"It was me that put my son in this situation," Mr Williams said.
"Parents like us make the decision to not vaccinate on very little factual information about the actual consequences of the diseases - massive pain, disability and death - and a lot of non-factual, emotive information from the internet stating inflated figures on the frequency and severity of adverse reactions and conspiracy theories about 'evil' doctors, governments and drug companies."
Seven-year-old Alijah is among the 90 per cent of people who get tetanus and survive, though he still gets spasms and will require ongoing medication and rehabilitation.
He was discharged in a wheelchair on January 8 after 26 days in hospital. He faces a 12-month recovery including having to learn to eat and walk again.
Mr Williams, a food technologist with a science degree, believed much of the information that convinced him and his wife not to vaccinate was misinformation and myths.
"Believing myths about vaccines is not the same as getting the facts. And that is the core problem."
Auckland Regional Public Health clinical director Dr Julia Peters said parents who did not immunise their children were making choices with potentially far-reaching implications for society.
They should think about whether they might infect someone without the same level of defence as them, for example, someone with cancer or a baby who was not yet immunised.
"So I think when people (think) 'it's just about me and my individual choice', actually that's not true."
The director of the University of Auckland's immunisation advisory centre, Dr Nikki Turner, said unless people had seen or experienced the effects of debilitating and painful diseases such as tetanus, whooping cough and measles, it was very hard to believe their severity.
One of the Government's six key health targets is for 95 per cent of 2-year-olds to be fully immunised.
By August last year 93 per cent were.
Dispelling some myths over the measles, mumps and rubella vaccine
Does the measles, mumps and rubella vaccine cause autism? Large international studies have found the MMR vaccine does not cause autism.
The original paper suggesting some association has been retracted by the Lancet, which first published it. Lead author Dr Andrew Wakefield has been investigated in Britain for failure to disclose a conflict of interest, publishing fraudulent research and ethics breaches. He has been struck off the medical register for serious professional misconduct.
How many germs/antigens is a baby exposed to with immunisation? Since 2000, infants have been exposed to about 50 antigens by immunisation. In 1980, children were vaccinated with 3041 antigens. Children today are exposed to far fewer antigens throughout an entire immunisation schedule than some of their parents and grandparents would have been exposed to in a single vaccine.
Are babies given too many vaccines all at once? Exposure to the environment gives much more foreign protein every day.
Can vaccines cause cot death? Vaccines are associated with a slightly lower rate of cot death than in non-vaccinated children.
Do vaccines cause allergies or auto-immunity? There is no evidence to prove this theory.
Is aluminium dangerous in vaccines? Aluminium has been used for more than 70 years to help stimulate immune response and quality, and decrease the toxicity of certain antigens such as pertussis, diphtheria and tetanus. Minor reactions occur often but there have been very few serious reactions. They are more likely if the injection is not delivered deep into the muscle.
Do vaccines cause febrile convulsions? Infants can have febrile convulsions after a vaccine but it is a result of a fever, which children can get after immunisations. Anything that causes a fever, such as a virus, can lead to a febrile convulsion.
Can pertussis/whooping cough vaccine cause brain damage? Three New Zealand children, now adults, were given ACC payouts after suffering brain inflammation following childhood vaccinations, including a whooping cough vaccine no longer in use. The payout was made despite it being unclear whether the inflammation was a direct result of the vaccine. A study of 2.5 million children in America since then could not show a link between vaccines and brain damage.
Source: Immunisation Advisory Centre, Auckland University