Yes, there are some data suggestive of a link between Zika and microcephaly but these are being massively overblown in the media.
Yes, Brazil has seen an unusually high number of babies born with microcephaly recently: according to the Brazilian Ministry of Health, between November 8th and January 30th, 404 babies were born with microcephaly and 17 were linked to Zika. 17 out of 404? To put it another way, 96% of these cases occurred without the mothers having been exposed to the Zika virus. It isn’t clear whether they used PCR or serology to determine this – if it’s serology that too could heave an impact: there will be false positives in there because Zika antibodies will cross-react with other flaviviruses such as dengue or West Nile.
But that’s not all: incidence of microcephaly has been rising in Brazil since 2012, two years before Zika is thought to have arrived. We need to remember microcephaly can be also caused by a number of other infectious agents: syphilis, toxoplasmosis, rubella, cytomegalovirus and herpes to name a few.
In Colombia over 3,000 pregnant women tested positive for Zika, yet no cases of microcephaly were reported. Ditto Nicaragua, Chile, the Dominican Republic and El Salvador – no microcephaly.
Also Zika has been around since at least the 1950s but so far it has never been found to correlate with birth defects. That’s not to say there isn’t a link but a connection between Zika and the increase in microcephaly still needs to be confirmed and other causes need to be ruled out. The WHO seems to be using the notion of ‘guilty until proven innocent – which is fine, they’re just doing their job – but it’s a lot more complex than it appears in the media.