Tom says his parents weren't anti-vaxxers, but for one reason or the other he never ended up getting his shots.

"It never really happened, basically."

By the time he figured this out, it was too late; he was too old to get free vaccines.

"I found it was going to cost a whopping big medical bill."

Vaccinations you might need: MMR — measles, mumps, rubella — approximately $35.00 per dose

Varicella (chicken pox) — approximately $80.00 per dose

Hepatitis B — approximately $20.00 per dose

Poliomyelitis (polio) — approximately $39.99 per dose

Tetanus/whooping cough/diphtheria — approximately $45.00 per dose

Influenza (flu) — approximately $20.00

Hepatitis A — approximately $65.00 per dose

Human Papillomavirus (HPV/Gardasil) — approximately $200.00 per dose

Haemophilus Influenzae Type B (Hib) — approximately $24.95 per dose

Meningococcal C — approximately $100.00 per dose

Because he was over 20, being vaccinated against tetanus, whooping cough, polio, hepatitis B, chicken pox and all the rest would cost $700.

The 21-year-old from Brisbane told Hack, "It's a pretty sucky situation."

"I feel pretty ripped-off."

Although public attention focuses on improving vaccination rates in Australian children, it's estimated almost 92 per cent (3.8 million) of unvaccinated Australians are adults.

Adults contribute substantially to ongoing epidemics of vaccine-preventable diseases.

Most cases of whooping cough, for example, occur in adults. About half of all cases of measles are in those aged 19 years or over.

Despite the cost of getting vaccinated, Tom says he's determined to get his shots.

But he says his main reason isn't to protect himself - it's because he has a moral obligation to others.

"I feel strongly about it," he said.

"Vaccinations work on the collective and I had a moral obligation to myself and society to be protected against these diseases."

"If there is an outbreak it would be very bad for our society - with the measles outbreak people were getting really worried because of the shortage of vaccinations."

Measles outbreak hits the US - anti-vaxxers blamed

The United States is in the grip of a 25-year-record measles outbreak, with almost 1000 cases reported so far, most of them in New York.

Authorities argue the crisis is being fueled by the anti-vaccination trend.

In Australia, too, there's been a spike in measles cases.

It's argued that with fewer people vaccinated, measles has been able to spread more easily.

This is the opposite of what's called 'herd immunity': Once enough people are protected, they help to protect vulnerable members of their communities by reducing the spread of the disease.

Australian Medical Association president Dr Chris Zappala told Hack he was concerned about the Australian anti-vaxxer movement affecting herd immunity, and therefore exposing others to vaccine-preventable diseases like measles.

"I think it's really important people have confidence that when they go and get their vaccines they're doing something helpful - not just for themselves but helping to protect the community."

We need to get vaccination rates up to those critical mass levels so these infectious organisms don't get a foothold and start to circulate.

'I got measles because others didn't get their shots'

As a child, Stephen Aitken relied on herd immunity to keep him safe.

Allergic to the egg white present in many vaccines, he couldn't be immunised. His only chance was to trust in other children's vaccinations.

One day, he came down with fever and itchy spots, and was diagnosed with both chicken pox and measles: A life-threatening combination.

He was out of school for 8 months, and, according to his doctors, almost died.

"I was an ill child through no fault of mine," he told Hack.

This happened due to lack of herd immunity.

As an adult, he grew out of his allergy and was able to get vaccinated. Fortunately, because of his history of allergies, it was still free.

Stephen says he sympathises with Tom and other children of non-vaxxers, who, through no fault of their own, are exposed to dangerous diseases.

Then as adults, they have to pay lots of money to get their shots. (The measles, mumps and rubella vaccination is free, however. The Government also provides free adult vaccines for influenza, pneumococcal pneumonia and shingles for people over 65, and selected vaccines for those with underlying medical conditions, Indigenous people older than 15, and pregnant women.)

Are you an unvaccinated adult? Here's what you need to know about catching up

Tom says that, given the public interest in achieving herd immunity for many diseases, vaccines should be made free for adults as well.

"In my personal situation I'm able to afford it, but the problem is more of an issue for people who can't afford it," he said.

Dr Nicholas Wood, an associate professor of paediatrics and child health at the University of Sydney, told Hack that Tom had a reasonable argument.

But he said there was also an argument for making adults pay for measles; it encouraged them to get their shots when they were children or teenagers.

"The younger you get them the better protection you get," he said.

For more information, the National Centre for Immunisation Research and Surveillance has published a plain-language guide to vaccines.