A doctor told me that even if a virus is injected directly into a bloodstream (such as, for instance, the HIV virus,) there is a less-than-20% chance that the disease will actually take hold in the human body.
I’m curious why this is the case (I’d always assumed that if you take a virus - be it HIV, Ebola, Zika, flu, etc. and inject it into someone’s body, they automatically get the disease.) The body shouldn’t have antibodies against the virus yet. Is this true of every virus - that there’s only a small chance that it will actually “lodge” in the body?
That lone virus needs to find a hospitable spot to do its work. It can’t attach to just any human cell; it needs a suitable environment, a “host” cell. Hepatitis C virus loves hepatocytes. HIV needs the right immune cell. Cold viruses love naso-pharyngeal epithelium.
And even if they hit the right host cell, the chance of successful entry and reproduction is far from certain. It might just get lodged in the cell membrane, might not make it to the right area in the cell to commandeer its reproductive processes, etc.
I actually think the odds of a single virus causing an infection are far, far less than 20%. I’d peg them as much less than 1% just because there are so, so many things that can go wrong.
We know that people who are positive for HIV, but with < 40 viral copies per ml of their blood are essentially non-infectious. And that people with < 400 copies/ml will cause 1 infection per 79 person-years, a very, very low rate of infectivity.
Interesting. (I didn’t mean a single lone virus, but rather, a dosage of it.)
So - I know this is the ultimate “it depends” question, but - generally, how big a dosage of virus is needed for there to be a 20% chance of sickness? Depends on the virus, but are we talking one syringe’s worth of injection? Half? One milliliter?
I don’t think there’s a conflict here, because an injection is going to involve much more than one virus. I don’t know how many would be expected to be in a typical “dose”, but the chance for any given one logically must be significantly less than the chance for at least one out of many.
As you noted “it depends”. Certainly less than a milliter in most cases. For instance a ml of snot from someone shedding cold virus would be an improbable amount to have enter your body. My body anyway.
The standard dose for the measles-mumps-rubella (MMR) vaccine is 0.5 milliliter. If that’s enough for the weakened virus to stimulate the immune system, then it should probably be in the ballpark for a full-strength load of virus to make you sick.
OK, but usually, after floating about for long enough, the virus would find that geartooth cell sooner or later? (If the human is unvaccinated, the white blood cells wouldn’t be antibody-notified to kill the virus)
You don’t need to have a primed immune system to mop up odd virus particles (or other pathogen). The immune system is a complicated beast and the “innate” immune system is constructed with components that will take down a non-specific virus infection early. Phages will eat the naked virus particles as they come across them, and NK (natural killer) cells will kill some infected host cells. So low numbers of an invading virus can be successfully controlled.
The problem is that this part of the immune system is limited in capability, and will become overwhelmed if there are either too many initial pathogen particles or the pathogen is able to reproduce fast enough. Which can mean infecting tissue that is heard to get too. This is when you need an existing immunity. Then the body is ready primed to produce the required antibodies in massive numbers quickly and can outrun the infection.
This is really all there is to immunisation. You give your body a head start in the exponential war. If your body has a couple of days head start the infection will be headed off. But if your body needs that couple of days to work out how to manufacture the needed antibodies before engaging in the war, and the innate system is overwhelmed by either initial numbers or an infection that is aggressive enough, you lose.
So, the precise dose needed to overwhelm the innate immune system depends upon a range of factors, but there is a dose below which you would expect should not normally cause infection.
This is a nonsensical question as asked. Viruses are not liquid, and have no standard dilution, so “volume” has no necessary relation to “dosage”. (Vaccines are usually in liquid form, but the liquid is just a carrier; the amount of actual virus in most vaccines would be invisibly small.)
If you mean how many individual viral particles you need to have a 20% chance of infection, it depends mightily on the exact virus under discussion. According to this chart from a paper on influenza strains, it takes about 61 viral particles* to have a 50% chance of causing a lethal influenza infection in mice. I doubt it would scale linearly to humans, but from the mouse numbers I’d ballpark we’re talking in the hundreds-to-thousands range for a 20% chance of influenza infection (of any severity) in humans.
Well, technically, what they’re counting is “plaque forming units”. I couldn’t be bothered to look up what the ratio of PFU to actual viral particles is, but it’s probably on the order of ones to tens.
