[EvaH (OP)]

Judi asks:

Do we know the dosage required to get Covid infection?  I know they can find load by looking at the amount of virus people have in their bodies, but how do you figure out how much it takes to make you sick?  Do we know this for other diseases? Is it different for viruses and  bacteria?

What do you think?

[evan_au]

At the lower limit, it only takes 1 virus particle to successfully infect a lung cell and cause a lethal infection.
- This one virus particle will then replicate, producing hundreds or thousands of other virus particles, many of which will go on to infect other cells in the lungs, and possibly spread into the bloodstream, infecting the lining of blood vessels and other organs like heart and kidneys
- Some of these virus particles will be breathed out, to be breathed in by other people.

Lets assume that an infected person indoors breathes out 1000 virus particles while meeting with another person for 1 hour at a restaurant. If the other person is 1.5 meters away, they will probably breathe in around 200 of them.
- How many of these virus particles will successfully infect a cell in the other person is something that I don't know.
- But if you are a health worker who spends 8 hours per day with many infectious people, the exposure is likely to be much higher, despite wearing a surgical mask.

[alancalverd]

despite wearing a surgical mask.

which is why you shouldn't use a surgical mask in an infectious area! The loose-fitting surgical mask (along with loose gowns and scrubs) is intended to prevent staff from infecting patients. A fully-fitted mask (and a disposable NBC suit) reduces the probability of patients infecting staff.

The extent of public and political confusion on this simple matter is remarkable. I am delighted by the courtesy that other people extend to me by wearing a surgical mask in public, but they aren't doing themselves any favors. 

We tend to think of "surgical" = good, clean, precise, and so forth, but to avoid catching COVID you need to think "industrial" or "military" - that stuff can kill you.

[Bored chemist]

The extent of public and political confusion on this simple matter is remarkable

The government is confused about many things, but it's aware that it doesn't particularly care which members of the public die.
There aren't enough PPE masks to go round, and the government knows it (because they decided not to maintain stocks)
So a cheap, readily available or home made mask that stops them spreading it is fine.
That way, the "good stuff" - the actual PPE masks- can be kept for the people who are exposed via work- notably in healthcare.

[syhprum]

I am shocked to learn that breathing in one virus particle could cause a lethal infection has my body no defenses or are we all doomed?

[alancalverd]

"Could". Yes, we have defenses, but one successful invader can in principle do unlimited damage.

Viral infection isn't like a plane dropping one bomb, but landing in a factory and building a hundred new bombers.

[set fair]

https://www.sciencemediacentre.org/expert-reaction-to-questions-about-covid-19-and-viral-load/

[alancalverd]

Excellent reference!

It's interesting to consider what might happen with a "homeopathic" dose of a single virus. If it managed to replicate before the innate immune system recognised a threat, the situation would be pretty much the same as inhaling a large dose. This is consistent with some models of cancer, particularly breast cancer, where a single malignant cell can replicate several times before the immune system gets round to rejecting it, by which time it has established a significant tumor.     

[Edwina Lee]

I remember that when the UK had Foot-and-Mouth, the number of viral particles required to infect was eventually worked out at 3, and that was eventually controlled by slaughting lots of cattle.   
I guess that covid-19 requires a far higher viral count to get infected. Otherwise, we would all be infected by now.

My hunch is that the prominent 5 day oscillation cycle we see in the infection data tells us important characteristics of the viral infection.

[Bored chemist]

My hunch is that the prominent 5 day oscillation cycle

What 5 day oscillation?

[Bored chemist]

I remember that when the UK had Foot-and-Mouth, the number of viral particles required to infect was eventually worked out at 3, and that was eventually controlled by slaughting lots of cattle.   

It could have been eradicated by vaccination, but the politicians were more concerned about money.

I guess that covid-19 requires a far higher viral count to get infected. Otherwise, we would all be infected by now.

What does the count matter?
How high or low would it need to be to get to the inhabitants of North Sentinel island?
https://en.wikipedia.org/wiki/North_Sentinel_Island

[alancalverd]

Foot and mouth, like swine vesicular disease, has had a long time to adapt to its host. COVID is relatively new in humans and, unlike Ebola, transferred from a very different species. Given enough time and stupidity, it may become as infectious as FMD. The center of stupidity seems to have moved from Wuhan to Washington DC, but the lower physical density of population in the new host country means it will take a bit longer to evolve the next level of infectivity. 

The UK's non-vaccination policy for farm animals was based on the historical export of breeding stock - or so we were told at school 60 years ago. The objective was to maintain an absolutely clean herd by slaughter, so no asymptomatic carriers could be exported and all (very few) imports were quarantined. Not sure the entire policy could be adopted for COVID, but quarantine has proven cheap and effective for pretty well every infectious disease since Leviticus. Pity that modern politicians wave the bible instead of reading it.

[Edwina Lee]

My hunch is that the prominent 5 day oscillation cycle

What 5 day oscillation?

Have a look at the daily infection graphs on this site. They all display a 5 dayish oscillation cycle.
https://www.worldometers.info/coronavirus/

[Edwina Lee]

I guess that covid-19 requires a far higher viral count to get infected. Otherwise, we would all be infected by now.

What does the count matter?
How high or low would it need to be to get to the inhabitants of North Sentinel island?
https://en.wikipedia.org/wiki/North_Sentinel_Island

I don't know what Bored Chemist is trying to say here, but viral load clearly matter according to Set Fair's article:-

https://www.sciencemediacentre.org/expert-reaction-to-questions-about-covid-19-and-viral-load/

[alancalverd]

My hunch is that the prominent 5 day oscillation cycle

What 5 day oscillation?

Have a look at the daily infection graphs on this site. They all display a 5 dayish oscillation cycle.
https://www.worldometers.info/coronavirus/

It's a 7 day cycle, with peaks on Fridays. Most people will work through a low-grade infection and report sick at the end of the working week, with maybe another peak on Monday. Labs may back up their results but post them all on Friday to clear the deck for next week.

The deaths graph is odd, as that also peaks on Friday. Whilst recorded deaths peak on Mondays (because register offices are often closed at the weekend) recorded cause of death seems to peak at the end of the doctors' working week. 

As foir viral load, it's pretty obvious that the more you have, the more likely you are to infect someone else, and the morf you inhale, the more likely it is that some will penetrate your first line of defence and set up shop before the immune system gets into gear. The lack of early symptoms is interesting. It seems as if the innate immune system is very insensitive to the virus, so you can build up an overwhelming viral load before the adaptive system adapts, by which time the innate inflammatory response suddenly turns on as well, producing the excessive inflammation that can become fatal.     

[Bored chemist]

I guess that covid-19 requires a far higher viral count to get infected. Otherwise, we would all be infected by now.

What does the count matter?
How high or low would it need to be to get to the inhabitants of North Sentinel island?
https://en.wikipedia.org/wiki/North_Sentinel_Island

I don't know what Bored Chemist is trying to say here, but viral load clearly matter according to Set Fair's article:-

https://www.sciencemediacentre.org/expert-reaction-to-questions-about-covid-19-and-viral-load/

What I was saying was that the number of viral particles needed to produce an infection is not, by a long way, the best predictor of whether or not "

we would all be infected by now.

If you had actually tried to answer the questions, you might have realised the problem with your assertion.
Is there some reason why you didn't try to answer them?

It's fairly easy to imagine two  viral diseases where the number needed was the same, but the shedding rates were very different and thus the overall infectivity was different.

[Bored chemist]

My hunch is that the prominent 5 day oscillation cycle

What 5 day oscillation?

Have a look at the daily infection graphs on this site. They all display a 5 dayish oscillation cycle.
https://www.worldometers.info/coronavirus/

So you think that 5 is the same as 7.

Well... it's a viewpoint.

My hunch is that the prominent 5 day oscillation cycle we see in the infection data tells us important characteristics of the viral infection.

If you find out what the actual facts are, it's easier to explain them.

[Edwina Lee]

My hunch is that the prominent 5 day oscillation cycle

What 5 day oscillation?

Have a look at the daily infection graphs on this site. They all display a 5 dayish oscillation cycle.
https://www.worldometers.info/coronavirus/

It's a 7 day cycle, with peaks on Fridays. Most people will work through a low-grade infection and report sick at the end of the working week, with maybe another peak on Monday. Labs may back up their results but post them all on Friday to clear the deck for next week.

The deaths graph is odd, as that also peaks on Friday. Whilst recorded deaths peak on Mondays (because register offices are often closed at the weekend) recorded cause of death seems to peak at the end of the doctors' working week. 

As foir viral load, it's pretty obvious that the more you have, the more likely you are to infect someone else, and the morf you inhale, the more likely it is that some will penetrate your first line of defence and set up shop before the immune system gets into gear. The lack of early symptoms is interesting. It seems as if the innate immune system is very insensitive to the virus, so you can build up an overwhelming viral load before the adaptive system adapts, by which time the innate inflammatory response suddenly turns on as well, producing the excessive inflammation that can become fatal.     

So it is 7 afterall.  My eyes must have gone funny a the past few days.

[alancalverd]

My eyes must have gone funny a the past few days.

You should get out more.
Or maybe not!

[Bored chemist]

My eyes must have gone funny a the past few days.

Test them by driving to Barnard Castle.