That would be here:
https://pubmed.ncbi.nlm.nih.gov/33378609/ and
https://pubmed.ncbi.nlm.nih.gov/34401888/
But of course, those who say this aren't interested in the science. They instead cling to a misunderstanding of science that proposes their genes are being modified.
All proteins in the body are synthesized from copies of DNA known as mRNA, and this mRNA is
destroyed after it has served its function.
To modify genes, the DNA (the master copy) would have to be modified. This simply does
not happen.
The mRNA vaccine harnesses this exact mechanism to deliver the spike protein (the part the virus uses to gain access to our cells - that is, to infect us) to immune cells so they can learn how to fight the virus without a full-fledged viral infection. And just as expected, the mRNA is then destroyed.
Since SARS-Cov-2 is an RNA virus, it also harnesses this exact same mechanism in its host to replicate itself.
So if the mRNA vaccine is "gene therapy", then so is being infected by a virus. ALL viruses use this mechanism to replicate***.
The only difference between an mRNA (or even DNA) vaccine and other vaccines is that the other vaccines either deliver the spike proteins directly (synthesized
outside the body from mRNA) to the immune cells OR they use a virus ("viral vector" - usually an adenovirus) to deliver mRNA to the body with instructions to synthesize the spike protein. The mRNA vaccine delivers the mRNA directly to the immune cells - that is, not through a viral vector.
This is the difference between buying a case for one's phone directly from the store, or buying/renting a 3D printer and loading instructions for the case to print the case, or using a 3D printing service (not owning/renting the 3D printer). The mRNA vaccine would be the latter - the body already knows how to produce proteins from genes, so let's just give it the instructions for the viral spike protein and let it do the rest.
*** Some viruses actually do modify DNA by inserting a copy of themselves into the DNA of the host - e.g., HIV - so if you're worried about gene modifcation, do your best to prevent being infected by viruses such as HIV.