Viral load - can someone explain?

[OverUnderSidewaysDown]

Can someone with expertise explain viral load? I used to understand it as meaning how much virus an individual was carrying and shedding , i.e. how infectious they were to other people. But in recent days I've heard it described as meaning how much virus a person is exposed to, and used as an explanation for why doctors and nurses are getting the virus more than other people.
Which is correct? A doctor could catch it from just one patient, but if they treat one hundred patients does their susceptibility increase?

Crikey this is really useful stuff, as you say OP!!

But herd immunity has only been proven in practice with vaccination programs not majority infection of a virus

Not exactly. The concept changed when vaccinations came in, because there was no longer any reason to leave a part of the herd vulnerable.

Before vaccination, you woukd end up with a scenario whereby enough if the population had the disease to stop it circulating rapidly, and althought it hadn't gone away totally the odd case that popped up didn't become an outbreak, because o many peripheral wwere immune.

But new people kept being born with no immunity and no means to vaccinate them in infancy. So every few years, there were enough non-immune people that the odd case could turn into an outbreak and sweep through the non-immune. That's why some diseases are known as childhood diseases. It's not because they had some peculiar affinity for children, or was more serious in them (indeed often quite the opposite). It was simply that they were usually seen in children, in waves every few years, as the herd had its immunity topped up

Good explanation of viral load, @AnotherMurkyDay

But herd immunity has only been proven in practice with vaccination programs not majority infection of a virus

There is evidence for naturally acquired herd immunity, for example, before vaccination it was observed that there were cycles of measles epidemics. If you took a city like Liverpool, if there was a measles epidemic one year, there would be very few cases the next year because herd immunity in the local population prevented the disease spreading but the next year there would be another epidemic because enough babies had been born to take the number of susceptible individuals below the herd immunity threshold.

We don't know whether infection and recovery from coronavirus confers immunity or how long any immunity would last so we can't rely on naturally acquired herd immunity as a way to stop the pandemic. And sacrificing millions of lives, many of them HCPs, to achieve herd immunity for the "greater good of society" would be totally unethical, as well as destroying the NHS. So stay at home.

Yeah sorry I over simplified that I meant that herd immunity protecting the most vulnerable has only been proven through vaccination programs. You are completely right about measles, I think the issue is that it has never been acceptable ethically to intentionally infect the majority of a population with any virus just to see whether herd immunity works on a longer term basis. Which is why measles outbreaks have always involved some level of quarantine because the mortality is too high.

And that's with something like measles which we know does usually result in immunity for individuals, with covid 19 we don't know whether immunity will last

Thanks for explaining anohermurkyday unfortunately I think I probably have it (no test so don't know) but now looks like my dh is coming down with it and I'm worrying that our children will get a really bad dose. I guess it also means I'm at risk of getting it again, and I'm still no where near 'better', have been self isolating but unfortunately that's only when I got the fever, I didn't realise early symptoms where possible corvid-19.

I wish there was more information out there about viral load, (and about symptoms too but that's another thread) but thank you again for explaining it so clearly

Another interesting thing about immunity is that once immune you need regular exposure to the virus or bacteria to maintain immunity. This is why we have booster vaccinations.
If we totally irradiate a disease the herd immunity will eventually reduce because no one is exposure to disease to boost immune response.
HepB is very infectious and like Covid-19 lives outside the body for a long time. It’s carried by most bodily fluids. HCP now have to have HepB immunity to work. Regular screening for immunity has shown most HCP have not needed boosters regularly because the virus is probably still rife in the general population so we are regularly exposed to it.

This is so interesting I am fascinated with the science behind it! Does this go for every virus?
Asking because last year one of my dc had a virus and was poorly for a week. Then next dc became poorly and was extremely poorly for 10 days. Then next was my turn and I felt like I was dying for 3 weeks. This would explain it if I was given two doses simultaneously, both of varying degrees of severity!

@tempestterra

Same in my house we are on the final person now. People 1 and 2 had it the least bad, person 3 had it a bit worse, person 4 most poorly.

So unfortunately my anecdata is that it did follow the predicted pattern

@bobstersmum

Maybe or it may be that your immune system was working less well by the time you caught it because you were run ragged caring for your DCs. Also, children tend to get things more mildly. That's why people used to throw "chicken pox parties" so that everyone would get it and therefore wouldn't get it when they were older when it would be worse. The risk with that is that although children do tend to fight things off quicker, if they take a turn for worse that can happen incredibly quickly too and can be harder to control (things like febrile convulsions and meningitis, as well as them being harder to medicate and intubate). With covid 19 so far children have mostly been less affected but obviously not all children have healthy immune systems. And we can't know how well somebody else's immune system will react. There are things we can use as predictors, like certain medications or pre existing conditions, but it's always going to be a gamble. One of my DCs has been really unwell, and I have had to seek medical advice (person 4). So it's not as simple as "kids don't get it" no pre existing conditions either or anything like that. We have all had very high fevers, and one of mine who used to have febrile convulsions I was very worried about too (but burnt through it quickly and better now) I am sure we have had covid 19 because it has followed the pattern perfectly. My first symptom was losing my sense of taste and smell, which they are only just calling a symptom

I'll just keep everything crossed DC have already had it, but very mildly. And hope DH manages to get through it.

Thinking about it last summer ds had chicken pox first then dd got the pox and was so much worse. Happened with a really nasty virus last November too, where ds got it mildly and dd was really unwell.

Thank you again another and hope your last member of the family makes a speedy recovery.

Thank you @tempestterra

Yes I was the chicken pox spreader in my family as a child and have never lived it down because my brother got it really badly and still has chicken pox scars to this day. I do too, to be fair, we both had it badly but he had to be hospitalised. It triggered a life long interest in illness for me.

And yes they are saying lots of people may have had it already really mildly and not even known! So fingers crossed for you @tempestterra

This is fascinating and explained really clearly. Thanks @AnotherMurkyDay and others.

Hi OP, I know a little about viral load, considering mine gets taken every 6 months to confirm it’s still behaving itself and undetectable!

So viral load is the measurement of a particular virus taken in the body from where it can be found - in the case of HIV it’s the blood and in the case of coronavirus via a throat or nasal swab as it affects the respiratory tract. It is measured as copies per ml and this can change massively depending on what stage illness the patient is at. Our viral load machines can measure from

HIVpos There is evidence that severe cases of COVID-19 have a higher viral load.

www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30232-2/fulltext

There is also evidence with SARS that initial viral load effects outcomes:

www.ncbi.nlm.nih.gov/pmc/articles/PMC527336/

It would also explain why HCP are being more severely affected than would be expected for their age group/fitness.

Yes there is correlation between repeated exposure, high viral load and unexpected poor outcomes, but as with most of the covid 19 science most of it is somewhat speculative because there are a lot of unknowns and all research is in the early stages. That's what's so worrying that we really can't predict things that clearly as we don't know the patterns yet.

I'm sorry but I'm not quite getting it.
So someone coughs on you, and you get 100 germs. Ok, most healthy people can.handle 100 corona 19 germs. Its aim is to spread and reproduce within the body, but one blast of a hundred germs is usually manageable. It gets killed before it gets out of hand, and the person is only mildly ill.
So, is it different if you are walking about, pick up 100 germs on Monday. 50 on Tuesday. 100 on Wednesday, etc?

Sorry if I'm being a bit daft

You've kind of got it @7days. Very simply, say you are infected by just one virus particle. Once it is in your body it starts to replicate and say it takes an hour to do that. Then you have 2 virus particles, in an hour, they have replicated into 4. If you started with 100 virus particles though, in 2 hours, you would have 400.

It takes time for your immune system to recognise what is going on and respond. The more virus particles you start with, the greater number there will be before your immune system starts to fight back. If you are exposed to more of the virus in small amounts after your immune system has kicked in, as in your example, that probably will make no significant difference.

The issue is being exposed to large amounts of the virus repeatedly over a short amount of time, such as a someone caring for someone with COVID-19, even more so if you are an HCP caring for multiple people who may have different strains of the virus. The closer you get to someone, the more likely it is you will take on large amounts of the virus from them - if I throw a glass of water at you from 50cm away, more droplets will hit you than if I throw it from 2m IYSWIM. The longer time you spend with someone contagious, the more virus you will pick up.

It is not the only factor that can affect severity of the disease but it is one.

I have a question about this too, if anyone knowledgeable knows the answer..

Does it work that the lower the viral load of the initial exposure, the longer the incubation period before symptoms??

It is one of the factors that affects incubation period @littlemissmuffins.

This kind of explained it for me and has been shared a lot on facebook (sorry its a long one):
Why do we need to shut places where people group?

Remember this: VIRAL LOAD

There will be a lot about this. Why is it important?

With this virus, the amount of virus in your blood at first infection directly relates to the severity of the illness you will suffer. This isn’t unusual - HIV management is all about reducing viral load to keep people alive longer. BUT it’s very important in COVID-19.

So if you are in, say, a pub or religious building or entertainment venue with 200 people and a large number don’t have symptoms but are shedding, you are breathing in lots of droplets per minute and absorbing a high load of the virus. In a crowded space. They become ill over the next 48 hours. You then three days later wonder why you can’t breathe and end up in hospital. You’d decided because you were young and healthy it wasn’t going to be a problem. Wrong.

Fortunately but unfortunately because the elderly are isolating quite well, the initial UK data suggests that all age groups above 20 are almost equally represented in ITUs in England. Most of the cases are in London but the wave is moving outwards.

This means that being under 60 and fit and well doesn’t seem to be as protective as we thought. Why? Viral load.

This may be skewed simply by the fact that too many Londoners didn’t do as asked and congregated in large groups in confined spaces and got a large initial viral load. They then went home and infected their wider families. Which is why, as London is overwhelmed, we need to shut everything down to save the rest of the UK. We are a week at most behind London.

Our sympathies go out to the families affected in London and the critical care teams battling right now to save as many as they can.

If I sit with one person and catch this virus, I get a small viral load. My immune system will start to fight it and by the time the virus starts replicating, I’m ready to kill it.

No medicines will help this process meaningfully hence there is no “cure” for this virus. All we can do is support you with a ventilator and hope your immune system can catch up fast enough.

If I sit in the same room with six people, all shedding I get six times the initial dose. The rise in viral load is faster than my immune system can cope with and it is overrun. I then become critically ill and need me (or an ITU/HDU specialist) to fix it instead of just being at home and being ok in the end.

THIS BIT IS IMPORTANT:

If you are a large family group, remember that by being ill and in the same room, you will make each other ill or “more ill”. If you get sick, isolate just yourself to one room and stay there. Don’t all sit in one room coughing. You will increase the viral load for all of you, reducing your survival rate.

A family of six people may produce double the droplets of a family of three in the same space. Maths is important.

If one of you is symptomatic, assume you are all shedding and make sure you keep some space.

Parents are getting it from their kids because no one is going to stop comforting their child (nor should they) so the parent gets a big hit as well as the child. I don’t think that can be helped.

REMEMBER: THINK ABOUT VIRAL LOAD

It could save your life or your child’s.

Britain, it’s up to you now. We are preparing for the worst but we are hoping for the best from you. Please help us to help you. Stay home and take this seriously now before we need the army on the streets to remind you. Yes I’m serious.

#flattenthecurve

Very untechy explanation.

We've all seen films with a look out on the wall of a castle or fort? That's our immune system, both on the walls and wandering around the castle. It keeps watch, challenges anything trying to get into the castle and if something does get in then the soldiers fight them.

BUT the soldiers on the wall can only challenge the enemy / virus / bacteria that they recognise.

Once they recognise a pathogen they create an antigen which is the best way to deal with the recognition.

Vaccines deliver a pathogen that is (usually ) dead, so using the army analogy, this is sending the soldiers on a training exercise and also why babies get a bit grouchy after vaccination.

We also come into contact with pathogens that our army recognises and we don't even know about it.

Now if the antigens can get into the castle they start to breed. The best thing for an antigen to do is to get into the castle without being recognised and to breed without destroying the castle.

This is why Covid 19 is dangerous, no one before this had any antigens to it and most people had a mild cold / sniffles, the kind of disease you get in winter but not bad enough to keep you out of work / school, so it had lots of opportunities to invade other castles ie people.

Ebola is easy to contract but as it almost always kills its host and at best makes a person severely ill so they are not out and about infecting others.

HIV is a sneaky little thing that has 2 mechanisms to enter the body without being noticed, it then goes about reproducing and quietly removing the body's soldiers, for better info watch 'Myles Power' on you tube, the one about the HUV / AIDS trial.

Sometimes the body's army is reduced, because it is fighting or has just fought an enemy - this is why you sometimes get ill and then keep getting ill.

Sometimes the army decides to attack its own castle, that would be an autoimmune disorder.

Obviously the best thing for the castle is to not have any pathogens around, so social distancing and isolation protect everyone in this way.

PPE is like building another wall around the castle, no soldiers on it but so high it's difficult to get through.

Thanks so much for the explanations on this thread, I had no idea about the importance of viral load. It explains why healthcare workers are at higher risk and makes me appreciate the risks they are taking even more. I will now be much more rigorous about hygiene, keeping apart etc if our fairly large family gets it. Really helpful.