[jfoldbar (OP)]

i was chatting to someone about the tiredness symptoms of covid. then i became curious.   
generally when people have the flu, they can feel tired. i always thought this is because the immune system is working overtime. this however results in other symptoms too, like high temp.
however with covid people can have extreme tiredness with no other symptom at all.
do we know what is going on biologically that causes extreme tiredness (especially that comes and goes) without any other symptom at all?

[evan_au]

Many of the symptoms we associate with "cold and flu" are actually caused by Interferon, a signaling molecule produced by human cells when they detect a viral infection; it is a defense mechanism which rallies the immune system and protects the infected individual.
- This cluster of symptoms often include tiredness, headache, running nose, fever and aching muscles.
- Because vaccination simulates a viral attack, these symptoms are often associated with vaccination.
- Studies in animals have shown that sick animals often refrain from contact with others, and sick-looking animals are shunned by their usual contacts.
- This ability of Interferon to signal to other members of the same species suggests that Interferon protects not just the individual, but the whole group of animals.
https://en.wikipedia.org/wiki/Interferon

SARS-COV2 is far more stealthy than SARS or MERS (two previous coronavirus outbreaks) because it manages to suppress  the Interferon response for a couple of days after you become infectious. This allows it to quietly infect your family and friends before you realize you are sick.
- But the sheer volume of virus eventually becomes too great for your body to ignore, and about half of infected people eventually come down with symptoms.

"Long COVID" can occur to people who were asymptomatic in the original infection, but is more likely in those with severe infection.
- One of the symptoms that can occur to Long COVID victims is "brain fog" and extreme tiredness
- In some ways this is similar to other post-viral syndromes, including glandular fever (Epstein-Barr virus).
- COVID-19 is known to cause loss of smell in up to 40% of victims. MRI studies have shown that it doesn't just infect the nasal neurones, it can cause brain damage. This is thought to contribute to the brain fog/tiredness.
https://en.wikipedia.org/wiki/Long_COVID#Symptoms

[chiralSPO]

Immune response sufficient to have high energy demand doesn't necessarily require fever.

Another important aspect of covid that can cause fatigue is reduced oxygen supply. the virus can infect/inflame/damage the lungs, and reduce how much oxygen is available. Even a slight decrease can have major implications on energy availability.

[jfoldbar (OP)]

Another important aspect of covid that can cause fatigue is reduced oxygen supply. the virus can infect/inflame/damage the lungs, and reduce how much oxygen is available. Even a slight decrease can have major implications on energy availability.

wouldnt this mean that the patient have trouble breathing?

[jfoldbar (OP)]

- This cluster of symptoms often include tiredness, headache, running nose, fever and aching muscles.

but can interferon cause only tiredness?

[chiralSPO]

wouldnt this mean that the patient have trouble breathing?

Yes, but it doesn't mean that they would know it! The human body is very good at noticing that it has too much carbon dioxide, but not so good at noticing that it doesn't have enough oxygen. Poor lung function can interfere with both carbon dioxide expulsion and oxygen absorption, but the carbon dioxide issues really only manifest in obvious ways when the body is working hard.

My wife's uncle had covid (he was unvaccinated, and most likely had delta variant, but we don't know for sure). He only noticed feeling short of breath when he exerted himself (like going up the stairs), but his resting O₂ stayed between 89 and 92 % for almost two weeks (it's supposed to be above 94 %). He was very tired, but did not always have labored breathing...

[jfoldbar (OP)]

but not so good at noticing that it doesn't have enough oxygen

so, the tiredness is from lack of oxygen?
so, person is breathing. seems like he is breathing normally, however not enough oxygen is getting in?

[chiralSPO]

but not so good at noticing that it doesn't have enough oxygen

so, the tiredness is from lack of oxygen?
so, person is breathing. seems like he is breathing normally, however not enough oxygen is getting in?

That's one possible explanation, at least for covid.

[evan_au]

wouldnt (lung damage) mean that the patient have trouble breathing?

Mountaineers attempting to climb Mt Everest go to a series of base camps at higher and higher elevations. They spend a few days to a week at each base camp to allow their body to adapt to the reduced oxygen. And your body responds to reduced oxygen, eg by making more red blood cells.

COVID damages lungs, but it often occurs very slowly, allowing patient bodies to adapt.
- It's not clear to me whether the virus does the damage directly, or if it is an auto-immune reaction that occurs after the acute viral infection is over.

I heard an interview with an emergency doctor in New York (recorded in May 2020) who dealt with COVID patients. He said that normally, if you measure an oxygen saturation level of 70%, the patient is unconscious; but he saw patients with these levels who were chatting to him and saying they felt quite all right - and surprised when he was trying to give them oxygen.
He reported one patient who came in with oxygen saturation around 55% (from memory), and when he said that we need to get you on a ventilator immediately, they said they wanted to finish their social media post first!
https://www.wnycstudios.org/podcasts/radiolab/articles/dispatch-5-dont-stop-believin