In Yap, Zika had been a curiosity rather than a major threat. Despite lots of people getting a fever or rash, nobody ended up in hospital. That changed when the virus arrived on the much larger islands of French Polynesia in late 2013. During the resulting outbreak, forty-two people with Guillain-Barré Syndrome arrived at the main city hospital in Papeete, on the northern coast of Tahiti. The gbs cases cropped up slightly later than the main Zika outbreak, which is what we’d expect for a syndrome that takes a couple of weeks to appear after an infection. Speculation about a possible link was confirmed when local scientist Van-Mai Cao-Lormeau and her colleagues discovered that almost all the gbs cases had recently been infected with Zika.[42]
As in Yap, the French Polynesia outbreak had been huge, with the majority of the population infected. And like Yap, the outbreak had been very brief, with the bulk of cases appearing over a few weeks. Given that our team had spent 2014–15 developing mathematical models to analyse dengue in the Pacific, we decided to turn our attention to Zika as well. Unlike the plain-coloured Anophelines that can fly miles to spread malaria, dengue and Zika are both spread by Aedes mosquitoes, best known for being stripey and lazy (‘aedes’ means ‘house’ in Latin). As a result, the infection generally spreads when humans move from one place to another.[43]
When we tried to get our model simulations to reproduce the dynamics of Zika in French Polynesia, we realised there must have been a large, dengue-like rate of spread to generate such an explosive outbreak.[44] The short span of the outbreak stood out even more when we considered the delays involved in the infection process. During each cycle of transmission, the virus has to get from a human into a mosquito then back into another human.
While analysing transmission rates in French Polynesia, we also estimated how many people were already infected when the first cases were reported in October 2013. Our model suggested there had been several hundred infections by this point, meaning the virus probably arrived in the country weeks if not months earlier. This result would link into another mystery: how did the Zika virus reach Latin America? After the first cases were reported in Brazil during May 2015, there was a lot of speculation about when the infection had been introduced to the continent, and by whom. One early hypothesis pointed to the FIFA World Cup, held in Brazil during June/July 2014, which had attracted over three million football fans from around the globe. Another candidate was the Va’a sprint canoe championship, held in Rio de Janeiro during August 2014. Unlike the World Cup, this smaller event had included a team from French Polynesia. So which explanation was most plausible?
According to evolutionary biologist Nuno Faria and his colleagues, neither theory was particularly good.[45] Based on the genetic diversity of Zika viruses circulating in Latin America by 2016, they reckoned that the infection was introduced much earlier than previously thought. The virus probably hit the continent in mid-to-late 2013. Although too early for the canoe championship or World Cup, the time range coincided with the Confederations Cup, a regional football tournament held in June 2013. What’s more, French Polynesia was one of the countries competing.
There was just one gap in the theory: the Confederations Cup occurred five months before the first Zika cases were reported in French Polynesia. But if the outbreak in French Polynesia had in reality started earlier than October 2013 – as our analysis suggested – it was just about plausible that it could have spread to Latin America during that summer. (Of course, we should be cautious about trying too hard to find a sport-shaped prologue for the Zika story: there’s always a chance that it was just a random person in the Pacific taking a random flight to Brazil sometime in 2013.)
As well as analysing past outbreaks, we can use mathematical models to look at what might happen in future. This can be particularly useful for health agencies faced with difficult decisions during an outbreak. One such difficulty came in December 2015, when Zika reached the Caribbean island of Martinique. A big concern was the island’s ability to handle gbs cases: if patients’ lungs failed, they would need to be put on ventilators. At the time, Martinique only had eight ventilators for a population of 380,000. Would it be enough?
To find out, researchers at Institut Pasteur in Paris developed a model of Zika transmission on the island.[46] The crucial thing they wanted to know was the overall shape of the outbreak. gbs cases who required a ventilator typically stayed on it for several weeks, so a short outbreak with a large peak could overwhelm the health system, while a longer, flatter outbreak would not. At the very start of the Martinique outbreak, there hadn’t been many cases, so the team used data from French Polynesia as a starting point. Of the forty-two gbs cases reported there in 2013/14, twelve had required ventilators. According to the Pasteur model, this meant they could have a big problem. If the outbreak in Martinique followed the same pattern as French Polynesia, the island would probably need nine ventilators, one more than they had available.
Fortunately, the Martinique outbreak wouldn’t be the same. As new data came in, it became clear that the virus wasn’t spreading as quickly as it had in French Polynesia. At the peak of the outbreak, the researchers expected there would be around three gbs cases needing ventilators. Even in the worst-case scenario, they estimated that seven ventilators would be enough. Their conclusion about this upper limit turned out to
