Unexpected Drop in Microcephaly Incidence in Brazil

The sudden increase in microcephaly cases in northeastern Brazil linked to an outbreak of Zika virus disease in 2015 sparked worldwide concern. Public health officials anticipated a similar number of cases of microcephaly in 2016 and did ongoing surveillance for this birth defect, comparing disease incidences in 2015 and 2016.

In 2015 the reported cases of suspected Zika virus infections in Brazil increased markedly, followed approximately 3 weeks later by a rise in the number of cases of Guillain Barré syndrome (GBS) and, 23 weeks after the Zika increase, by a marked increase in cases of microcephaly in newborns. In 2016, the number of Zika virus disease cases again increased, as did GBS case numbers. In contrast, whereas the incidence of microcephaly was anticipated to peak at about 9000 cases per week and be at more than 3000 cases per week for about 20 weeks, instead there was never a week with more than 1000 cases, and in general fewer than 500 cases were seen per week.

The authors offer three hypothetical reasons for the decline: first, that apparent Zika cases in 2016 were due not to Zika but to another virus that could lead to GBS but not cause microcephaly, most likely chikungunya virus; second, that an unrecognized cofactor acted along with the Zika virus to cause microcephaly in 2015 that was not present in 2016; and third, that in 2016, fear of microcephaly may have led to fewer pregnancies or an increase in pregnancy terminations.