nose, sore or red eyes and vomiting. Low mood can also occur, as it does in many viral illnesses.

Coronavirus symptoms - woman coughing into hand

Moderate COVID-19

If your illness gets worse, you may become a moderate case - this is common. This may last for around 7 to 14 days.

People with moderate illness are more breathless and tend to have an increased heart rate, particularly if they’re moving around. This is caused by inflammation further into the lungs, so symptoms like coughing and breathlessness may be worse.

In addition to the symptoms you may get with a mild illness, you may also experience:

  • a temperature higher than 37.8C
  • feeling breathless when doing moderate exercise (such as walking up stairs)
  • soreness from coughing, but no pain
  • a more persistent cough, several times an hour
  • a headache, particularly if you’re hot
  • tiredness and a need to stay in bed
  • a dry mouth

Severe COVID-19

People with a severe form of the illness may develop pneumonia, which is inflammation of the lungs (caused by infection), right down into the tiny air sacs.

You’re far more likely to become a severe case if you’re older or have any of the health conditions that make you vulnerable. It’s not impossible in those who are healthy, just much less common.

Symptoms of severe COVID-19 infection and the resulting pneumonia include:

  • extreme breathlessness - you can't do much or speak easily
  • pain in your chest, tummy or back when you breathe
  • a high temperature
  • a tight chest
  • lacking the desire to eat or drink
  • seeming confused to others
  • bluish lips or face

Other common symptoms of pneumonia caused by the illness include (and you don’t have to have all or even most of them):

  • rapid and shallow breathing
  • fast heartbeat
  • unwell appearance
  • lowered blood pressure

Critical COVID-19

People with a critical illness are very unwell and develop severe pneumonia. The chances of this happening to someone who is otherwise healthy are extremely low.

A condition called severe acute respiratory syndrome (SARS) can develop -- when the small air sacs in the lungs become so inflamed and wet that they stick shut and the surfactant can’t hold the tiny air sacs open.

If this happens, you’ll need a ventilator to help inflate your lungs.

In the worst cases, people can develop sepsis, which causes other organs to stop working.

It's possible to survive critical COVID-19 if you have intensive care help, but even that is not always enough.

Although most people (including older, more vulnerable adults) will not develop sepsis, the number of coronavirus cases occurring in the world means that even a tiny percentage of people who develop sepsis amounts to a large number of people affected.

If you think you may have coronavirus, you can use our COVID-19 Symptom Mapper to check your symptoms and compare them with others around the world.

This should give you a better understanding of how the illness is affecting you and will help us to map the spread of the outbreak.


Coronavirus Disease 2019: Resources [Internet]. American Academy of Otolaryngology-Head and Neck Surgery. 2020 [cited 1 April 2020]. Available here.

Day M. Covid-19: identifying and isolating asymptomatic people helped eliminate virus in Italian village. 2020. Available here. 2020 [cited 1 April 2020]. Available here.

Qiu H, Wu J, Hong L, Luo Y, Song Q, Chen D. Clinical and epidemiological features of 36 children with coronavirus disease 2019 (COVID-19) in Zhejiang, China: an observational cohort study. 2020. Available here.

Coronavirus Disease 2019 (COVID-19) – Symptoms [Internet]. Centers for Disease Control and Prevention. 2020 [cited 1 April 2020]. Available here.

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Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.

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