Is this the first time Zika has been linked to problems for unborn babies?
In 2013 and 2014 Zika spread through French Polynesia, infecting an estimated 20,000 people. While an increase in birth defects were not noticed at the time, health officials have taken another look in light of the news from Brazil, and found an increase in central nervous system malformations.
Source: NPR's "Zika in French Polynesia"
Before Zika arrived, how many babies had microcephaly each year in Brazil?
In 2010 to 2015, prior to the arrival of Zika, Brazil reported between 139 to 175 cases of babies born with microcephaly annually. It is very strongly suspected that microcephaly was under reported in these years.
- UNICEF reports that just over 3,000,000 babies were born in Brazil in 2012. Assuming that the risk of microcephaly in Brazil was the same as in the U.S., 0.14% for severe cases, we would expect around 4500 babies to be severely effected. This doesn't even include children with more moderate microcephaly!
- Researchers looking over back over birth data prior to the arrival of Zika, from 2012-2014, have found many unreported cases of babies with microcephaly.
How many babies have been born with microcephaly in Brazil since the arrival of Zika?
There have been 4180 suspected cases of microcephaly since the beginning of October. 732 of these cases have been examined more closely. Of these 732 cases, 462 were rejected as false diagnosis, while 270 were confirmed as microcephaly. If this ratio of cases accepted were to continue, we would expect around 1600 cases to be confirmed.
Source: Nature's "Zika virus: Brazil's surge in small-headed babies questioned by report."
There are other reasons to suspect over-diagnosis since 2015, including:
- Broadening diagnostic criteria: Brazil increased the head circumference criteria in order to ensure any babies who might be effected received help. This is a good move if the goal is to make sure babies receive care, but it also means healthy children may be identified.
- The Brazilian government started requiring doctors to report babies born with microcephaly in October, after which there were more reported cases.
Do we know what the risk of microcephaly is for a baby born to a mother who was infected during pregnancy?
No. The numbers aren't good enough to say anything definitively.
Source: ECLAMC Summary and Conclusions