As reports of Boris Johnson entering intensive care surfaced last night, it can be pretty confusing when we read about the increasing number of COVID-19 cases in hospital whilst also seeing the NHS advice that most cases are mild and can be treated at home.
So why are some coronavirus cases more serious than others? At what point do COVID-19 patients end up needed hospital care?
To understand why some COVID cases need more intensive medical care, we first need to understand how our immune system reacts when faced with a new virus.
How does our immune system react to viruses?
There are two parts to our immune systems response: a quick-fire initial reaction and a longer, more specific response to the exact virus or bacteria it’s fighting.
The first part is called the innate immune system. Its job is to detect the virus and create an initial response aimed at limiting its spread.
This first response isn’t specific to the virus, but a broad anti-viral response. The small proteins released cause side effects that can make us feel under the weather, such as headaches, fever and muscle aches.
“This response serves two purposes: to slow down the replication and spread of the virus, keeping us alive until the ‘acquired immune response’ kicks in (which, for a virus we haven’t seen, is about 2 to 3 weeks),” explains Dr Michael Skinner, Reader in Virology at Imperial College London.
Why do some coronavirus patients end up in intensive care?
The majority of COVID-19 patients only have mild to moderate symptoms; this means their innate and acquired immune responses have managed to work together to successfully fight the virus.
However, it’s expected that around 5% of the UK population will need additional medical support in hospital, with one third of those admitted needing intensive care.
There are three main ways that a more severe coronavirus infection can develop.
A slow acquired response
After the initial ‘innate’ immune response, the second ‘acquired’ phase kicks in once it has a response that’s specific for that virus.
In older people or those whose immune systems don't work as well as they should, this second stage may take longer to kick in, meaning the virus has more time to replicate.
If it’s an illness where higher viral load means more severe symptoms, this additional time means that viral load is increased and the person will become more and more unwell.
Medical opinion is currently divided on whether higher viral load does cause more severe symptoms in COVID-19 patients.
To try and contain the virus, one of the things our initial innate immune response does is to cause inflammation.
Whilst this can be helpful in the short term, inflammation can have a serious impact on other parts of the body if it continues for too long.
As well as providing a virus-specific solution, the ‘acquired’ immune response is also supposed to tell the innate response to stand down now that a better solution is available. This includes telling it to stop causing inflammation.
According to Dr Skinner, “[inflammation] is useful in containing the virus early in an infection but can result in widespread damage of uninfected tissue if it becomes too large and uncontrolled, a situation named ‘cytokine storm’ when it was first seen with SARS and avian influenza H5N1. It is difficult to manage clinically, requiring intensive care and treatment and carries with it high risk of death”.
Pre-existing health conditions
Another reason that COVID-19 cases may become more serious is if someone has a pre-existing health condition.
This could be something respiratory related, such as asthma or COPD, or may impact a completely different part of the body, like diabetes.
If someone has other underlying health conditions, this means that the body’s systems may already be under a lot of stress.
The added challenge of fighting a virus may cause too much strain and they may need more serious medical help.
By proactively self-isolating, those with existing health conditions reduce their risk of being exposed to the virus and the stress on their body becoming too much.
These vulnerable groups are all more likely to require critical care if they become ill with COVID-19. If too many people need hospital treatment at the same time, the NHS will be unable to cope.
Taking care not to expose vulnerable people to the virus through shielding should help reduce the overall number of people needing critical care and help the NHS cope during this pandemic.
If you have symptoms, it’s important to isolate yourself. Use our interactive isolation tool to figure out how long you should stay isolated for.