evidence: Both vaccines are made from messenger RNA, the molecule that cells naturally use to carry DNA’s instructions to cells’ protein-building machinery. A vaccine based on mRNA has never been approved by the FDA before. However, many years of research have gone into RNA vaccines, which is one reason why scientists were able to start testing such vaccines against Covid-19 so quickly. Once the viral sequences were revealed in January, it took just days for pharmaceutical companies Moderna and Pfizer, along with its German partner BioNTech, to generate mRNA vaccine candidates. [...] In subsequent years, Langer, Anderson, and others have developed fatty molecules called lipid nanoparticles that are also very effective at delivering nucleic acids. These carriers protect RNA from being broken down in the body and help to ferry it through cell membranes. Both the Moderna and Pfizer RNA vaccines are carried by lipid nanoparticles with PEG. [...] One drawback to mRNA vaccines is that they can break down at high temperatures, which is why the current vaccines are stored at such cold temperatures. Pfizer’s SARS-CoV-2 vaccine has to be stored at -70 degrees Celsius (-94 degrees Fahrenheit), and the Moderna vaccine at -20 C (-4 F). One way to make RNA vaccines more stable, Anderson points out, is to add stabilizers and remove water from the vaccine through a process called lyophilization, which has been shown to allow some mRNA vaccines to be stored in a refrigerator instead of a freezer.