A threshold for measles was pinned down in 1985 after a college dorm was exposed to the disease shortly after a blood drive. Researchers checked antibody concentrations in the students’ blood donations and identified 0.02 international units per milliliter as the level needed to prevent infection.

With these diseases, the magnitudes of response to the vaccines combined with the antibodies’ rates of decay produce durable immune responses: Measles antibodies decay slowly. Tetanus antibodies decay more quickly, but the vaccine causes the body to produce far more than it needs, offsetting the decline.

“We’re fortunate with tetanus, diphtheria, measles and vaccinia,” Dr. Slifka said. “We have identified what the threshold of protection is. You track antibody decline over time, and if you know the threshold of protection, you can calculate durability of protection. With Covid, we don’t know.”