Parents are being urged to ensure their children's vaccination status is up to date following an outbreak in the Northern Territory of diphtheria, a serious respiratory bacterial infection.
The NT's Centre for Disease Control has confirmed three cases of respiratory diphtheria in Darwin and one in Alice Springs within the past two weeks.
Adelaide University epidemiologist Adrian Esterman said Australia had at most a handful of diphtheria cases each year and outbreaks were rare.
"We need about a 95 per cent coverage of the diphtheria vaccine to get herd immunity going," he told AAP.
But as with the measles-mumps-rubella (MMR) vaccine, vaccination rates have dipped for diphtheria.
MMR coverage in Australia was 95 cent before the COVID pandemic, but was now down to 92 per cent, Professor Esterman said.
"It doesn't sound like a huge drop, but it's enough for there to be a slight impact on herd immunity and then you see these small outbreaks in under-vaccinated communities."
People arriving from overseas bring in the infection and it is picked up by people who are not vaccinated.
Parents were becoming more reluctant to get their kids vaccinated, Prof Esterman said.
"We need to make sure that people are aware that diphtheria can be very nasty."
Diphtheria can easily spread person to person through inhalation of respiratory droplets when an infected person coughs or sneezes.
Respiratory diphtheria symptoms can include a sore throat, mild fever, loss of appetite and in severe cases, trouble breathing, in some cases leading to death if untreated.
Diphtheria was a feared childhood disease and common cause of death in children until the 1940s, when vaccines were rolled out.
NT Health said the risk to the general public was "extremely low" and contact tracing was underway for people who may have come into contact with diphtheria cases.
"NT Health encourages parents to check their child's vaccination status to ensure they are up to date according to the NT immunisation schedule," a spokesperson said.
The other less harmful strain of the disease is cutaneous diphtheria, spread by direct skin contact on lesions of infected people, with symptoms including sores or ulcers and slow-healing wounds.
In the NT there have also been 33 cases of cutaneous diphtheria notified since 2025.
Vaccination is free under a national program for children aged six weeks to two months, four months, six months, 18 months, four years and 12 years.
Pregnant women from 20 weeks of pregnancy are also eligible and adults are encouraged to get a booster vaccine every 10 years.
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