People who got all their vaccinations against tetanus and diphtheria in childhood don’t need booster shots to remain protected against the two rare but dangerous diseases, researchers conclude in a new study that found no difference in disease rates between countries that recommend adult revaccination every 10 years and countries that say completing childhood vaccinations is enough.

As of 2017, the World Health Organization recommends vaccinating adults against tetanus and diphtheria only if they didn’t finish their childhood immunization series or don’t know whether they did. The guidelines make exceptions for pregnant women, some types of international travel, and tetanus-prone injuries. But in the U.S., the Advisory Committee on Immunization Practices, which makes recommendations to the Centers for Disease Control and Prevention, currently favors booster shots every 10 years for adults.

The CDC declined to comment on this new study, published Tuesday in Clinical Infectious Diseases.

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In what the study’s senior author called an “embarrassingly simple” experiment of nature, researchers compared disease incidence over 15 years in 31 industrialized countries in North America and Europe with similar health care infrastructure and socioeconomic status. They analyzed data from the WHO, adding up to more than 11 billion person-years. The paper looked particularly closely at France, which recommends 10-year booster shots, and the United Kingdom, which has never done so.

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Adult booster vaccination offered no benefit, the researchers concluded after combing through WHO data from 2001 through 2016: There was no decline in tetanus or diphtheria incidence in France or in other countries that vaccinated adults who’d had all their vaccine doses as children, compared to the U.K. and other countries that didn’t.

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Before this sweeping analysis, Mark Slifka, professor of microbiology and immunology at Oregon Health and Science University, and his colleagues had tracked immunity in fewer than 50 people over 25 years, reporting in a 2007 paper that both tetanus and diphtheria vaccines offered long-lived protection. For a 2016 follow-up paper, his team measured antibody levels in 500 U.S. patients over 10 years, coming up with the same result: People who got all their childhood vaccine doses for each disease had antibodies above a protective threshold, meaning their coverage was durable, too.

“What we had in 2016 was immunologic data. Now we have epidemiologic data that shows that the immunology is correct,” Slifka said in an interview. “What’s exciting about this is it’s a direct measure of whether there was more disease or less disease in [childhood] vaccination or revaccinating as adults.”

Slifka estimates the U.S. could save roughly $1 billion in health care costs every year if it dropped booster shots of these “legacy vaccines” for covered adults.

Both tetanus and diphtheria are now rare in industrialized nations, but the two diseases have a deadly pre-vaccine history. Tetanus, a local bacterial infection sometimes called lockjaw that brings rusty nails to mind, had a fatality rate of 91% in 1947. Diphtheria, a bacterial infection of the throat and upper airways, is still fatal in 5% to 10% of cases.

In the current study of industrialized countries, deaths were recorded in people who were unvaccinated or under-vaccinated as children in developing countries or in Latvia, which had a period of poor vaccination coverage before 1995.

When the tetanus and diphtheria vaccines were first widely administered in the 1940s and 1950s, booster shots were recommended after three years, then in the 1960s that shifted to five years before settling at the 10-year intervals now standard in the U.S. Nine other vaccination series given in childhood — measles, mumps, rubella, polio, hepatitis A virus, hepatitis B virus, Haemophilus influenzae B, rotavirus, and human papillomaviruses — were studied and then limited to childhood.

Two recent examples of changing guidelines are the chickenpox vaccine, first conceived of as “one and done” before vaccinated children developed breakthrough infections after one dose. Now two doses are standard. The vaccine against human papillomavirus, or HPV, moved in the opposite direction. It started as a three-dose regimen but later research showed a single dose may work just as well.

Speaking in general terms about tetanus and diphtheria, Stanley Plotkin, an emeritus professor of pediatrics at the University of Pennsylvania and a veteran vaccine researcher, cautioned that “diphtheria incidence is low because the organism no longer circulates in the U.S., even if the vaccine is not boosted; and that tetanus antibodies persist well but the [levels] decrease with time and there are plenty of reports of tetanus in vaccinated people long after immunization, so that low [levels] may not always be protective.”

Shaun Truelove, an assistant scientist at the International Vaccine Access Center of Johns Hopkins Bloomberg School of Public Health, said the question of booster vaccinations is a little more complicated than it may appear.

“Looking at incidence of these diseases over the last decade does not exactly tell the full story,” he said. “In this case, the lack of transmission in these highly vaccinated populations, particularly among children, may be limiting exposure among older individuals for whom immunity has waned.”

Slifka added a caveat, emphasizing that his paper’s conclusions apply only to countries with strong health care systems.

“If it’s a developing country where it’s unclear what vaccination coverage is, unclear what their infection rates or incidence rates are, those countries need to gather that information before they can make this type of decision,” he said.