Several weeks since the Zika threat exploded onto the global mediascape we may not be any closer to understanding the risk.

Consider this: just hours after Tom Frieden, Director of the US Centers for Disease Control and Prevention, reported to the White House Foreign Affairs Committee that Zika virus had been identified in the tissue of two babies in Brazil who died from microcephaly, the CBC and other media organizations broke a developing story about a new report by a Brazilian paediatric cardiologist whose research on congenital heart disease found that several thousand babies were born with microcephaly as early as 2012, long before the current outbreak of Zika virus.

Similar ostensibly conflicting reports raise questions about why the highest concentration of microcephaly cases were confined to northeast Brazil and don’t appear in other Latin American countries with similar climates, and why not a single case of microcephaly has been reported in Colombia where more than 5,000 women have tested positive for the virus. One epidemiologist quoted in the CBC story suggested that Zika virus may not have been the cause for the clustering of microcephaly cases at all: “I soon got the idea that blaming the Zika virus for this epidemic does not really get to the point.” He then proposed that agrochemical exposure could be the underlying reason for the unusually high rates of microcephaly cases in Brazilian babies.

But amidst the ever-present Zika uncertainty, there has been one remarkable moment of risk communication clarity. Perhaps surprisingly, that moment was not based on clear messaging about what we know about the disease and how best to manage our exposure to the virus and the threat it may pose. Rather, it was based on what we don’t know and how best to manage the uncertainty of the situation.

On February 1, the Director-General of the World Health Organization declared Zika to be a Public Health Emergency of International Concern (PHEIC), a formal designation constituted through the revised International Health Regulations (IHR). A global health treaty was first introduced at the Paris International Sanitary Conference in 1851, and has been updated several times since then in response to changing international political arrangements, the evolution of infectious diseases, and our collective global risk exposure. The IHR were most recently revised in 2005 and appeared at a time when international public health, security and democracy were becoming more deeply interconnected, and provided a crucial governance framework appropriate for an era characterized by accelerated global communication, international trade and transnational population flows.

The WHO has declared a PHEIC only three other times: in 2009, with the outbreak of H1N1; in 2014, during an outbreak of wild poliovirus across Central Asia, the Middle East and Central Africa; and in 2015, in response (however late) to the Ebola outbreak in West Africa.

The declaration of a PHEIC is a remarkable risk communication tool. It’s dramatic, calling to mind the image of a global public health star chamber that comes together to assess our common vulnerability. And yet, the WHO is under pressure to exercise this power only rarely to ensure that when it does, it’s able to truly command international attention. In the case of Zika, the PHEIC declaration moved the CDC to elevate its emergency operations center in Atlanta to its highest level of alert; it gave the Obama administration justification to request $1.8B to fund eradication efforts abroad and preparation efforts at home; and it precipitated a global conversation about the need for modernizing reproductive health rights in the largely Catholic Latin American countries most acutely affected by the outbreak.

In terms of design, a PHEIC is, in theory, an independent process which involves the bringing together of independent, international experts recognized in their fields of specialization to assess evidence and offer the WHO and its member nations collective guidance. The model takes advantage of a broad swath of expertise in areas like virology, epidemiology, medical anthropology and risk communication, but, crucially, also shares responsibility for what are invariably difficult decisions.

But a PHEIC is also a blunt and potentially clumsy instrument. Put forward as part of the revised IHR, it is an “on or off” proposition – the crisis is either a five alarm emergency or it’s something which may raise worry and heightened alarm, but doesn’t pose imminent, international danger. Indeed, in May 2014, an emergency committee advising the WHO called on countries in the Arabian Peninsula to strengthen hospital hygiene and assist in information sharing and research on the Middle East Respiratory Syndrome (MERS), a coronavirus similar to SARS (less contagious but more deadly) which first appeared in 2012 and has killed approximately half of the 1,100 people it has infected. But the committee stopped short of declaring the MERS outbreak a global health emergency, noting that measures to slow the outbreak, including contact tracing, quarantine and monitoring, had contributed to declining rates of transmission.

As MERS illustrates, international health risk communication demands a nuanced approach. The rules and regulations surrounding the PHEIC are part of the broader framework represented by the IHR, and provide an effort to bring discipline, rigor, and resolve to the management of global health threats. But, and truth be told, like the IHRs, the role of the PHEIC continues to evolve. Discipline, rigor and resolve are all crucial, but so too are flexibility and adaptability. Each of the four PHEICs declared since 2007 – H1N1, Polio, Ebola, and, now, Zika – has been fundamentally different in many ways, and the role and nature of the PHEIC have similarly been distinct. The factors informing the decision about H1N1 differed from Polio and Ebola—each are, in many ways, unlike Zika. Nevertheless, the politics encircling each declaration no doubt hangs over all future decisions and becomes part of the evolving narrative of a PHEIC and its role in combating global health risks.

This brings us back to the remarkable decision by the WHO to designate Zika a public health emergency of international concern. The decision is remarkable given that it clearly wasn’t based on scientific evidence of a serious threat—rather, it was based on a lack of clear evidence altogether. As the Emergency Committee that advised the WHO reported, “Our advice to declare a PHEIC was not made on the basis of what is currently known…. Our advice to declare a PHEIC was rather made on the basis of what is not known.” Think about this for a moment: how many other health threats of potentially international significance do we not fully understand? How many other threats “could” be much more serious than we may imagine or know them to be?

As discussed in a previous analysis of the Zika risk communication dilemma, scientific experts, public health officials and political leaders generally loathe uncertainty. Risk perception research tells us that the more uncertain we are, the more afraid we will be—and helping populations manage their fear is no easy feat, often demanding higher levels of institutional resources than a given situation actually demands. Uncertainty is also problematic for experts and officials because it threatens their professional identity, decision-making norms, and illustrates the limits of their expertise. Finally, uncertainty creates fertile ground for the proliferation of competing accounts and explanations—some of these will have scientific merit, but others will be founded on little more than conspiracy theory. Regardless, in a context of scientific uncertainty such competing accounts provide grist for the media mill, accelerating and amplifying public fear and anxiety.

For the WHO the Zika uncertainty poses challenges on multiple levels with respect to its international role and responsibility, its status as a global public health leader, and its credibility, which had been badly damaged through the failed response to Ebola. In a context where the very future of the WHO is itself highly uncertain, the stakes of its Zika response—both how it assessed the threat posed by the virus and how it communicated that assessment publicly—are very high indeed.

On the one hand, the Emergency Committee might have erred on the side of caution, choosing to defer a decision about declaring a PHEIC to a later stage when it would be armed with more definitive evidence. It might have opted to bristle in the face of scientific uncertainty and not risk its credibility and collective reputation. On the other hand, as Lisa Rosenbaum persuasively argued, “if you don’t communicate uncertainty and end up being wrong, you risk losing even more credibility.” The risk of not marshalling every possible tool or approach necessary, as occurred during Ebola, no doubt placed enormous pressure on officials to at least be seen to be responding swiftly and decisively. As images of babies born with microcephaly continued to circulate, and with some global health experts publicly pressuring the WHO to act, the pressure to wait for more conclusive evidence was no doubt enormous.

This is what makes the Committee’s analysis so interesting from a risk communication perspective. Publishing details of its decision in The Lancet, one of the most important medical journals in the world, the members gave a full overview of their meeting, walking readers through the debate and discussion about how it assessed the available science and balanced what little they knew against the rising levels of risk perception. Whether the PHEIC declaration was ultimately the best decision to make, this was a remarkable demonstration of decision-making transparency.

Ultimately, the decision by the WHO to declare the Zika outbreak a PHEIC is significant in two potentially conflicting ways, and raises important issues for scholars interested in health risk communication, journalists who cover these events, and, most importantly, for health authorities responsible to protect populations from harm.

First, it has afforded the WHO an opportunity to establish the definitional parameters of the Zika narrative as a potentially serious, global public health threat. While achieving a position of narrative control is important, the uncertainty of the scientific assessment means that other possible accounts will persist. Suggestions that microcephaly has less to do with Zika than larvicide exposure could seriously harm the credibility of the WHO and undermine the IHR and the entire PHEIC process.

Second, and at the same time, the WHO’s skillful embrace of uncertainty and the emergency committee’s adherence to transparency in accounting for how it reached the decision to recommend a PHEIC declaration, represents a new high standard in risk communication. The pressure to appear resolute and confident, i.e., to reassure the public in the face of frightening health risks is tremendous. Failing to do so risks diminishing public faith in an organization’s expertise and its capacity to carry out its mandate to protect the public from harm. However, being precise about the level of uncertainty any decision-makers face can, perhaps counter-intuitively, strengthen trust and credibility. For the most part, the WHO, CDC and other international health organizations have done a fair job in conveying what they know, of distinguishing what they know from they think is possible, and of describing the steps they are taking to close the uncertainty gap. In declaring the Zika outbreak a global health emergency, and permitting its emergency committee to provide a detailed account of its reasoning, the WHO has, for the time being, struck an effective balance between showing leadership and control in its communications while acknowledging that there is much yet to be discovered.