By middle age, only half of adults maintain antibody protection without a booster
A disease once rendered nearly silent by modern medicine has returned to Australia with uncommon force, exposing the fragility of collective immunity when vaccination falters. More than 220 cases of diphtheria have emerged across four states since January — the worst outbreak in decades — falling with disproportionate weight upon Aboriginal and Torres Strait Islander communities whose access to routine healthcare has long been uneven. The resurgence is not a failure of the vaccine, which remains among the most effective tools in medicine, but a consequence of waning immunity and declining immunisation rates in the years following the pandemic. It is a reminder that public health is not a permanent achievement but a continuous practice.
- A bacterium that once killed one in ten of those it infected has re-emerged in Australia at a scale unseen for decades, with over 220 cases recorded and one life already lost.
- The outbreak is striking Aboriginal and Torres Strait Islander communities with particular severity — 94% of cases — where overcrowding, skin infections, and limited healthcare access have accelerated transmission.
- Vaccination rates have slid to five-year lows since the COVID pandemic, and roughly 90% of current cases are occurring in people who were vaccinated but whose immunity has since waned.
- The federal government has committed A$7.2 billion in emergency support, while health authorities are now recommending boosters every five years for high-risk groups rather than the standard ten.
- Containment depends on speed: early detection of sore throats and skin sores, updated immunisation schedules, and the urgent restoration of vaccination rates in affected communities.
Australia is facing its worst diphtheria outbreak in decades. Since January, more than 220 cases have been recorded across the Northern Territory, Western Australia, South Australia, and Queensland — a number that has forced a national reckoning with vaccination rates that have been quietly eroding since the pandemic.
Diphtheria is caused by a bacterium that produces a toxin capable of damaging the airways, heart, and nerves. It takes two forms: a respiratory version that can be fatal even with treatment, and a cutaneous form that causes skin ulcers and is less deadly but still contagious. The two can cross-contaminate through close contact, meaning a skin sore in one person can spark a throat infection in another.
The outbreak's concentration is striking. Around 94 percent of cases have affected Aboriginal and Torres Strait Islander people, in communities where higher rates of skin infection, overcrowding, and limited healthcare access have created conditions for rapid spread. Most cases have been cutaneous, though roughly 30 percent have been respiratory. One person has died.
The DTP vaccine remains extraordinarily effective — more than 99 percent of vaccinated infants develop sufficient antibodies — but immunity fades with time. By middle age, only half of adults retain protective antibody levels without a booster. This biological reality, combined with falling childhood immunisation rates that hit a five-year low last year, explains why approximately 90 percent of outbreak cases have occurred in people who were vaccinated: protection had simply worn thin. Most experienced only mild illness, but the vulnerability was real.
In response, the federal government has announced a A$7.2 billion emergency support package. Health authorities are now recommending booster shots every five years — rather than ten — for Aboriginal and Torres Strait Islander people and healthcare workers in affected areas. Pregnant women, close household contacts, and children are also being urged to update their immunisations. Northern Territory Health and Western Australia Health have both released updated schedules for communities in outbreak zones.
For those in affected areas, the guidance is clear: any sore throat or skin sore warrants a clinic visit. Difficulty breathing, swallowing problems, or a greyish membrane in the throat require immediate emergency care. The outbreak is still unfolding, and its resolution will depend on how swiftly vaccination rates can be rebuilt.
Australia is confronting a diphtheria outbreak of historic proportions. Since the start of the year, health authorities have documented more than 220 cases across four states—139 in the Northern Territory, 82 in Western Australia, seven in South Australia, and three in Queensland. It is the worst outbreak the country has seen in decades, and it has forced a reckoning with vaccination rates that have been sliding, particularly since the pandemic.
Diphtheria is caused by a bacterium called Corynebacterium diphtheriae, which produces a toxin capable of damaging airways, nerves, and the heart. The infection spreads through respiratory droplets or contact with skin sores. It comes in two forms: respiratory diphtheria, which affects the throat and airways and can be fatal even with treatment—killing up to one in ten people who contract it—and cutaneous diphtheria, which causes skin ulcers and is generally less severe but still contagious. The two forms can cross-contaminate; bacteria from a skin sore can trigger respiratory infection in another person through close contact.
What makes this outbreak particularly stark is its concentration. According to the Australian Centre for Disease Control, roughly 94 percent of cases since January have been Aboriginal and Torres Strait Islander people. Most have been cutaneous cases, though about 30 percent have been respiratory. Health investigators are still working through the contributing factors, but the picture is becoming clear: waning immunity in communities with lower routine immunisation rates, combined with higher prevalence of skin infections, overcrowding, and limited access to healthcare, has created conditions for rapid spread.
Vaccination is the most effective shield against diphtheria. Before vaccines became standard in the 1950s, about one in ten people with respiratory diphtheria died. In the decades since, Australia recorded only four diphtheria-related deaths between 1999 and 2025—a testament to the power of the DTP vaccine, which protects against diphtheria, tetanus, and pertussis. Children receive it at two, four, and six months of age, again at 18 months, at four years old, and in early adolescence. But immunity fades. Research shows that more than 99 percent of vaccinated babies develop sufficient antibodies to fight the diphtheria toxin, yet by middle age, only half of adults maintain those antibody levels without a booster.
National immunisation data reveals a troubling trend. Vaccination rates have declined significantly since COVID, and last year Australia's childhood immunisation rate hit a five-year low. The current outbreak has exposed the consequences. Roughly 90 percent of cases have occurred in people who were already vaccinated—a fact that underscores both the vaccine's protective power and the reality that waning immunity leaves people vulnerable. Most of these vaccinated individuals developed only mild disease. But one person has died.
The federal government has announced a A$7.2 billion emergency support package aimed at boosting vaccination rates and strengthening the health workforce in affected states. Health authorities are now recommending that all children, adolescents, and adults over 50 stay current with routine immunisations. For Aboriginal and Torres Strait Islander people and healthcare workers in outbreak zones, the new guidance calls for booster shots every five years instead of the standard ten-year interval. Pregnant women should receive a booster between 20 and 32 weeks of pregnancy. People with mild diphtheria or those in close household contact with infected individuals are eligible for additional doses.
For those living in outbreak areas, the advice is straightforward: any sore throat or skin sores warrant a visit to a local clinic for early detection. Fever, breathing difficulties, swallowing problems, or a greyish membrane in the throat demand immediate emergency care. Local and state health departments are working to contain the outbreak, and this week both Northern Territory Health and Western Australia Health released immunisation schedules for people living and working in affected communities. The outbreak is still unfolding, and the path forward depends on how quickly vaccination rates can be restored.
Notable Quotes
By middle age, only half of adults maintain antibody levels if they don't have a booster dose of DTP— Health authorities on vaccine immunity decline
More than 99% of babies who get the relevant vaccinations develop enough antibodies to fight against the diphtheria toxin— Research on childhood vaccine effectiveness
The Hearth Conversation Another angle on the story
Why is this outbreak hitting Indigenous communities so much harder than others?
It's not random. You have lower routine immunisation rates in these communities, which means less baseline protection. Add in higher rates of skin infections, overcrowding, and gaps in healthcare access, and the conditions are perfect for a disease like this to spread fast.
But you said 90 percent of cases were in vaccinated people. So the vaccine isn't working?
It's working—just not perfectly, and not forever. The vaccine is 99 percent effective when you're freshly vaccinated. But immunity decays. By middle age, half of adults have lost their antibody protection without a booster. In an outbreak, that matters.
One person died. Was that someone unvaccinated?
The reporting doesn't specify. But what's notable is that most vaccinated people in this outbreak got mild disease. The vaccine is still doing its job of preventing severe illness, even when it doesn't prevent infection entirely.
Why did vaccination rates drop so much since COVID?
That's still being unpacked, but the pandemic disrupted routine healthcare everywhere. People missed appointments. Clinics were overwhelmed. Trust in institutions wavered. The result is a five-year low in childhood immunisation rates.
What does a five-year booster schedule actually mean for people in affected areas?
Instead of getting a DTP booster every ten years, Aboriginal and Torres Strait Islander people and healthcare workers in outbreak zones need one every five years. It's a tighter safety net while immunity is waning faster in these communities.
Is this outbreak going to end?
It depends on how quickly vaccination rates climb back up. The government has committed A$7.2 billion to that effort. But it's a race—the disease is spreading now, and immunity takes time to build.