In the pre-immunisation era diphtheria was predominantly a disease of children aged under 15 years; most adults acquired immunity without experiencing clinical diphtheria. Asymptomatic carriage was common (3–5 percent) and important in perpetuating both endemic and epidemic diphtheria. The global incidence of diphtheria dropped dramatically during the 20th century. Immunisation played a large part, but may not be wholly responsible for this reduction (see Figure 5.1).
* DTP3 refers to the third dose of diphtheria, tetanus and pertussis vaccine.
Source: World Health Organization. Immunization Surveillance, Assessment and Monitoring. URL: www.who.int/immunization_monitoring/data/data_subject/en/index.html
Immunisation leads to the disappearance of toxigenic strains, but a bacteriophage, containing the diphtheria toxin gene, can infect and rapidly confer toxigenicity to non-toxigenic strains. This makes the return of epidemic diphtheria a real threat when there is insufﬁcient herd immunity, as happened in the states of the former Soviet Union during 1990–97. Factors contributing to this epidemic included a large population of susceptible adults, decreased childhood immunisation, suboptimal socioeconomic conditions and high population movement.1 Diphtheria remains endemic in these countries, as well as in countries in Africa, Latin America, Asia, the Middle East and parts of Europe, where childhood immunisation coverage with diphtheria toxoid-containing vaccines is suboptimal.2
Diphtheria is rare in developed countries such as New Zealand due to active immunisation with diphtheria toxoid-containing vaccine. However, continuing endemic cutaneous diphtheria in indigenous communities has been reported from the US, Canada and Australia. Small diphtheria outbreaks still occur in developed countries.3 These often appear to be caused by unvaccinated or partially vaccinated individuals travelling to endemic countries.4–6
The overall case fatality rate for diphtheria is 5–10 percent, with higher death rates (up to 20 percent) among persons younger than 5 and older than age 40 years. The case-fatality rate for diphtheria has changed very little during the last 50 years.7
Diphtheria infection was common in New Zealand until the 1960s (see Figure 5.2). In 2009 a case of toxigenic diphtheria was reported in an adult male who developed a cutaneous infection after being tattooed in Samoa. A secondary case of toxigenic cutaneous diphtheria was subsequently identified in a fully immunised 11‐year‐old household contact.8 The last case of toxigenic respiratory diphtheria was reported in 1998.9
Source: Ministry of Health and the Institute of Environmental Science and Research
The 2005–07 National Serosurvey of Vaccine Preventable Diseases found that 61 percent of 6–10-year-olds, 77 percent of 11–15-year-olds, 71 percent of 16–24-year-olds, 48 percent of 25–44-year-olds and 46 percent of ≥45-year-olds had presumed protective levels of diphtheria antibody.10 The decline apparent with age suggests there is likely to be a large and increasing pool of adults who may be susceptible to diphtheria in New Zealand, despite the introduction of adult tetanus diphtheria (Td) vaccination in 1994.