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In the context of Covid-19, in Denmark all ferrets/minks in farms were killed, as there is infection in humans by the ferret corona-subtype.
Contrary heightened concerns, a virus transferred from ferret might be less dangerous than its human subtype that is transmitted within humans. That is my inference from Jenner introducing vaccination (vacca means cow): pox virus from cow is not as dangerous to humans as the human small pox virus, in spite of its strong effect of immunization against the latter. In other words, it seems possible that the true principle of vaccination is cross reactivity and not attenuation. To attenuate one and the same antigen that causes the disease seems different from "being lucky" (compare comments, this is an edited version of my question) as Jenner is considered to have been when using antigen A as a cross-reactive antigen instead an attenuated identical antigen.
Is the following reasoning coheherent? The general principle of vaccination is cross-reactivity, not attenuation, as even in case of apparently identical antigen used after attenuation there is a broad specifity of anti-body reaction that - this is important and new, in this edited version of my question - "of course" is more specific that Jenner's vaccine as it indeed is one and the same antigen - however, attenuation must be seen in reference to a very narrow specifity of target cells, i.e. non-immune cells, that Jenner's dosis just did not reach as he was using a cross-reactive vaccine of a different species of virus the cross-reactivity of which was effective against in that case small pox, not cow pox. So, is the underlying principle of vaccination not attenuation and identity of antigens but attenuation and - on the other hand - very narrow specifity of target cells, that Jenner's vaccine gladly was not able to surmount?
We cannot say if the virus in ferrets gets less or more dangerous than the one which actually circulates in humans. There are two problems with this infection:
- The ferrets can build a reservoir for the virus making it possible for circulation to occur among these animals and re-introduce it into the human population. It may also be possible that the virus jumps into other animals, also enabling transmission.
- It is possible that the virus mutates in the animals (it does this anyways) and changes, so acquired immunity against the original SARS-CoV-2 it will not be present or at least less effective, enabling further infections. This can also render a vaccine directed against a mutated part ineffective.
So far, these things are concerns only, but to avoid them, swift action with the killing of all Danish ferrets has been taken.
About your example with Jenner: He was simply lucky that the cow pox he used for this experiment where close enough to the small pox to generate immunity. For the SARS virus we still talk about the same virus, not a different one.