Reassurance of population immunity to diphtheria in England: Results from a 2021 national serosurvey

Background: Diphtheria is rare in England because of an effective national immunisation schedule that includes 5 doses of a diphtheria-containing vaccine at 2, 3, 4 months, preschool and adolescent boosters. However, in recent years there has been a notable increase in cases due to Corynebacterium ulcerans among older adults and evidence of endemic transmission of C. diphtheriae (normally associated with travel to endemic countries). We aimed to update 2009 estimates of diphtheria immunity considering the evolving epidemiology. Methods: Residual sera collected from diagnostic laboratories and general practitioners in England in 2021 were randomly selected and tested for diphtheria antibody, to estimate proportions protected per age group. Diphtheria antibody levels were defined as susceptible (<0.01 IU/mL), basic protection (0.01–0.099 IU/mL) and full protection (≥0.1 IU/mL). Immunity estimates were standardised to the England population and compared to 2009. Results: Based on 3,745 residual sera tested, 89% (95%CI: 87%–90%) of the 2021 England population had at least basic diphtheria protection (vs. 90% [88%–92%] in 2009) and 50% (48%–52%) full protection (vs. 41% [38%–44%]). Higher antibody levels were observed in those aged 1 and under, 10–11, 12–15, 25–34 and 35–44 years compared to 2009. The largest proportion susceptible were observed in those aged 70+, 26% (21%–31%) vs 12% (7%–18%) in 2009. Conclusions: Basic diphtheria protection is comparable between 2021 and 2009. The increase in immunity in working age adults is likely due to the school leaver booster introduced in 1994. The current vaccination schedule is maintaining sufficient population immunity. However, we recommend clinicians remain vigilant to severe diphtheria outcomes in older adults, because of their observed susceptibility.