The global trend of parents shunning vaccinations for their children has become deadly serious.
Japan is battling its worst measles outbreak in the past 10 years, with more than 170 cases reported.
And the World Health Organisation says an epidemic of the disease in Madagascar has caused more than 900 deaths.
The WHO says there have been more than 68,000 cases of the disease in the African island state, in which 553 deaths were confirmed and another 373 suspected from measles since the outbreak began in September.
Those most at risk are infants from nine to 11 months old.
The epidemic is blamed on a low immunisation rate for measles across the island nation over many years, WHO spokesman Tarik Jasarevic said. The vaccination rate is estimated to be less than 60 per cent, Mr Jasarevic said.
Anti-vaccine propaganda has been spread widely on social media, to the extent that networks including Pinterest and YouTube have detailed measures they are taking against misinformation on vaccines. They have been accused of enabling the propaganda.
Pinterest confirmed to AFP Monday it changed its policy last year on anti-vaccine content.
The company said it had started blocking certain searches related to vaccinations and cancer cures last year, because results were leading to harmful misinformation.
“We want Pinterest to be an inspiring place for people and there’s nothing inspiring about misinformation,” a spokesman said.
“That’s why we continue to work on new ways of keeping misleading content off our platform and out of our recommendations engine.”
The site is blocking search results and barred accounts and removed “pins” that violate its rules on medical disinformation, but the spokesman was unable to give specific numbers.
YouTube announced on Friday that it was removing ads on anti-vaccine videos to stop them making money.
“We have strict policies that govern what videos we allow ads to appear on, and videos that promote anti-vaccination content have been and remain a violation of our longstanding harmful or dangerous advertising policy,” YouTube said.
Buzzfeed had drawn attention to instances in which YouTube’s automated recommendations system had allowed some anti-vaccine videos to slip through.
Pressure is growing on social networks in the US and elsewhere to take action in the face of criticism that they have enabled the development of an anti-vaccine movement.
Several outbreaks of measles in the US have sickened 159 people since the start of the year, including 65 in Clark County, Washington state. Most of the cases involved non-vaccinated children.
US health authorities say the percentage of children aged two or over who have not been vaccinated has gone from 0.9 per cent of children born in 2011 to 1.3 per cent of those born in 2015.
In 2017-2018, the number of requests for vaccination exemptions rose for the third school year in a row in the US.
Democratic politician Adam Schiff wrote to the heads of Facebook and Google about the issue on February 14.
Facebook responded that it was looking at ways to reduce the visibility of anti-vaccine content.