Australia has recorded 538 cases of diphtheria in 2026, the largest outbreak since national reporting of the disease began in 1991. Cases are concentrated in the Northern Territory, Western Australia, South Australia and Queensland.
About one in five people diagnosed has been admitted to hospital. The previous high under national reporting was 31 cases, in 2022.
Numbers have fallen since the peak. Around 80 cases were reported in a fortnight in late May and early June. In the most recent fortnight there were 26.
Megan Campbell, a public health physician at the National Aboriginal Community Controlled Health Organisation, said the decline may not mean what it appears to mean. "We do worry with that drop in numbers that part of that may be due to a lack of testing in some areas," she said.
The Australian Centre for Disease Control has declared the outbreak a Communicable Disease Incident of National Significance. The Commonwealth is providing $13 million to the jurisdictions running the response, including $3 million to NACCHO for community liaison and health support.
A high proportion of those diagnosed had already received at least three valid doses of diphtheria vaccine. Stephanie Williams, an epidemiologist at the Australian Centre for Disease Control, said the protection those doses give does not disappear as antibody levels fall. "Whilst those antibody levels generally do decline over time, they are still providing some protection," she said.
Adrian Esterman, professor of biostatistics and epidemiology at Adelaide University, said vaccination on its own will not end the outbreak. "We can vaccinate until the cows come home, but it won't help unless they do something about housing and sanitation," he said.
More than 90 per cent of cases in this outbreak have been among Aboriginal and Torres Strait Islander people, according to the NSW Health fact sheet current to 24 July. No cases have been reported in New South Wales this year.
Diphtheria is caused by the bacterium Corynebacterium diphtheriae and affects either the throat and nose or the skin. Most cases in this outbreak have been mild, which is part of why testing rates matter to the count.
Boosters are recommended for anyone travelling to an affected area who has not had one in the past five years, and are funded for Aboriginal and Torres Strait Islander people in New South Wales. Whether the recent fall in case numbers reflects fewer infections or less testing has not been settled.
