Instructions Included

Vaccines from the U.S. company Moderna and its German rival BioNTech use RNA as a messenger inside cells. The message: Produce an immune reaction, stat

- Robert Langreth and Naomi Kresge

Almost every antiviral vaccine ever sold works in a similar way: A dead or weakened virus, or a piece of one, is introduced into a healthy person. The weakened virus stimulates the immune system to generate antibodies, protecting the person when the real pathogen threatens to infect them.

Over the decades, this tried-and-true approach has vanquished polio, eradicated smallpox, and reined in chickenpox, measles, and mumps. But vaccine production has never been simple or fast. Many flu vaccines are still grown in chicken eggs. Newer approaches draw on genetic engineering to eliminate the need for whole viruses, but their viral proteins are still grown inside live cells.

The coronavirus vaccines from Moderna Inc., in Cambridge, Mass., and its German rival BioNTech SE propose to immunize people in a radically different way: by harnessing human cells to become miniature vaccine factories in their own right. Instead of virus proteins, the vaccines contain genetic instructions that prompt the body to produce them. Those instructions are carried via messenger RNA, or mRNA.

Moderna’s mRNA-1273 consists of a strand of mRNA that tells the body to produce the spike protein the coronavirus uses to latch onto human cells. The strand is like one side of a zipper; the “teeth” are a sequence of chemical letters that cells read to produce the 1,273 amino acids that make up the spike protein. If the vaccine works as intended, the body will start producing the proteins soon after injection, prompting the immune system to react and build up protective antibodies against them.