Since the Pfizer-BioNTech COVID-19 vaccine is patented, one could assume that patent and the used mRNA-string are published somewhere.
If that is the case, where can one find it?
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Patent applications typically publish 18 months after filing. Since vaccine's specific to COVID-19 were started within the last year, it is likely that a patent application hasn't yet published. Reports of patent protection are probably related the the underlying mRNA technologies developed by BioNTech.
However, you can search the patent literature for items related. For instance on Google Patents (as one example) I've searched with "BioNTech RNA Pharmaceuticals GmbH" as the assignee and mrna and vaccine as keywords returning about 269 results. Here are the results. I used "mrna" since messenger RNA is the underlying technology for the vaccine and "vaccine" since I wanted more relevant hits. This is likely not a complete list of relevant patents and applications since it is possible Pfizer or BioNTech have licensed other intellectual properties.
Regarding patents on the technology behind the vaccines, I found this on PatentlyO
The components of the new mRNA vaccines of Pfizer, Moderna, CureVac and others are heavily patented. There are 2 main estates: 1. the modified mRNA estate held by UPenn; 2. the lipid nanoparticle components held by Arbutus, Moderna, Acuitas and others. Most patents have been licensed where appropriate, there is one major (public) dispute (see Arbutus/Moderna).