The incidence of bacterial meningitis in Belgium increased 10-fold between 1988 and 1993, mainly due to meningococcus. Immigrants and non-white people got infected more often.Response to diphtheria booster vaccination in healthy adults: vaccine trial. 2000, Vellinga, BMJ
Adults in Belgium got revaccinated against diphtheria. 24% of them did not get their antibodies above the ‘protective’ level. Among those, whose antibodies level was initially low it remained insufficient in 42% of the cases. The authors conclude that one booster dose for adults is not enough.
Death from diphtheria in developed countries is so rare that every such case is widely reported in the media. In 2015, a boy died of diphtheria in Spain, and in 2008, a girl died in England. These seem to be the only deaths of children from diphtheria in developed countries in the last 30 years.