> Well, if the solution is vaccine-based, no because it *does* train the immune system
In the old days when we were injected with dead versions of actual viruses, yes.
For a robust immune response, you'll generally want an attenuated virus, not a dead one. I mean, it doesn't matter much for something like flu, because it mutates so quickly that any immunity approaches zero after two or three years anyway, but for any vaccine that you want to actually last for decades, unless your exposure risk is low (e.g. polio in the western world), a live, weakened infection is probably a better option.
Today when they're programming our own cells to create parts of real viruses... what exactly are we training it to do?
See above. The reason attenuated viruses are so much more effective is because they trigger multiple levels of immune response by infecting cells. Programming cells to create parts of real viruses differs only from attenuated vaccines mainly in that the resulting products do not then go on to infect more cells, and that the mRNA bits are usually time-bombed so that they stop producing those virus parts after a period of time, thus minimizing the rate of actual cell death.
But either way, the continued exposure over a longer duration, coupled with the involvement of cellular stress signals, help trigger both the innate and adaptive immune systems, resulting in a stronger immune response than if you just had bits of unexpected dead virus material floating around in isolation.
The immune system is incredibly complicated and we're pretty much just injecting people and hoping it doesn't train the immune system to attack their own bodies instead of the virus.
That's actually way more true with the dead virus vaccines you think are so great. For a classic example, the flu vaccine that caused a detectable uptick in narcolepsy (an autoimmune condition) in Europe, called Pandemrix, was an inactivated, adjuvanted vaccine. The adjuvant somehow triggered autoimmunity in some people. And the adjuvant was needed precisely because without that, the inactivated vaccine did not produce an adequate immune response.
IMO, the odds of an mRNA vaccine causing an autoimmune response are likely orders of magnitude less than an adjuvanted, attenuated vaccine doing so, because an adjuvant causes the immune system to pay more attention to whatever is nearby, including your own cells.
Processionals are literally paid to tell you to use their products. Why would you listen to anything they say?
Not all of them. Some of them are independent research scientists, some of them immunologists working in the public sector or academia, etc. The percentage of professionals in this area who work for the vaccine companies is tiny compared with the percentage of independent researchers studying viruses and vaccines. That said, I'd trust even the research teams at the vaccine companies over some random person on YouTube or other Internet sites who shows no actual sign of understanding immunology, but bulls**ts just well enough people to convince a lot of other people who also don't understand it. And sadly, I've seen so much of that level of noise that I have a standing "No, I won't watch your YouTube video about medical subjects; if it were legitimate, it would have been published in a properly peer-reviewed journal" policy at this point. :-)