WASHINGTON, Feb 15 - Trump in the past has promoted debunked theories linking vaccines to autism, and shortly before his inauguration was considering a commission on the matter.

Such comments have alarmed health professionals across the world.

Only last week, the American Academy of Pediatrics and dozens of other health organisations signed a letter to Trump stating claims that vaccines aren't safe "have been disproven by a robust body of medical literature," and offering to meet with him to explain that science.

A look at his statement at a forum on Tuesday (February 14) and what is known about the prevalence of autism in children is revealing.

"Tremendous increases ... really a horrible thing to watch the tremendous amount of increase," Trump said.

In fact, about 1 in 68 school-aged children has autism or related disorders in the US, a rate that has stayed roughly the same for two years, the Centers for Disease Control and Prevention reported in March last year.

That's far more than in 2000, when the CDC estimated that about 1 in 150 children had autism. That increase is explained in large part by more awareness of the developmental disorder and changes in practice that broadened the definition for an autism diagnosis.

While Trump during one primary debate insisted he was "totally in favour of vaccines" he has subscribed in the past to theories unsupported by scientific evidence linking vaccines to autism.

He tweeted in 2012: "Autism rates through the roof - why doesn't the Obama administration do something about doctor-inflicted autism. We lose nothing to try."

In 2014 he tweeted: "If I were President I would push for proper vaccinations but would not allow one time massive shots that a small child cannot take - AUTISM."

A similar assertion in a 2015 presidential primary debate brought a rebuke from the American Academy of Pediatrics, which said it is "dangerous to public health" to suggest that vaccines are linked to autism.