WHO refers to the threat as "vaccine hesitancy" and says it "threatens to reverse progress made in tackling vaccine-preventable diseases".
"Vaccination is one of the most cost-effective ways of avoiding disease - it currently prevents 2-3 million deaths a year, and a further 1.5 million could be avoided if global coverage of vaccinations improved," WHO says on its website.
According to WHO, about 21 million lives have been saved since 2000 thanks to the MMR vaccine, for example.
WHO says anti-vaxxers are directly to blame for measles outbreaks all over the globe. There has been a 30 per cent increase in cases globally, including in countries that were very close to eliminating the disease completely.