How Long Do I Retain Immunity?

Q: Having had all the common childhood illnesses, such as measles and mumps, 70-plus years ago, how much immunity do I still retain?

A: You are probably immune to some of these illnesses. But it is difficult to be definitive about the duration of immunity, because definitive studies, known as challenge studies, are rarely conducted. In a challenge study, subjects are intentionally exposed to an infection to which they are thought to be immune. The subjects prove they are immune by not getting sick.

Challenge studies have been conducted for some respiratory viruses, such as influenza and the common cold. But for ethical and safety reasons, challenge studies cannot be conducted for more serious infections, such as meningitis, pneumonia and polio.

In the absence of challenge studies, doctors rely on antibody levels as a surrogate marker of immunity. In 2007, doctors at the University of Oregon conducted what may be the best study of antibody levels to common infections. They followed 45 subjects for as long as 26 years, measuring their antibody levels to eight common pathogens: measles, mumps, rubella, Epstein-Barr virus, varicella zoster virus (chickenpox), diphtheria, tetanus and vaccinia (the cowpox virus that eradicated smallpox).

The results were remarkable. Antibody half-life — the time required for antibody levels to decrease by 50 percent — was 50 years for varicella zoster virus and, they estimated, more than 200 years for measles and mumps. The half-lives of tetanus and diphtheria were much shorter, 11 years and 19 years, respectively. That’s why, for example, it’s recommended you get a booster tetanus shot every 10 years.

An important caveat about these data is that immunity from natural infection may last longer than immunity from vaccination. Individuals born before 1957, who grew up in a time when measles was common, may have more durable protection than those born later who got the measles vaccine, which became widely available starting in 1963.

This hypothesis is supported by recent outbreaks of mumps. More than 6,000 cases of mumps were reported in the United States in 2016. Most occurred in young people who had been vaccinated as opposed to older people who were born before the introduction of mumps vaccine in 1967.