DURING THE covid-19 pandemic new vaccines were rolled out with unprecedented speed. The fastest to arrive were jabs built from molecules of messenger RNA (mRNA) designed to teach the body how to fight off the disease-causing virus. By late 2021, mRNA vaccines had saved an estimated 7.7m lives globally, including most of the 3m Americans whom the Commonwealth Fund, an American health-care charity, estimates were saved by vaccines before 2023.