In seven areas that recently experienced Zika outbreaks, there were also sharp increases in the numbers of people suffering from temporary paralysis, researchers reported on Wednesday.

The analysis, published online in The New England Journal Of Medicine, adds to substantial evidence that Zika infections - even asymptomatic ones - may bring on a paralysis called Guillain-Barre syndrome.

The condition can be caused by a number of other factors, including infection by other viruses.

Researchers studying the Zika epidemic in French Polynesia had estimated that roughly one in 4,000 people infected with the virus could develop the syndrome.

The Centres for Disease Control and Prevention has said the Zika virus is "strongly associated" with Guillain-Barre, but stopped short of declaring it a cause of the condition.

The new data suggests a telling pattern: Each site in the study saw unusual increases in Guillain-Barre that coincided with peaks in Zika infections, the researchers concluded.

"It's pretty obvious that in all seven sites, there is a clear relationship," said Dr Marcos Espinal, the study's lead author and director of communicable diseases at the Pan American Health Organisation, which collected data on confirmed and suspected cases of Zika infection and on the incidence of Guillain-Barre.

"Something is going on," he said.

Overall, Dr Espinal and his authors found increases in Guillain- Barre that were two to 10 times what would normally be expected.

Roughly 500 million people in Latin America and the Caribbean are at risk of Zika virus infection.

The nations in the study comprised the Dominican Republic, El Salvador, Honduras, Suriname, Venezuela and Colombia, along with the state of Bahia in Brazil.

Collectively, they reported a total of nearly 1,500 cases of temporary paralysis.

NEW YORK TIMES