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Covid-19. What is mild, moderate and severe disease?

Everyone is talking about mild, moderate and severe case, and critical cases. What does this mean?

There are, as yet, no clear guides for patients to tell them what doctors mean by mild, moderate or severe Covid. Some guidance on classifying illness is appearing in research papers and epidemiological reports, but it is not very specific. For example, the broad definition of moderate disease seems to be that is worse than mild disease but not severe. What has been written on classifying illness is incorporated into the advice here.


What this page can and cannot do:

This page is to offer you information regarding how you may experience Covid. It not a substitute for a doctor’s direct advice to you. It is not a substitute for seeking help if you feel you need it. It is written by a GP but it is not intended to replace one.


When to seek help:

There is no substitute for being assessed by a health professional, and this may need to be on the phone or face to face so that your blood pressure can be taken, your oxygen levels measured and your general condition assessed.

If you are in a vulnerable group, seek advice: If you are over 70 or have significant heart, lung or kidney disease, immune deficiency, are on oral steroids or have had a solid organ transplant, and you develop symptoms of Covid, please seek advice. Covid can worsen swiftly. Vulnerable people should not manage their Covid alone by relying on written advice. Written advice cannot check up on you. Government advice is constantly updated. Additional advice for the older and vulnerable is also found here.

If you have symptoms that are worsening, seek advice: This is particularly true if you have any of the severe features. People with Covid, particularly those at risk, can deteriorate from mild to moderate to severe. Likewise, if you are afraid please seek advice. That is what health professionals are there for.


How people have been ‘classifying’ Covid?

You will have heard mention in the public health announcements and in the media that some people have had mild covid, others moderate, severe or critical covid. Yet, when you look up the symptoms, you are told to expect a cough and a temperature. How bad a cough is a mild case? How much of a temperature is severe?

This article classifies Covid into:

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

These classifications are consistent with published descriptions so far. When formal, agreed guidance on what we call mild, moderate and severe cases are published, they may differ slightly. For now this seems to fit the way the terms are being used.

Each of these ‘levels’ of illness considerably less common than the previous one.

Most people who are young and healthy are likely to be in group 1 or 2. However there are reports emerging that some young healthy people have become very unwell. In percentage terms this appears to be very low, but the chance is not zero,

If you are over 60, or you are vulnerable because of other health conditions, then you have a greater chance of moving further down the list.

You may well not exactly fit any one classification exactly. You may be between two. You may move from mild to moderate symptoms, then back to mild. You may have symptoms from more than one group.  Illness is like that. When huge numbers of people are affected they will not all fit perfectly into simple group classifications of mild, moderate or severe. But if your symptoms are predominantly of mild disease with, say, one or perhaps two moderate features, you may be ‘mild to moderate.’ If your symptoms are predominantly moderate, but you are clearly getting worse and start to get symptoms that seem to fit ‘severe’, do not wait to seek urgent help until you have more of the severe symptoms.

In each category most people will not progress to the next. You are more likely to progress if you are vulnerable, particularly if you are over 60. If you are in the very vulnerable group and you think you have even mild Covid, you should seek advice. Remember, each of these ‘levels’ of illness considerably less common than the previous one. Most people who are young and healthy are likely to be in group 1 or 2.

If you are over 60, or if your health is normally compromised by heart disease, lung disease, cancer and chemotherapy, immune deficiency, or if you have had a solid organ transplant, then you have a greater chance of moving further down the list. Even in the over 80s group, however, very significantly more people will have mild or moderate disease than severe disease.

Professor Kevork Hopayian has posted this video on youtube showing how the virus attacks:

Classifying symptoms : how bad is my Covid?

What is Asymptomatic Covid-19?

It is possible that a large proportion of the population will have Covid-19 yet have no symptoms at all, but we don’t know that yet. It seems likely that this will occur more often in the healthiest and the younger age groups, including most children. The government are hoping to roll out immune testing in order to discover how many people have had Covid unknowingly, without symptoms. Early research on small numbers suggests it could be 40-50% of cases. It could be even more – figures of 80% have been mentioned. We don’t know.

Being asymptomatic means that you have no symptoms. (There are reports of loss of sense of smell in asymptomatic people. Technically even that is a symptom).

If you live in a house with people with probable Covid infection and you have no symptoms, you may be an asymptomatic case. You will not know for sure until we have an antibody test. The government is working on this. At the moment the test is being evaluated for accuracy. An inaccurate test is worse than no test.

Remember, this entire pandemic probably started with one case. Don’t be a spreader.


What is Mild Covid?

The virus affects mainly your upper respiratory tract, mainly the large airways. Main symptoms are temperature, with or without cough.

Patients with mild illness have fluey symptoms. These may include dry cough and mild fever, but the fever may not reach 37.8, and there may sometimes be little or even no cough. Patients might notice a feeling of being a bit more breathless than normal on exercise, but they are not out of breath on normal household activity.

With Mild Covid:

  1. You may have a fever, including one that doesn’t reach the 37.8 mark.
  2. You may lose your sense of smell
  3. You may have tiredness, muscles aches or a headache.
  4. You are not highly likely to have sore throat or runny nose, but they do occur in some cases.
  5. You do not have marked breathlessness.
  6. Your self-care, cooking, eating and drinking are not affected. Your appetite is normal or fairly normal.
  7. You may feel sad or weepy.
  8. Most normally-healthy people under 60 who have symptoms will have this form.
  9. The symptoms typically seem to last about 7-10 days.

Most (81%) symptomatic COVID-19 cases are mild and remain mild in severity. However patients with mild disease can deteriorate, sometimes quickly, and this is more likely in at risk groups.


What is Moderate Covid?

If your illness gets a worse than this then you may move towards being a moderate case, with inflammation lower down in the lungs, so lung symptoms like cough are more marked.

The lungs consist of large airways (bronchi), smaller airways (bronchioles) and then the tiny air sacs on the end (alveoli) where oxygen is extracted from the air. They contain a fluid called surfactant which keeps the lungs stretchy and compliant and helps keep the air sacs open. Patient with moderate Covid may have inflammation moving down into the bronchioles. They are more breathless and tend to have an increased heart rate, particularly if they are moving around.

With moderate Covid:

  1. You may have a more troublesome cough than those with mild symptoms.
  2. Your temperature is more likely to reach or exceed 37.8.
  3. You may be breathless on exercise, even on walking up the stairs, but not to a degree that alarms you and if you potter around your house or sit still you are not breathless.
  4. It may be a little sore to keep coughing but you are not in pain.
  5. The cough may be, for a few days, very persistent, coughing many times an hour.
  6. Your sleep may be slightly disturbed by your cough, but you do get some sleep, and you are not breathless in bed.
  7. You may get diarrhoea. Nausea and vomiting are, however, unlikely.
  8. You may have a headache, particularly if you are hot.
  9. You may show early signs of inflammation of your lungs – you are more breathless than a mild case, where breathlessness is only slight and on exercise.
  10. You may feel tired, but still able to move about your own home comfortably, and you can shower and self-care if you must, even though you may not want to.
  11. You may feel dry-mouthed, from breathing through an open mouth. But when you pass urine it is still pale yellow, the colour of a glass of lager, so you are not dehydrated.
  12. You can sit and watch TV or read a book without feeling you are struggling to breathe, or worrying that you can’t breathe.
  13. If you talk to others nobody thinks you are confused or not making sense.
  14. You may be able to prepare your food and drink, or you may feel too weary to do so, but you are still able to eat, even if it is less than usual. Your ability to eat is not prevented by the need to breathe.
  15. For a few days you may feel so tired that you want to stay in bed.
  16. You may feel miserable, weepy or low.
  17. Moderate Covid is very common. It seems to last about 7-14 days.

If you have these symptoms but you are becoming increasingly breathless, if you cannot manage basic things like showering and eating, or if you cannot speak in whole sentences without taking extra breaths, seek medical advice by phone at once.


What is Severe Covid?

Severe Covid means that you have pneumonia, which is inflammation (caused by infection) of the lungs themselves, right down into the tiny air sacs (alveoli).

Severe Covid is much more likely if you are older or have any of the health conditions that make you vulnerable. It is not impossible in the healthy, although but it is much less common

Patients with severe disease are very breathless (and may be unable to breathe at a comfortable rate on slight moving around or even at rest), and breath faster than usual, even when sitting still. They cannot finish a sentence without extra breaths. They may even avoid speaking. Their oxygen levels may have fallen so the urge to breathe faster is strong. Doctors will measure breathing rates when assessing this condition. Normal adults breathe at about 12-18 breaths per minute when they are not thinking about it. In pneumonia the rate rises, sometimes markedly. (Note however that these are adult rates. Young children breathe much faster than adults).

If you think that you or someone you know has symptoms like this you should seek urgent medical help.

In Severe Covid

  1. You are noticeably breathless, you can do very little.
  2. You may be breathless even when sitting still.
  3. You may be unable to complete a sentence in speaking.
  4. You may feel you are having to work hard to breathe.
  5. Your chest, tummy or back hurt when you breathe.
  6. Your temperature is high.
  7. Your chest is tight, as if you can’t expand it properly.
  8. You can’t keep up with your breathing, as if you had just sprinted hard (except you haven’t).
  9. You are not eating or drinking normally.
  10. You can’t read or watch TV, you are too focussed on breathing or feel too unwell.
  11. Others think you are confused.
  12. Other common symptoms of Covid pneumonia include (and you do not have to have all or even most of them):
    1. rapid and shallow breathing
    2. rapid heartbeat
    3. unwell appearance
    4. lowered blood pressure

If you or someone you know have symptoms like this, seek medical advice by telephone. If you are too breathless to talk to someone on the phone call 999.

What is Critical Covid?

In severe pneumonia, a condition called SARS (Severe Acute Respiratory Syndrome) can develop, when the small air sacs in the lungs become so inflamed and wet that they tend stick shut, surfactant cannot do its job as there is too much inflammatory fluid in the alveoli responding to the virus. You need help inflating your lungs. This is what a ventilator is for. People with critical covid are very unwell.

The most critical of all of these patients develop sepsis and other bodily organs stop working, a very dangerous situation for the body.

The chances of this happening to any previously healthy individual are extremely low, but not zero. Whilst the vast majority of healthy younger people do not develop critical Covid, you can never say never in medicine. Worldwide amongst the hundreds of thousands of cases, there have been some young people who have become extremely sick and, even, some who have died.

The majority of elderly patients also do not develop critical Covid, but a significant proportion do. The risk rises with age and with the health conditions that we know make you vulnerable.

The number of Covid cases that we now have in the world means that even a small percentage of patients developing sepsis makes for a large absolute number of patients. Critical covid can be survived, with intensive care help, but even that is not always enough, as we are seeing worldwide.

The difference between severe and critical Covid is one a hospital health care professional will make. Both need to be treated in hospital urgently.


Please share this article if you found it useful. there are a lot of worried people out there. This is not a commercial blog, and this article is written by an NHS doctor for information.

The contents of this blog, amended in style, have now been posted by the author on the patient and health professional website at:

Mild Covid

Moderate, Sere and Critical Covid

Research links, for interest

On asymptomatic cases on a cruise ship

Italian paper confirming that 44% of Italy’s lab confirmed cases were asymptomatic (in Italian)

On risk assessment, March 12th, European centre for Disease Prevention and Control

WHO research trials page

WHO Covid home page

UN Covid humanitarian planning

UN and WHO coronavirus portal

UK government public information site

UK emergency legislation: Health Protection Coronavirus Regulations (2020)

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Hergest the Hound

I am a dog of many thoughts.

5 replies

  1. Thanks, a very sensible and pragmatic summary which as you say works in parallel with NHS advice. Glad your hound found something useful to do during lockdown, ours is getting exhausted by each member of the house taking him out separately for a run or walk every day,

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