For years, the FDA has known about the risk posed by residual DNA in vaccines. Its own guidance to industry states:

"Residual DNA might be a risk to your final product because of oncogenic and/or infectivity potential. There are several potential mechanisms by which residual DNA could be oncogenic, including the integration and expression of encoded oncogenes or insertional mutagenesis following DNA integration."

Put simply, the FDA acknowledges the possibility that fragments of DNA left over by the manufacturing process can be incorporated into a patient's own DNA, to potentially cause cancer.

FDA and WHO guidelines consider the amount of residual DNA in a single dose of traditional vaccine should not exceed 10 ng (one billionth of a gram).

But this limit – used for traditional vaccines – is unlikely to be relevant to the mRNA vaccines whose lipid nanoparticles can penetrate inside cells to deliver the mRNA efficiently.