evidence: "We were making RNA within a week or so" of the SARS-CoV-2 sequence being published, said Drew Weissman, MD, PhD, who researches mRNA vaccines at the University of Pennsylvania Perelman School of Medicine. That speed propelled development: according to Weissman, both groups currently testing nucleic acid-based vaccines in phase 3 trials licensed his team’s mRNA formulation from the university. [...] The first 4 COVID-19 vaccine developers with published clinical trial data all used either a non–replicating adenovirus or mRNA platform. The US government is betting on some of these new technologies. Under the auspices of its Operation Warp Speed vaccine development initiative, it has already purchased hundreds of millions of doses of ChAdOx1 nCoV-19, mRNA-1273, BNT162b2, and an investigational non–replicating viral vector vaccine in early trials from Johnson & Johnson–owned Janssen Pharmaceutical Companies, as well as other candidates. Doses should be standing by if or when any of these are approved. [...] Speaking at the July 27 media briefing, Collins addressed concerns: "Yes, we’re going fast. But, no, we are not going to compromise safety or efficacy." Experts say several factors argue for mRNA vaccines’ safety. For one, mRNA can’t cause an infection. It also doesn’t enter the cell’s nucleus, so the chance of its integration into human DNA is believed to be very low. In addition, the body breaks down mRNA and its lipid carrier within a matter of hours, assuaging some concerns about long-term risks.