Well .... it's not really gene therapy. The way the vaccine works is that dendritic cells take up the mRNA particles and express the spike protein on their surface. Our own immune system then responds to these dendritic cells with spike proteins just like it would respond to a sars virus with spike proteins and kills them, and produces antibodies. The antibodies arising from this approach is no different from the antibodies that would occur if you injected virus. And in fact, that is exactly what J&J vaccine does, it injects a harmless adenovirus that has been altered to express the spike protein. So ... just for your benefit of making the argument ... the dendritic cell, the adenovirus, or the sars virus, all expressing the same spike protein will stimulate the production of antibodies to said protein.