Areas with the Zika virus had a higher rate of birth defects in the last half of 2016 than in the first half, but it is too soon to tell if the spike is caused by the virus, according to the Centers for Disease Control and Prevention.

The CDC found that areas with local transmission of the mosquito-borne virus — southern Florida, a part of southern Texas and Puerto Rico — saw a 21 percent increase in births with outcomes linked to Zika in the last half of 2016 from the first half of 2016. Zika is linked to birth defects including microcephaly, which causes babies to be born with abnormally small heads and brain damage.