This weakens the argument. If people who contract hepatitis E have already been exposed to the whole virus, not just a single part of it, they should have antibodies that recognize the virus. Since they are able to contract the disease again, they are clearly not permanently immune to it. This suggests that the vaccine, which just works by exposing people to part of the virus, will also not make them permanenly immune.

Irrelevant. The doctors never make claims about different strains of hepatitis. The conclusion strictly deals with exposure to hepatitis E and immunity to the same disease, not different varieties of it.

This gives us an example of another hepatitis vaccine successfully being developed from isolated viruses. This might very weakly support the doctors’ claim, though we still don't know if all the necessary conditions for a hepatitis E vaccine have been met. But this doesn't weaken their claim.

Irrelevant. Knowing how widespread the virus is doesn't tell us whether a vaccine can be produced from the isolated virus.

This just tells us that exposure to certain viruses does in fact produce permanent immunity. This might slightly strengthen the doctors’ argument, but certainly doesn't weaken it.