A high proportion of adults in Europe do not have protective antibody concentrations against diphtheria

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In a recent study published in the Eurosurveillance Journal, researchers conducted a retrospective analysis of seroprotection against tetanus and diphtheria among residents in Austria following an increase in diphtheria cases since 2022 in Europe.

Study: Lack of seroprotection against diphtheria in the Austrian population, in light of reported diphtheria cases in Europe, 2022. Image Credit: BillionPhotos/Shutterstock.comStudy: Lack of seroprotection against diphtheria in the Austrian population, in light of reported diphtheria cases in Europe, 2022. Image Credit: BillionPhotos/Shutterstock.com


Diphtheria is an infection caused by Corynebacterium diphtheriae resulting in the inflammation of mucous membranes in the throat and nose.

While an increase in the incidence of diphtheria among migrants in Europe was observed beginning in August 2022, recent studies also found that the concentrations of antibodies against diphtheria in adults in approximately 18 countries in the European Union were negligible.

Although no diphtheria outbreaks have been reported among the general population of European Union countries, the low seroprotection increases the risk of disease outbreaks.

Estimating the seroprevalence among the population could help determine the risk of a diphtheria outbreak and increase awareness about the importance of vaccines to ensure continued protection against diphtheria and various other diseases.

About the study

In the present study, the researchers measured the concentrations of antibodies against tetanus and diphtheria to estimate the seroprotection prevalence among Austrian residents.

The study included 10,247 individuals who volunteered to get tested for concentrations of anti-diphtheria and anti-tetanus toxoid immunoglobulin G (IgG) between 2018 and 2022.

Additionally, the extent of diphtheria toxoid and tetanus toxoid antibody waning was also examined for individuals who had antibody concentration measurements for two-time points since the last vaccination. Enzyme-linked immunosorbent assay (ELISA) was used to determine the concentrations.

The results were categorized as non-protective, inadequately protective, or adequately protective against tetanus and diphtheria. A generalized linear model was used to log-transform the antibody concentrations with sex and age categories as independent variables.

Additionally, geometric mean concentrations were calculated, and the waning of antibody concentrations was analyzed separately for the diphtheria toxoid and tetanus toxoid, with covariates including sex and age and the within-subject variable of time elapsed since the last vaccination.


The results indicated that the overall seroprotection prevalence against diphtheria was 63.96%, and that against tetanus was 95.99%. Furthermore, the protection against tetanus was seen to be long-term based on the antibody concentrations, but the protection against diphtheria did not show similar patterns.

When analyzed according to sex, males were seen to have 1.13 times higher seroprotection against diphtheria than females.

The seroprotection levels also varied according to age groups, with individuals above 60 years having the lowest seroprotection against diphtheria and individuals between the ages of 15 and 59 years having the highest seroprotection prevalence.

The geometric mean concentrations of the antibodies against the tetanus toxoid were 7.9 times higher than that against the diphtheria toxoid. Additionally, the concentrations of the antibodies against the diphtheria toxoid waned in 89 individuals by approximately 2.9%.

Furthermore, when age groups analyzed the waning of antibody concentrations, the findings reported that the diphtheria toxoid antibody concentrations decreased at 16.5% for each increase of 10 years in age.

Austria introduced childhood vaccination against diphtheria in 1945, and the occurrence of diphtheria in the general population had drastically reduced by the late 1960s.

However, 72 cases of diphtheria were reported among migrants in Austria in 2023, and the low levels of protection against diphtheria due to the waning of vaccine-induced immunity pose a risk of diphtheria outbreaks.

The researchers believe that the low levels of protection could be due to various reasons, such as missed booster vaccine doses, waning antibody levels, and low levels of vaccine-induced immunity.

While the antibody concentrations against the tetanus toxoid decreased annually by 6.9%, the high initial concentration of antibodies against the tetanus toxoid implied that the antibody concentrations would not reach the inadequate protection threshold before 50 years from the last vaccination.

Furthermore, the drastic difference in the levels of anti-diphtheria toxoid antibodies and anti-tetanus toxoid antibodies indicated that either the diphtheria toxoid content in the booster vaccine doses for adults is low or that individuals preferred the monovalent tetanus vaccine for emergency care.


Overall, the findings suggested that the level of seroprotection against diphtheria among the residents of Austria was not adequate to grant protection against the disease, although the seroprotection prevalence against tetanus toxoid was high.

Given the increase in diphtheria cases among the migrant population in Austria, the researchers recommended increasing the awareness about booster vaccinations among the resident and migrant populations to ensure protection against the disease.

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