Vitoria Evillen (R) blows soap bubbles near her 4-month-old sister Luana Vieira, who was born with microcephaly, at their home in Olinda, Brazil, February 3, 2016. The suspected link between the Zika virus and a birth defect known as microcephaly appears "stronger and stronger" as researchers study whether there is a causal connection, the head of the U.S. Centers for Disease Control and Prevention said on Friday. Picture taken February 3, 2016. REUTERS/Ueslei Marcelino