The whooping cough vaccine may be allowing people to transmit the disease without showing any symptoms, which researchers say could be contributing to its comeback in the last few years.

Children have been receiving the current version of the vaccine since the 1990s, which uses a handful of pertussis bacteria proteins, rather than inactivated cells of the bacteria, in order to bypass adverse side effects of the vaccine's original version that was introduced in the 1950s.

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"There could be millions of people out there with just a minor cough or no cough spreading this potentially fatal disease without knowing it," said Ben Althouse, a Santa Fe Institute Omidyar Fellow and one of two authors of the study, in a press release. "The public health community should act now to better assess the true burden of pertussis infection."

The whooping cough vaccine used from the 1950s to the 1990s drove down infection rates to 1 in 100,000 people, however side effects motivated scientists to create an acellular version of the vaccine that did not cause the side effects because it only contains protein from pertussis bacteria, rather than whole, inactivated cells.

Researchers examined health records from the U.S. and U.K. of all reported whooping cough cases from 1922 to 2012 and health records from patients in both countries, taking into account anti-vaccine movements in the 1970s and 1980s, as well as the anti-vaccine movement of the last several years.

With knowledge from a 2014 study in baboons that acellular vaccines would prevent the symptoms of whooping cough, but not transmission, researchers found that the recent outbreaks -- the 50,000 cases in the U.S. reported in 2012 were the most since 1955 -- were made worse by vaccinated people who show no symptoms of the disease.

"There are lots of people out there who may be transmitting pertussis unknowingly," said Sam Scarpino, also a Santa Fe Institute Omidyar Fellow and the other author of the study. "Not vaccinating your own child puts her or him at increased risk of severe disease, even death."