Zika epidemic likely to burn itself out within three years

The Zika virus is carried by the Aedes aegypti mosquito, but the team cautioned any large-scale government programs to target the mosquitoes may have limited impact. “The virus is very similar to the dengue virus, and transmitted by the same mosquito. But previous experience with dengue has shown controlling spread to be incredibly difficult. Also, efforts to contain the epidemic would have needed to have been implemented much earlier in the current Zika epidemic to have a major effect - but by the time we realized the scale of the problem it was too late. ”

He added any efforts to slow spread of the virus may in fact prolong the current epidemic: “Slowing transmission between people means the population will take longer to reach the level of herd immunity needed for transmission to stop. It might also mean that the window between epidemics - which we predict may be over a decade - could actually get shorter.”

And while the potential end of the epidemic is no doubt positive, it does raise issues for vaccine development, adds Professor Ferguson: “If our projections are correct, cases will have dropped substantially by the end of next year, if not sooner. This means by the time we have vaccines ready to be tested, there may not be enough cases of Zika in the community to test if the vaccine works.”

He suggests one option may be to recruit ‘sleeper sites’ for vaccine trials across the globe. These centers would obtain, in advance, the lengthy legal and ethical approval needed for a trial. Then if there is a Zika outbreak in its area, a center would be ready to begin a vaccine trial straight away.

However, Ferguson highlighted there are still many questions to answer about Zika — and therefore many caveats to making predictions: “In a worst case scenario Zika would become endemic in Latin America in the long-term, which would mean smaller, frequent outbreaks. A key issue is we don’t understand why the Zika virus affected Latin America in such an explosive way. One possibility is climate may have in some way aided spread of the virus, as spread coincided with an El Nino event. Genetic mutation of the virus might also have played a role, although early data currently give limited support for this hypothesis.”

Ferguson added that previous exposure to dengue might also have played an important role in the current Zika epidemic. Some research, including recent studies from an Imperial team, has suggested prior dengue exposure may amplify Zika infection in a person.

“This is an effect called Antibody Dependent Enhancement and is of significant concern. It is too early to say whether dengue exposure affects the risk of getting Zika or the clinical consequences of infection, but this needs to be urgently examined in future research. We also need to understand why South East Asia, which also has high rates of dengue, has not experienced a similar Zika outbreak.”

He added more research is urgently needed. “One research priority is to fully understand the extent of Zika transmission, and what proportion of people in Latin America - and across the globe have been infected. To do this we need to assess past exposure to Zika by testing blood from representative samples of at-risk populations for the presence of antibodies to the virus. We and other groups are working on such studies at the moment.”

“There are currently more questions surrounding Zika than answers - and only through a coordinated global research effort will we find the answers we desperately need.”

— Read more in Neil Ferguson, “Countering Zika in Latin America,” Science (14 July 2016) (doi: 10.1126/science.aag0219)