Travellers rather than anti-vaxxers are most likely responsible for the spike in measles cases in Australia, where 92 cases of the disease have been confirmed in 2019, close to the number of cases for the whole of 2018.

The sudden rise in cases has prompted the health minister, Greg Hunt, to reiterate the importance of vaccinations. “Immunisation saves lives,” he told reporters. “The measles vaccine is very effective at protecting lives.”

But it was unlikely there had been an increase in the number of Australians questioning the vaccine’s efficacy and who opposed the vaccination, infectious disease experts say. Rather, some Australians are unaware that they are no longer immune to the disease, while others haven’t got around to getting their children vaccinated due to logistical reasons or language barriers.

Australia has had worse outbreaks in the past decade. In 2012, a 25-year-old man returning to Australia from Thailand and carrying the virus led to 167 Australian cases over the eight months that followed, largely in southwest Sydney. Numerous studies examining that outbreak found approximately 20% of cases were in people of Pacific Islander descent. Aboriginal and Torres Strait Islanders were also over-represented.

“Many students of Pacific Islander background appeared to have missed routine childhood vaccinations both before and after their arrival in Australia,” a study published in the World Health Organisation’s Western Pacific Surveillance and Response Journal found. It highlighted the importance of community outreach programs informing those with English as a second language about the availability of vaccines, and revealed the disease is so viral that young travellers can cause measles outbreaks even in non-endemic countries like Australia. In other words, outbreaks occurred in pockets where people were under-immunised, but those people were not necessarily anti-vaccination.

Associate Professor Anita Heywood from the University of New South Wales, who evaluates immunisation programs and immunisation gaps, said a similar phenomenon was occurring with the most recent Australian outbreak.

“Australians travel a lot and no matter how well we control measles in Australia, if the world isn’t controlling it as well we will always have people coming back with it, whether that’s Australians returning from overseas or tourists,” she said.

A 3D graphical representation of the measles virus particle. Photograph: Reuters

“It’s not until we reduce the incidence globally that we will prevent these cases in Australia. There are always a couple of cases in those opposed to vaccination, but the anti-vaccination movement gets so much airtime, particularly on social media, that many people think that movement is a much bigger contributor to the outbreak than it is.”

The issue in Australia is that those born between 1966 and 1992 may only have had one dose of the vaccine, with two needed for it to be fully effective. Those born before 1966, when the vaccine was not available, likely had measles or were exposed to it and so are immune. A second dose was fully-funded from 1992 onwards and was administered to high school students.

Those who had only received one dose should get another, especially if they were planning to travel overseas or be in contact with travellers, Heywood said. Many travellers did not know they were under-immunised, she added, and did not think of measles when getting their travel vaccines against diseases more commonly associated with travel such as diphtheria or tetanus.

The other potentially susceptible group is babies. Babies are protected by their mother’s antibodies until the age of about nine months. They are routinely given the vaccine at 12 months; giving the vaccination too early risks the maternal antibodies still being present and killing the virus in the injection, rendering it useless.

“I think we get a bit complacent in Australia, we think everyone does vaccination as well as we do,” Heywood said. “But France is also grappling with measles and that’s because unlike in Australia, there is not much coordination of the vaccine program nationally so overall coverage rates are a little over 80 per cent and have been like that for a long time.”

Herd immunity for measles requires at least 90-95% of the population be vaccinated, and that’s because it is one of the most infectious diseases. Droplets of the virus can hang in the air for about two hours, long after the infected person may have left the room. It is three times more infectious than polio or small pox.

Heywood said the US was a country where the vaccination movement was having an impact on infection rates. While there would be pockets there under-immunised for logistical, accessibility and language-barrier reasons, there have also been numerous reports of anti-vaccine campaigns spreading misinformation, especially in Orthodox Jewish communities.

It led to health authorities taking the highly unusual step on Wednesday of ordering nearly everyone in a heavily Orthodox Jewish New York City neighbourhood to be immunised against the disease or face fines.

But in Australia, Professor of infectious diseases with the Australian National University, Peter Collignon, said anti-vaccination numbers hadn’t changed significantly overall, though he did say there were pockets that contained more anti-vaccinators than in other areas that were potentially putting their children at significant risk during the outbreak.

“We are not going to get a measles epidemic in Australia,” he said.