Health experts have been looking at 4,180 suspected cases of microcephaly reported since October in Brazil, where authorities said the birth defect could be linked to the virus and announced that 220,000 military personnel were being deployed to help eradicate the Aedes aegypti mosquito that transmits Zika.

But yesterday, Health Ministry officials said they had done a more intense analysis of more than 700 of those cases, confirming 270 cases and ruling out 462 others.

What this means is hard to say, according to some experts. It does not answer whether the tropical Zika virus is causing the babies to have unusually small heads. Nor does it really tell us how big the problem is.

Brazilian officials still say they believe there's a sharp increase in cases of microcephaly and strongly suspect the Zika virus is to blame.

The concern is strong enough that the US Centers for Disease Control and Prevention this month warned pregnant women to reconsider visits to areas where Zika is present, and officials in El Salvador, Colombia and Brazil have suggested women stop getting pregnant until the crisis has passed.

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