Diphtheria toxoid is prepared from cell-free puriﬁed diphtheria toxin treated with formaldehyde. It is a relatively poor immunogen, which, to improve its efﬁcacy, is usually adsorbed onto an adjuvant, either aluminium phosphate or aluminium hydroxide. Diphtheria toxoid is only available as a component of combination vaccines (in New Zealand as DTaP-IPV-HepB/Hib, DTaP-IPV, Tdap and Td).
See Appendix 1 for the history of diphtheria toxoid-containing vaccines in New Zealand.
The diphtheria toxoid-containing vaccines funded as part of the Schedule are as follows.
Other diphtheria toxoid-containing vaccines registered (approved for use) and available (marketed) in New Zealand are:
Immunity against diphtheria occurs via an antibody‐mediated response to the diphtheria toxin and is primarily of the IgG type. Antitoxin antibodies can pass through the placenta to provide passive immunity to the newborn.
Although there are no randomised controlled studies on the efficacy of the vaccine, between 87 and 98 percent protection has been demonstrated using population-based analyses. Immunised cases have been shown to have less severe disease, as highlighted during the outbreak in the former Soviet Union.
Vaccines combining pertussis antigens with diphtheria and tetanus toxoids have been gradually introduced into immunisation schedules throughout the world. Immunogenicity data for these combination vaccines is discussed in section 14.4.2.
Although immunisation is more effective at preventing disease than preventing infection, it does create herd immunity and reduces carriage and therefore transmission.11 To prevent major community outbreaks, it has been suggested that 70 percent or more of the childhood population must be immune to diphtheria.12, 13 This may explain the control of diphtheria in New Zealand despite historically relatively poor coverage.
Diphtheria antitoxin levels decline over time in children after they have received a primary series of vaccines and a booster dose is given. In countries where diphtheria immunisation is common practice and high coverage rates are achieved, there will be no natural boosting from circulating disease, and antitoxin levels declining with increasing age may result in a susceptible adult population.14
Despite this, there has been minimal disease in developed countries, suggesting that antibody levels may not be a reliable guide to protection and that other factors may be operating.15 For example, a high proportion of the adult German population have low antibody levels, indicating susceptibility, yet this has not led to diphtheria outbreaks despite Germany’s relative geographical proximity to the former Soviet Union.16
The duration of protection after Tdap boosters is unknown, but the results of an ongoing Australian study have shown that five years after the Tdap booster dose, 94.4 percent of adults had seroprotective levels of antibodies, compared with 93.7 percent who received Td vaccine.17
Transport according to the National Guidelines for Vaccine Storage and Distribution.18 Store at +2oC to +8oC. Do not freeze.
DTaP-IPV-HepB/Hib and Td should be stored in the dark.
The dose of DTaP-IPV-HepB/Hib, DTaP-IPV, Tdap or Td vaccine is 0.5 mL, administered by intramuscular injection (see section 2.3).
DTaP-IPV-HepB/Hib, DTaP-IPV, Tdap or Td vaccine can be administered simultaneously (at separate sites) with other vaccines or immunoglobulins. (See also section 14.4.4 for information about co-administration of DTaP-IPV-HepB/Hib and PCV13.)