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Beware overblown Zika estimates. So far, most reports of birth defects have not been confirmed.

In the widespread coverage of Zika, many news outlets (including this one) have reported on Brazil's terrifying surge in birth defects following the arrival of this mysterious virus.

According to Brazil's health ministry, since last year doctors have reported some 4,000 cases of microcephaly, a devastating birth defect that's characterized by a shrunken head and incomplete brain development. That's much higher the usual number of cases in pre-Zika Brazil — a very worrisome data point.

But what if that number is wildly wrong? What if the number of microcephaly cases in Brazil is actually much smaller?

This is very much a live question. There's a lot we don't know about Zika and its link with birth defects. But what we do know is that the 4,000 figure represents unconfirmed reports.

The Brazilian health ministry is still working through all the microcephaly cases in the country and trying to figure out which ones are valid, which aren't, and how many are linked to the virus.

That's a slow process. As of January 29, of the 4,180 reported cases just 270 have been confirmed as microcephaly, and 462 have been thrown out.

Of the confirmed cases, only six have so far been linked to Zika. Six. That's much different from the 4,000 figure that's been making the rounds, and there are a couple of reasons for the disconnect.

1) An awareness bias. When doctors began to sound the alarm last year over the impact Zika might have on the brains of fetuses, Brazilian health officials massively ramped up the effort to find babies who may have been harmed by the virus. People were freaked out, and surely so were doctors.

In that context, doctors were looking for microcephaly and may have been more likely to report the birth complication — perhaps including borderline cases that would have never been reported in the past and cases that aren't actually microcephaly at all. So this search for microcephaly probably led to an "awareness bias," meaning health officials are finding more cases now because they are looking for them.

2) An incorrect baseline. Conversely, there's now some suspicion that Brazilian doctors actually underreported microcephaly before Zika's arrival. That means their baseline — a couple hundred cases per year — was too low. (Before the arrival of Zika in 2014, the health ministry reported 147 cases of microcephaly, which observers note is "surely a vast underestimate" given the size of the population and general frequency of the condition.) All of that makes it difficult to determine if the uptick in microcephaly is real.

3) Difficult diagnosis. Microcephaly is difficult to diagnose. As you can read in our explainer here, it's a condition that's caused by a range of things — from maternal alcoholism to Down syndrome — and a diagnosis involves measuring a baby's head and identifying a potential cause. In a recent announcement from the World Health Organization, the agency's director noted that the case definition for Zika needs to be standardized, since right now estimates might be capturing a lot of noise.

4) Confusion over suspected and confirmed cases. The last is the simplest explanation: We in the media sometimes fail to appreciate or communicate that initial reports of infectious disease numbers don't always represent real cases. That's why health officials work to validate and confirm them. (The same confusion over suspected and confirmed cases emerged during the Ebola outbreak, and probably many other epidemics that came before.)

As health officials work through all the microcephaly reports, it's possible many more will be linked with Zika. It's also possible many will get thrown out. But right now, the correct figure of confirmed microcephaly linked to Zika is six.

Expect these numbers to change again and again

There are still a lot of questions about the link between Zika and birth defects. Before last year, the research world knew very little about this virus. As late as 2007, there had only been 14 documented Zika cases in the world. So we're very much learning about the virus, and as more people get Zika, the picture of this disease and its potential for complications will continue to evolve.

Right now there's a lot of good evidence to suggest there is a link between Zika and microcephaly. In January, for example, scientists at the CDC found Zika in the brains of two microcephalic babies who died within 24 hours of being born. They also found evidence of Zika in two pregnancies that ended in miscarriage.

What's more, similar clusters of microcephaly have been found in other Zika outbreaks. A look back at the data from a Zika outbreak in the French Polynesian islands in 2013-'14 revealed a rise in birth defects following the arrival of Zika.

There's something going on here, and Zika wouldn't be the first virus to cause birth defects. (Rubella caused an epidemic of birth defects before the advent of the vaccine.) But researchers still haven't confirmed this link. And it's likely that the frequency of birth complications may look much different as this outbreak continues.

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