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A review in the Clinical Microbiology Reviews highlighted the remarkable success of vaccines in nearly eliminating tetanus and diphtheria. Today, widespread immunization has made these diseases so rare that a person is up to 1000 times more likely to be struck by lightning than to develop either infection. In the US, tetanus occurs at a rate of less than one case per 10 million person-years, whereas diphtheria is even rarer, at approximately one case per billion person-years. In recent decades, nearly all deaths from these diseases have occurred in individuals who have not completed the primary vaccination series. The authors pointed out that the exceptionally rare deaths from tetanus or diphtheria “are not due to vaccine failure and instead are mainly due to a failure to vaccinate.”

Booster Debate

Tetanus and diphtheria vaccines are unusual because many countries, including the US and Italy, recommend routine boosters in adults every 10 years — a policy not applied to any other childhood vaccine. However, the authors argued that these boosters were not necessary to maintain population-level immunity. They presented serologic, epidemiologic, and historical data showing that adult booster doses could be safely avoided with substantial cost savings. Mark K. Slifka, PhD, professor at Oregon Health & Science University (OHSU), Portland, Oregon, and senior scientist at the OHSU’s Oregon National Primate Research Center, based in Hillsboro, Oregon, and colleagues emphasized that adult booster doses, which have already been phased out in several European countries, can be safely discontinued if high vaccination coverage in children is maintained and vaccines remain available for specific clinical needs.

UK vs France

A striking comparison between the UK and France supports the argument that adult booster vaccinations may be unnecessary. Both countries have similar population sizes and comparable pediatric vaccination schedules for tetanus and diphtheria, with five doses each compared with four in Italy, with coverage rates between 94% and 96%. The UK had never implemented routine boosters for adults, whereas France required boosters every 10 years until 2012. Since 2014, France has recommended boosters every 10 years for adults older than 65 years and every 20 years for younger adults at ages 25, 45, and 65 years.

Despite these differing booster policies, disease incidence remains extremely low in both countries, with fewer than one tetanus case per 4 million person-years (0.21 vs 0.12 cases per million person-years in the UK and France, respectively) and fewer than one diphtheria case per 16 million person-years (0.06 vs 0.03 per million person-years in the UK and France, respectively). Notably, the incidence was slightly lower in the UK, where boosters were not administered to adults.

A study of 30 high-income countries found no significant difference in tetanus or diphtheria rates between countries that administered boosters to both children and adults and those that administered boosters only to children. Eight countries did not offer adult tetanus boosters, and nine did not offer adult diphtheria boosters.

Cost Savings

The authors estimated that eliminating routine 10-yearly tetanus-diphtheria boosters in the US, in alignment with the World Health Organization (WHO) guidance, which has not recommended adult boosters since 2017, could save approximately $1 billion annually. In Europe, eight countries besides the UK do not schedule routine tetanus and diphtheria boosters: Spain (which stopped boosters in 2009 but offers a single dose around the age of 65 years), Croatia (one booster after the age of 60 years), Ireland, Iceland, Malta, the Netherlands, Poland, and Hungary.

The WHO continues to recommend a tetanus and diphtheria booster during each pregnancy, administered as the diphtheria-tetanus-pertussis vaccine to protect newborns from whooping cough, as well as for travelers to diphtheria-endemic areas and for wound management in potential tetanus cases. Vaccination is also advised for individuals who are unvaccinated or whose immunization status is uncertain.

Long-Term Immunity

Why should routine booster shots be stopped for adults? This is because the antibodies last longer. Studies have shown that tetanus antibodies have a half-life of 11 years, whereas diphtheria antibodies have a half-life of 19 years. In a study of over 500 adults in the US, 99% of those younger than 60 years and 97% overall had protective tetanus antibody levels; comparable results were found for diphtheria. Other studies have shown no significant decrease in antibody levels with age, even in the absence of booster doses.

The researchers emphasized that the goal of an immunization program is to prevent disease and not just to produce antibodies.

Unlike tetanus, diphtheria is contagious; however, herd immunity remains strong in the absence of adult booster shots. In 2022, the UK reported 73 cases of diphtheria among asylum-seeking immigrants, the same number as that reported over the previous 20 years. None of the healthcare workers were infected. UK health officials concluded that the existing vaccination coverage was sufficient to prevent the spread of diphtheria and that the risk to the general population remained low.

This story was translated from Univadis Italy.