MIAMI, United States (AFP) — About one in 10 women in the United States who were infected with the Zika virus while pregnant had a foetus or baby with deformities last year, US government scientists said Tuesday.

The study released by the US Centers for Disease Control and Prevention (CDC) tracked 250 pregnant women who had confirmed Zika infection in 2016.

Twenty-four of these women either had a foetus or baby with defects, said the CDC Vital Signs report, the first research of its kind to be released since the mosquito-borne virus made its way into the United States last year.

Zika can cause brain defects such as microcephaly, an irreversible condition in which the child’s brain and skull are unusually small and misshapen. 

But even though some babies are born without any outward signs of defects, they may go on to show developmental delays, so researchers cautioned that their estimates may be lower than reality.

Since Zika erupted on a large scale in mid-2015, more than 1.5 million people have been infected, mostly in Brazil and other countries in South America. Some 70 countries have been impacted. 

Zika may lead to an itchy rash but often causes no symptoms.

Its effects on the foetus were particularly acute when the virus attacked a woman in the first trimester of pregnancy.

“Confirmed infections in the first trimester posed the highest risk — with about 15 per cent having Zika-related birth defects,” said the CDC Vital Signs report.

Most of the cases involved women who acquired the virus while travelling outside the country.

Local cases of Zika have been reported in Florida and Texas, and the virus can also be transmitted through sex.

“Zika continues to be a threat to pregnant women across the US,” said CDC Acting Director Anne Schuchat.

“With warm weather and a new mosquito season approaching, prevention is crucial to protect the health of mothers and babies.”

The CDC continues to track pregnant women who may be infected with Zika, and the current count is about 1,600, she said.

Experts urge pregnant women to consider avoiding travel to places where Zika is spreading, to use condoms or practice abstinence with partners who have been in areas of active Zika transmission, and take steps to prevent mosquito bites.

“All pregnant women should be educated about how to avoid Zika virus exposure during prenatal visits,” said James Ducey, director of maternal-foetal medicine at Staten Island University Hospital.

The World Health Organization declared a global health emergency in February last year due to Zika, and declared it over in mid-November.

The WHO has said no vaccine is likely to be available until 2020. 

“Unfortunately, there is no treatment for this infection,” said Michael Grosso, chief medical officer at Northwell Health’s Huntington Hospital in New York.

“We should hope that efforts at a vaccine will be successful so that parents and children are spared the neurologic and other disabilities associated with this virus.”