Zika in Perspective (not so scary)

Having a baby is scary enough, for oh-so-many reasons.  But the recent Zika virus explosion has made it even scarier.  How scary should it be?  The Washington Post has an article on the difficulties of confirming Zika-related microcephaly cases in Brazil.  As of last week, 4,180 cases had been reported since October, but few had been confirmed by experts.  So a sample was explored in depth, and…

After experts scrutinized 732 of the cases they found that more than half either weren’t microcephaly, or weren’t related to Zika.

To be precise, just 270 cases of the 732 examined seem to be confirmed as related to Zika or something similarly infectious.  If that ratio holds, of the 4,180 cases reported since October, only about 1,542 will prove substantiated.  A bit less scary… but still a big number.

Except, in the context of Brazil, that’s actually not a big number at all.  I can’t readily find births data covering the period from October, 2015 to the present.  But I can easily find data on how many infants there were in Brazil in 2010.  Not a perfect proxy, but close enough for a rough estimate.  There were 2,713,244 infants, implying at least that many births.  Now THAT is a big number, and a good reminder that Brazil is the 5th largest country in the world, just two spots behind the USA.

The estimate from 2010 is a year’s worth of births, of course.  To make that comparable with reports from October, let’s make a heroic assumption that we’ve got about four months worth of reporting in (October through January).  So how many Brazilian babies would likely be born in four months time?  904,415.  Still a very large number.  What percentage of those babies look like might have Zika related microcephaly?

I get somewhere between 0.17% (using the ratio of confirmed cases) to 0.46% (using reported cases).  That’s between 1.7 cases per 1,000 births to 4.6 per 1,000 births.  Less scary.

But again, everything is scary to prospective parents.  What helps, perhaps, is perspective.  The pre-existing infant mortality rate in Brazil is estimated to be 19 deaths per 1,000 births (according to Population Reference Bureau data).  By contrast, the infant mortality rate in the USA is an estimated 6 deaths per 1,000 births, and for Canada, 4.8 deaths per 1,000 births.

These kinds of statistics lead me to ask: why don’t Canadians have a travel warning issued for pregnant women (or for mothers of infants) visiting the USA?  After all crossing the border to spend a year in the USA would appear to add an additional risk of dying of an extra 1.2 deaths per 1,000 births.  This is pretty close to the low end 1.7 cases of Zika-related microcephaly per 1,000 births we might be seeing in Brazil.

Not really so scary after all.

Of course there are two caveats here.  1) It would appear this is just the start of the Zika outbreak, and knowledge about its size and ultimate effects remains in flux, as witnessed by the shifting case count, and 2) We shouldn’t really equate the risks of microcephaly (Zika-related or otherwise) with the risks of infant mortality, as suggested by this CBC report.

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