The first two COVID-19 vaccines to be distributed, one made by Pfizer Inc. and BioNTech SE, the other by Moderna Inc., are the first such drugs to consist of genetic instructions in the form of RNA: ribonucleic acid.

Cells in the arm use this blueprint to make harmless fragments of the coronavirus, prompting the immune system to make antibodies and other customized defenses should the person ever be exposed to the actual virus.

Both prevented more than 94% of infections in clinical trials. Some recipients experience a temporary fever, arm pain, or headache, but there is no evidence of any long-term complications.

Here is a step-by-step description of the process, written with guidance from University of Pennsylvania scientist Drew Weissman, whose work with former colleague Katalin Karikó, now at BioNTech, helped make the vaccines possible.

1. The person administering the shot depresses the syringe into the recipient’s arm, injecting billions of tiny spheres made of waxy molecules called lipids. Each one contains several copies of the RNA blueprint for making the spike on a coronavirus particle.

2. When the lipid spheres come into contact with white blood cells in the person’s arm, they deposit their cargo inside. The RNA travels across the cell membrane into the gel-like cytoplasm. It does not enter the cell nucleus, where DNA is located, so it does not have any effect on the person’s genetic code.

3. The RNA molecules are fed through roundish, machinelike structures called ribosomes, a type of cellular assembly line where proteins are made. In an infected person, viruses hijack this machinery to make copies of themselves. But in the vaccines, the RNA contains the recipe only for the exterior “spikes” of a virus particle, not an entire virus. They cannot cause an infection.

4. The cell then “presents” the spike proteins on its surface and travels to the lymph nodes. Though the proteins cannot cause an infection, they are recognized by the immune system as a foreign presence.

5. In the lymph nodes, immune-system cells respond by making antibodies: little Y-shaped proteins that are customized to latch onto the virus spike. If the person is ever exposed to an actual virus, the antibodies latch onto its spikes so that it cannot penetrate a cell — like putting gum on a key so it cannot penetrate a lock. Some antibodies remain in the bloodstream, ready to serve as first responders. In addition, specialized cells form a memory of the spike, retaining the ability to make more antibodies and other defenses in a hurry.