Most people who caught and spread measles in our worst epidemic in decades were unvaccinated, a new analysis shows – highlighting a problem experts say is taking New Zealand closer to a “tipping point”.

The just-published study also found that, while immunised people could still catch the dangerous and fast-spreading virus, they were much less likely to pass it on or get sick enough to need hospital care.

The insights come from the 2019 Auckland epidemic, in which 2200 cases were reported and more than 600 people were hospitalised.

A further 83 measles-related deaths were reported in Samoa, largely among young children, after the outbreak spread there.

Study lead author and public health registrar Dr Imogen Evans said the MMR vaccine remained highly effective at preventing measles, with around 95% of people protected after one dose, and 99% protected after two.

Of the 1450 Auckland cases her team analysed in the study, just 10% had received one dose of the vaccine, and 5% two. The rest were either unvaccinated (70%) or unsure of their status.

The average age of unvaccinated cases was about 20 years. The 20 to 29 age group had the highest number of cases (39%), followed by those aged 10 to 19 (23%) and children aged 1 to 4 years (16%).

The results, published in the journal Vaccine, echoed what other global studies had found: that vaccinated people, even if not completely immune to infection, were much less likely to contribute to wider outbreaks.

Complications from measles can include ear infections that cause hearing loss, pneumonia, and encephalitis which can cause brain damage. Photo / Supplied

The risk of onward transmission was shown to be much reduced among those with at least one dose, and especially compared to their unvaccinated counterparts.

“We also found that the time since vaccination was longer for people who passed measles on - suggesting there may be an element of waning immunity,” Evans said.

Those who’d been vaccinated years prior were shown to have a greater risk of onward transmission than those recently vaccinated.

That was something observed in a 2011 outbreak in Quebec, Canada, where 88% of cases among adolescents who’d received two doses still contracted measles, likely due to the gradual decline of their immune response over time.

“However, in this study, the vast majority of children and young people who got measles had received no vaccination, showing that it is the unvaccinated who are more infectious, and who potentially transmit measles in outbreaks,” Evans added.

“It was also clear that even though there were people who had one or two measles vaccines and still got measles, they were much less sick from measles and didn’t need to stay in hospital.”

Unvaccinated patients had the highest rate of hospitalisation (60.3%), while those with two vaccine doses had the lowest (11.7%).

In April, Otago University researchers warned the number of measles-susceptible children was rising by about 1000 every month - and had now moved close to a threshold observed before the 1980, 1985, 1991 and 1997 measles epidemics.

"Compared to those years, we now have more susceptible young adults, and hence may be primed for a large epidemic,” they said.

In 2023 coverage at 2 years was 83% overall - but only 69% for Māori, 81% for Pacific children, and 75% for children in the most deprived communities.

That was a worrying trend also seen in the latest study, with Māori and Pacific people accounting for 21.8% and 46.9% of the analysed cases.

The National Public Health Service’s prevention director, Alana Ewe Snow, said a range of initiatives were under way to lift coverage rates, including actioning more than 50 recommendations from last year’s Immunisation Taskforce Report.

In the most recent step, pharmacies were authorised to give “whole of life” immunisations for children from 6 weeks of age.

“Collectively, these initiatives, alongside our dedicated health professionals who deliver the vaccinations, are intended to support an increase in childhood immunisations across the motu.”

Measles symptoms

• - The first symptoms of measles are similar to Covid-19, or the common cold.
• - The illness begins with a fever, cough, runny nose, and sore, red eyes (conjunctivitis).
• - A rash appears two to four days after the first symptoms, beginning on the face and gradually spreading down the body to the arms and legs. The rash lasts for up to a week.
• - If you or someone in your family has measles symptoms, stay at home and call Healthline immediately on 0800 611 116 so you can get free advice and public health support

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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