Big Pharma behemoth Pfizer (NYSE: PFE) has dosed the first participants in a pivotal Phase 3 clinical trial to test its quadrivalent modified RNA (modRNA) influenza vaccine candidate.  The mRNA flu vaccine study will enroll roughly 25,000 healthy U.S. adults. Pfizer is not the only company to develop an mRNA flu vaccine. In December, Moderna (Nasdaq:MRNA)…