Recent studies show vaccines for mumps, pertussis, meningococcal disease, and yellow fever also lose their effectiveness faster than official immunization recommendations suggest. Vaccines have been a crucial public health tool for decades, so it may seem strange that their durability isn't well understood. But vaccines are approved and come to market years before it's clear how long protection lasts. Later, fading protection can go unnoticed because a vaccine in wide use has largely eliminated transmission of the microbes it protects against, making "breakthrough" infections rare. Even if viruses or bacteria are still in circulation, people vaccinated against them will sometimes receive natural boosting of their immunity. And declining vaccine immunity is not an all-or-nothing phenomenon: A breakthrough infection often leads to much less severe symptoms of the disease.

Researchers are ramping up efforts to figure out why some vaccines protect for mere weeks but others work for life. "We simply don't know what the rules are to inducing long-lasting immunity," says Plotkin, who began to research vaccines in 1957. "For years, we were making vaccines without a really deep knowledge of immunology. Everything of course depends on immunologic memory, and we have not systematically measured it."