A rise in birth defects in the Americas is increasingly linked to Zika virus, previously undetected in that part of the world. Regardless of the underlying cause for these congenital abnormalities, the key to success lies in strong global health leadership. While some lessons from the Ebola outbreak can be applied, this new threat presents a different challenge and needs a different response.

In December 2015, the journal Nature asked infectious disease experts to predict which pathogens would trigger the next global crisis. None suggested Zika virus, a mosquito-borne disease first identified 70 years ago in Africa. Yet, a month later, the World Health Organization (WHO) predicts up to four million cases in the Americas over the next year, and on Feb. 1, declared the cluster of microcephaly cases, strongly believed to be associated with Zika virus infection, a Public Health Emergency of International Concern (PHEIC).

The WHO decision represents an extraordinary step: It’s only the fourth time since the revised International Health Regulations came into force in 2007 that a PHEIC has been declared.

Zika virus infection causes mild, flu-like symptoms in most cases. What prompted concern was not the infection, but Brazil’s live birth information system (a system not readily available in less-developed countries) detecting a 30-fold increase in the number of babies born with microcephaly, a congenital defect limiting brain development. It is this increase, rather than Zika virus itself, that prompted WHO to declare a PHEIC.

If the spreading virus is associated with microcephaly, as evidence increasingly suggests, the global social and economic repercussions could be severe. A large increase in the number of children born with profound learning disabilities worldwide would have severe human as well as socioeconomic repercussions globally, causing productivity loss and high associated health care costs.

Very much like the West African Ebola outbreak, the spread of Zika virus was an almost unpredictable event—a characteristic common to most emerging infectious diseases that end up causing global crises. Unlike Ebola, it occurred in a part of the world where surveillance capacity enabled prompt detection of an unusual event.

If the spreading Zika virus is associated with microcephaly, as evidenced, the global repercussions could be severe.

When Brazil detected Zika virus in May 2015, there was no strong evidence of a link with microcephaly, and there is still no definite causal relationship, although evidence now suggests that infection during pregnancy is associated with microcephaly. Zika is also suspected to cause severe neurological symptoms in a small minority of infected adults. Regardless of the underlying cause—infectious, chemical or environmental causes are all plausible at the onset of the event—such a large-scale, unusual health event should prompt the WHO to take rapid action, particularly in light of the criticism the agency faced for delaying the Ebola response.

The decision to declare a PHEIC will help galvanize international attention and encourage research into the link between microcephaly and Zika virus infection. It will also encourage countries inside and outside the affected areas to improve surveillance, control mosquito populations and expedite the development of diagnostic tests and vaccines.

An earlier convening could have affirmed WHO’s position as the global leader during health emergencies, a much-needed message after the Ebola outbreak; however, the WHO’s Regional Office for the Americas (also known as the Pan American Health Organization, or PAHO) has already been involved in supporting the investigations in Brazil. The Brazilian Ministry of Health declared a public health emergency on Nov. 11, 2015, and PAHO issued an “epidemiological alert” on Dec. 1, but neither action garnered much global attention.

Working Outside Ebola’s Footprint

Although the WHO also declared Ebola a PHEIC, Zika is not Ebola; it does not spread from person to person, has a low mortality and does not kill health care workers. Zika therefore warrants a different response. It does not require health care worker mobilization, treatment centers, contact tracing or safe burial teams. Rather, the immediate needs are research to determine the cause of the microcephaly, diagnostic capacity building and sustained efforts to reduce the mosquito population.

Much of this is already underway. Brazilian authorities inspected more than seven million households looking for mosquito breeding sites. Their public health agency has generated key evidence for an association between Zika virus and microcephaly and has developed a rapid diagnostic test. The spread of Zika virus, detected in more than 20 countries so far, has led to extraordinary measures, such as Brazil, Colombia and El Salvador asking women to delay pregnancy for up to two years, as well as widespread travel warnings to affected countries for pregnant women.

What is needed now is strong leadership to ensure coordinated, consistent and proportionate advice to the public and a real focus on the critical research to help us understand what is really happening in Brazil and elsewhere. Declaring the event a PHEIC will help achieve these goals; however, the declaration is not linked to any funding and countries with less resources and weaker public health systems may still struggle to control the threat without external help. This again emphasizes the urgent need for a global contingency fund to be developed in support of the IHRs and global health security.

While infections with Zika virus are currently largely concentrated in South and Central America, a rapid spread and mounting evidence of association with microcephaly means it could become a global crisis. Aedes egyptus, the mosquito that carries the virus, can be found in Southern Europe, Africa and the Southern United States. There is additional concern that aedes albopictus, the highly aggressive tiger mosquito, could become a vector for Zika virus, further expanding areas at risk. It is unclear how Zika virus spread to the Americas. But like West Nile Virus, another mosquito-borne disease unknown to the Western Hemisphere until 1999 but now endemic in North America, Zika could be here to stay.

As the Ebola crisis wanes, it is clearer than ever that the nature or location of the next health crisis cannot be accurately predicted. The potential consequences of the Zika virus outbreak highlight, once more, the need for decisive and prompt global leadership, with robust surveillance and a flexible response capacity ready to face unexpected rather than predicted threats. Lessons from Ebola will help, but they are not the whole answer. The international community has to stop planning for last year’s problems and start solving this year’s.

A version of this piece first appeared as an expert comment on the Chatham House website.