Babies infected with Zika virus may suffer severe brain damage even if they do not display the signature symptom of an unusually small head, a study in monkeys suggested Monday.

This meant that brain-damaged children may be walking around undiagnosed and missing out on life-bettering therapy, scientists reported in the science journal Nature Medicine.

"Current criteria using head size to diagnose Zika-related brain injury fail to capture more subtle brain damage that can lead to significant learning problems and mental health disorders later in life," said the study's lead author Kristina Waldorf of the University of Washington School of Medicine in Seattle.

"We are diagnosing only the tip of the iceberg," she said in a statement.

Waldorf and a team analysed the brains of five growing macaque foetuses whose mothers they infected with Zika virus.

Macaques are considered a close animal model for human pregnancy.

Only one of the monkey foetuses displayed physical abnormalities early on, but later MRI scans revealed that the brains of four of the five were not developing as they should.

Particularly hard hit were regions of the brain where new brain cells are generated.

"Subtle damage caused by this virus during foetal development or childhood may not be apparent for years, but may cause neurocognitive delays in learning and increase the risk of developing neurological disorders such as schizophrenia and early dementia," said Waldorf's colleague and study co-author, Lakshmi Rajagopal.

"These findings further emphasise the urgency for an effective vaccine to prevent Zika virus infection."

Since Zika erupted on a large scale in mid-2015, more than 1.5 million people have been infected with the virus, mostly in Brazil and other countries in South America.

In most people, it causes no symptoms, or light ones such as an itchy rash.

But it is very dangerous for foetuses -- more than 2,200 babies have been born with Zika-related microcephaly, a shrinking of the brain and skull, according to the World Health Organization.

Many others died before birth.