my understanding of primitive/early vaccines is this:
1. take a virus that is highly adapted to humans, and inject it in another animal (like a rabbit)
2. the virus will cause a very low-grade infection in the rabbit, since the virus is adapted to humans, not rabbits - but it will still be a non-zero level of effectiveness
3. at the peak level of the rabbit's low-grade infection, the mutants of the virus that are more effective against rabbits will be expressed more than any others, and replicate themselves more than any other mutant
4. capture these variants from the rabbit, and inject them into another rabbit.
5. step 3 and 4 repeat as the virus becomes less human-specific and more rabbit-specific
6. if all goes well, you will have transformed the human-effective virus into a rabbit-effective virus. But the virus still has enough of the same characteristics that it will provoke an immune response in humans that will also protect against the human-specific variant
I imagine if you wanted to do the opposite, and craft a virus that is more effective against humans instead of less, you would simply follow the steps in reverse.
Since these steps are so simple, and require no direct modification of the genetic material (you simply encourage it to drift in a certain direction) it seems like it would be hard to tell which viruses were manipulated in this way.
1. take a virus that is highly adapted to humans, and inject it in another animal (like a rabbit)
2. the virus will cause a very low-grade infection in the rabbit, since the virus is adapted to humans, not rabbits - but it will still be a non-zero level of effectiveness
3. at the peak level of the rabbit's low-grade infection, the mutants of the virus that are more effective against rabbits will be expressed more than any others, and replicate themselves more than any other mutant
4. capture these variants from the rabbit, and inject them into another rabbit.
5. step 3 and 4 repeat as the virus becomes less human-specific and more rabbit-specific
6. if all goes well, you will have transformed the human-effective virus into a rabbit-effective virus. But the virus still has enough of the same characteristics that it will provoke an immune response in humans that will also protect against the human-specific variant
I imagine if you wanted to do the opposite, and craft a virus that is more effective against humans instead of less, you would simply follow the steps in reverse.
Since these steps are so simple, and require no direct modification of the genetic material (you simply encourage it to drift in a certain direction) it seems like it would be hard to tell which viruses were manipulated in this way.