Non-medical vaccine exemptions correlate geographically with spiking numbers of whooping cough, according to a new Harvard study.

The link between pertussis and children who are not inoculated is strong enough to suggest states require the shots for those who do not have a medical reason to not get them, they write in The Proceedings of the National Academy of Sciences.

“The results suggest that states should reconsider allowing non-medical vaccine exemptions,” the team concludes. “The data also suggest a need for the recreation of a longer-acting pertussis vaccine or improved regimen and a revised vaccination schedule for the current acellular pertussis vaccine.”

The study was based on 2012 Centers for Disease Control and Prevention numbers. That year, there were 48,277 cases of pertussis reported in the United States – the greatest number since 1955.

The overall incidence rate for pertussis in the U.S. was 15.4 per 100,000 people in 2012.

Nineteen states had rates higher than that average, but only five kept county-by-county statistics, which would allow the researchers to assess non-vaccine/disease clusters: New Jersey (15.7 per 100,000 people), Arizona (17.2), Oregon (23.2) – and the biggest pertussis states – Utah (55.7) and Washington (71.3).

Forty-five percent of the counties that featured whooping cough clusters in 2012 also had high rates of non-medical vaccine exemptions in kindergarteners, they conclude.

“We found evidence of spatial clustering of non-medical vaccine exemptions in Arizona, Utah, Oregon and Washington,” they write. “Forty-five percent of the counties in this study were identified as having high rates of non-medical vaccine exemptions.

“The proportion of kindergarteners with non-medical vaccine exemptions was 2.8 times larger in exemption clusters,” they add.

School immunization requirements are in effect in all 50 states – but their scope and enforcement vary widely. Pertussis has been a standard vaccine since the 1990s, when it was covered in the DTaP vaccination (diphtheria-tetanus-acellular pertussis).

But so-called “herd immunity” only takes effect of 90 percent or more of a population is covered by inoculation, the Harvard team writes.

California had a major measles outbreak three years ago – but responded by “eliminating” non-medical vaccine exemptions, the Harvard team writes.