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How long do vaccines last? The surprising answers may help protect people longer


Like millions of people in the United States last year, Stanley Plotkin and his wife got vaccinated against influenza at the start of the Northern Hemisphere's flu season, in early October. Plotkin, a physician and emeritus professor at the University of Pennsylvania, knew well the value of the shot: He is one of the world's most renowned vaccinologists, having had a hand in several vaccines on the market, including the one for rubella. He's even the co-author of the standard medical textbook, Vaccines.

In January, just 3 months later, the couple got a second flu shot.


That was an unusual choice, one not recommended by the U.S. Advisory Committee on Immunization Practices (ACIP), which guides the country's vaccine usage. But a growing body of evidence that the protective immune responses triggered by flu vaccines wane in a matter of weeks persuaded Plotkin to return to the clinic. "The time and cost was trivial compared to the importance of influenza at my age," says Plotkin, 86. "With flu, we're not talking about getting a case of the sniffles."


It's not just flu. 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.


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