Scientists have figured out how measles obliterates the body's ability to protect itself against other diseases and infections.
The discovery is prompting calls to outlaw 'measles parties' and prosecute parents who deliberately expose their children to preventable diseases.
'Immune amnesia' is a little-known side-effect of measles - past studies have shown after exposure to the virus, people are more likely to catch other diseases, even ones they've had before.
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Two studies published on Friday have outlined how it works. Measles decimates antibodies produced by the body's immune system - tests done on unvaccinated children who caught measles found two months after the their bodies had fought off the virus, their antibody counts were up to 73 percent lower than normal, leaving them open to other infections.
In tests on animals, five months after infection nearly two-thirds of their antibodies were still missing in action.
"This immune-suppressant effect lasts for years - and is still present up to five years later," said Prof Michael Baker from the University of Otago's Department of Public Health.
In contrast, the immune systems of vaccinated kids exposed to measles were unaffected.
"The risks of being unvaccinated are not only that of measles, which is a potentially fatal infection, but also other infections because of the effect it has on the immune system," said University of New South Wales biosecurity researcher Prof Raina MacIntyre.
"This study yet again dispels the dangerous myths perpetuated by homeopaths and other 'natural' healers who claim that exposure of infants to natural infection is important to 'strengthen' children's immune systems," said Prof Nikolai Petrovsky at the Flinders University College of Medicine and Public Health.
"This terrible misinformation has been used to justify not just refusals of parents to vaccinate their children but even the shocking practice where parents have infection parties where children are deliberately exposed to other children with measles or other viral infections in order to deliberately infect them.
"Such practices should clearly be globally outlawed and parents subjecting their children to deliberate infection should arguably be faced with criminal charges as should any practitioners advocating or supporting such misguided behaviour."
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Prof Petrovsky said other vaccine-preventable diseases such as influenza "almost certainly" pose similar hidden risks to measles over the long-term.
While there has been an 80 percent reduction in measles infections since 2000, measles still manages to kill about 100,000 a year - most of them pre-school children and babies. The World Health Organization has named the anti-vaxxer movement one of the top 10 threats to public health, as it only takes a small percentage of the population not to vaccinate to weaken herd immunity.
"If you protect enough people in the population you actually interrupt transmission of the virus or the bacteria altogether," said Prof Baker. "There are a whole lot of benefits to individuals and the community from vaccination."
Kiwis are considered fully protected from measles if they have had two doses of the MMR vaccine, have had clinically confirmed measles or were born before 1969, when the disease was in widespread circulation. If you're not sure whether you've been vaccinated before it's perfectly fine to have a catch-up jab, experts say.