Detection means there is enough virus physically present in the sample for the detection technology to identify it.
Infection requires virus particles to be physically present in transmission vectors (fluids, droplets, etc). There's generally also a dose-response effect where more particles means more chance of evading the immune system well enough to establish replication in the victim.
So a lack of anything to detect, means a lack of anything to spread an infection.
> So a lack of anything to detect, means a lack of anything to spread an infection.
One should add that for that the threshold for detection has to be lower than the threshold for infection.
Example: let the detection threshold be 10 particles/ml and the infection threshold be 100 particles/ml (*) -> then undetectable implies that it is very improbable that an infection will take place.
(*) This is a very crude description. Think of it like this: Every single virion (virus particle) has a very low probability of causing an infection itself but there is a high number of them and for one them it might just work out (higher viral load -> higher risk of successful infection)
I'm not a biologist and this is based on general knowledge and some quick google searches but:
We're quite good at detecting viruses if we really want to. PCR can amplify DNA so much that we can detect even 50 viruses per milliliter of blood. A milliliter seems like actually a lot of blood to get into someone else and your innate immune system is capable of finding and neutralizing small amounts of contagions relatively well even if it's never seen it before. I do suspect this is a statistical impossibility though probably you could somehow get incredibly unlucky, for example in your blood momentarily all the free floating viruses end up in the same bit of blood and that somehow gets into someone else, but I think we can all realize that probability is tiny and in practice I don't think there are any examples of transmission with undetectable levels of HIV.
Well put. We can actually detect well below 50 copies/mL in the laboratory setting as well; we don’t go below this to maintain adequate test specificity and sensitivity in the clinical setting.
Not to put you on the spot but from my cellular bio lab I took it felt like PCR should be capable of detecting literally a single example of the targeted bit of DNA in the sample. Is the problem with detecting below 50 that the sample is too easily contaminated or is it that the primers can spontaneously bind to things they aren't supposed to at a high enough rate to invalidate the results?
Infection requires virus particles to be physically present in transmission vectors (fluids, droplets, etc). There's generally also a dose-response effect where more particles means more chance of evading the immune system well enough to establish replication in the victim.
So a lack of anything to detect, means a lack of anything to spread an infection.