For example, it is not clear how common Zika infections are in pregnant women, or when during a pregnancy a woman is most at risk of transmitting the virus to her fetus.
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It is now spreading locally in some 20 Latin American and Caribbean countries, as far north as Mexico.
US health officials are stepping up efforts to study the link between Zika virus infections and birth defects, citing a recent study estimating the virus could reach regions where 60 percent of the USA population lives.
Romit Jain, from the European Centre for Disease Prevention and Control in Stockholm, says there have been confirmed cases of imported Zika virus infections in Germany and Britain.
Most patients recover, but the syndrome is sometimes deadly.
One reason why World Health Organization is examining the Zika virus so quickly may be because the agency was criticized for its slow response to Ebola; almost 1,000 people had died before World Health Organization declared it to be an global emergency.
Health officials in several of those countries have told female citizens to avoid becoming pregnant, in some cases for up to two years.
Because so little is known about the virus – nothing has been published in scientific journals – the best approach is to assume the U.S.is vulnerable, said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.
The spread of the virus in Brazil has coincided with a sharp increase in the birth of babies with abnormally small heads.
In Brazil, cases of microcephaly, which can cause brain damage or death, have risen from 163 per year on average to 3,893 since the Zika outbreak began last year.
Brazilian health officials did not detail what they found in the 462 cases that were ruled out, but many of them were just premature and under-sized, a health ministry spokeswoman said.
American Airlines spokesman Ross Feinstein said customers can receive a refund on purchased plane tickets to San Pedro Sula, Honduras; Tegucigalpa, Honduras; Panama City, Panama; or Guatemala City, Guatemala, with a doctor’s note demonstrating they are unable to travel to those cities due to pregnancy.
The WHO has said that Canada and Chile are the only two countries in the Americas where the virus is not likely to spread. He says that for women who are not pregnant and for men, the impact of contracting the Zika virus is generally mild.
But Wednesday’s report says only 270 of those cases have been confirmed. There is no vaccine or treatment for the virus, a close cousin of dengue, which normally causes mild fever and rash.
“We have to be cautious about what is happening in other countries”, said Dr Farida Al Hosani, director of communicable diseases at Health Authority Abu Dhabi.
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There is now no vaccine for the virus, but scientists at the University of Texas Medical Branch have visited Brazil to carry out research in order to find one.
