Got a symptom but not sure what's causing it? Use our award-winning symptom checker to find out – it's free!

×
31st March, 20207 min read

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

Medical reviewer:Healthily's medical team
Author:Dr Mary Lowth
Last reviewed: 22/02/2021
Medically reviewed

All of Healthily's articles undergo medical safety checks to verify that the information is medically safe. View more details in our safety page, or read our editorial policy.

More than 100 million 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 doctor who also trains doctors.

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.

The chances of getting severely ill are higher for some people than others. This risk rises with age – the older you are, the higher your risk. People aged 85 or over are at the greatest risk.

Certain medical conditions also increase your risk, and this applies to adults of any age. These conditions include cancer, chronic kidney disease, chronic obstructive pulmonary disease (COPD), Down’s syndrome, heart conditions, having a weakened immune system from an organ transplant, severe obesity (a body mass index of over 40), pregnancy, sickle cell disease, smoking, and type 2 diabetes mellitus.

Some other medical conditions may also increase your risk, but this isn’t known for certain yet.

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.

Research suggests that about 1 in 5 people is 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.

Mild COVID-19

Most healthy people who have symptoms will have this form of the illness. The symptoms typically last about 7 to 10 days. And most people with mild COVID-19 get better in 2 to 4 weeks.

Most mild cases will stay 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 are:

  • temperature
  • a dry cough
  • tiredness
  • loss of sense or taste or smell
  • feeling slightly breathless
  • muscle pain
  • headache
  • sore throat
  • diarrhoea

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

Moderate COVID-19

If your illness gets worse, you may become a moderate case - this is common. The majority of people who have moderate COVID-19 recover in 2 to 4 weeks.

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.

Was this article helpful?

We include references at the end of every article, so you know where we get our facts. We only ever take evidence from medically-recognised sources, approved by the UK National Health Service's The Information Standard, or certified by Health On the Net (HON). When we talk about popular health trends or claims, we'll always tell you if there's very little or no evidence to back them up. Our medical team also checks our sources, making sure they're appropriate and that we've interpreted the science correctly.

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.