It is not a new caveat (I've repeated it ad nauseam for the past few months), but it bears repeating.
Despite growing evidence linking maternal Zika virus infection to increased rates of microcephaly in Brazil, causation has not yet been established. Nor is it beyond the realm of possibility that there are other, as yet unidentified, causes (or perhaps, co-factors) involved.
Investigations take time, particularly when the 9-month gestation period of at-risk mothers must be waited on. While Brazil is the only country in the Americas reporting a surge in microcephalic births, there are reportedly 5,000 Zika exposed pregnant women in Columbia - many of whom should deliver in the next month or two.
The incidence of microcephaly there, and a month or two later in Venezuela, should tell us a good deal more.
Until then we are reliant upon a small but growing supply of recently developed laboratory tests, incomplete and inconsistent surveillance (both past and present), and a limited number of fetal and placental tissue samples upon which officials must make critical - and potentially life changing - decisions.
Complicating matters, microcephaly (and reports of GBS - Guillain Barre Syndrome) are occurring in places where Zika isn't the only plausible culprit. The Latin American and Caribbean countries currently battling Zika are also plagued by Dengue (and a more recent arrival), Chikungunya.
Many of their inhabitants also suffer from the diseases of poverty, including malnutrition, limited medical and prenatal care, possible substance abuse, and many live in blighted areas where environmental exposures to chemicals, pesticides and pollution are common.
All of which has raised legitimate questions over whether it is truly Zika that is behind Brazil's reported rise in microcephaly, and even whether their elevated rate of microcephaly is a new phenomenon (see Pondering The Great Zika Unknowns).
All valid questions that deserve serious study. And I certainly don't know the answers.
But in the face of apparent immediate public harm - even when there is something less than proof absolute as to the cause - it is incumbent upon the public health system to take whatever steps it can to mitigate the risks.
As a paramedic, if I had a patient with sudden, unexplained chest pains (which happened a lot) . . . I treated them as if they were having a possible heart attack . . . even if I had doubts that was the cause.
If someone fell from a distance, or was involved in a serious accident, I immobilized their neck, back, and limbs under the assumption they might have a spinal injury or broken bones.
The list of precautionary steps I took to protect my patients (and admittedly, myself), is too long to list here, but it was something I did every day.
Because the potential harm in not proactively treating your patient is far greater than any inconvenience doing so might cause them. And believe me, some patients weren't happy about it. But you always assume a `reasonable worst case' based on the limited evidence you have at the time, and act accordingly.
Anything less is reckless disregard.
In many ways the CDC and WHO are faced with the same sort of dilemma I dealt with so often in my EMS days, decades ago.
They are responding to an apparent medical emergency. One with confusing symptoms, unknown etiology, with only a limited (or unreliable) `family history' to go on, a dearth of proven diagnostics, and an urgent need to stabilize the situation.
And on top of that, they have the entire world watching (and second guessing) their every move. Something I never had to contend with.
Whether Zika turns out to be the ultimate cause of Brazil's microcephaly epidemic (or increased rates of GBS), taking immediate, proactive steps against the most obvious suspect - mosquito vectors - was the logical first step for public health agencies.
The downsides of waiting for absolute proof of Zika's involvement - potentially thousands of maternal infections and GBS cases - was simply unacceptable.
And the upside - reducing other mosquito-borne illnesses (Dengue, CHKV) by following their advice - wasn't inconsequential.
Of course, should Zika somehow be exonerated, I expect there will be huge criticisms lobbed at the CDC and WHO for acting too quickly, and without solid evidence.
Mostly - I'm guessing - from armchair critics who have never been faced with making life and death decisions for others, and who had absolutely nothing to lose by being wrong.
I don't know if the CDC and WHO are ultimately going to be right about Zika. But I'm convinced they were right in acting swiftly - while still emphasizing the unknowns - based on the limited information available at the time.
Hopefully this is one good deed that will go unpunished.