The Zika virus epidemic in Latin America is likely to burn itself out within three years, new research suggests. The findings, from scientists at Imperial College London, also indicate that the current epidemic cannot be contained with existing control measures.
The investigators, who published their findings in Science, predict that the next large-scale epidemic is unlikely to emerge for at least another 10 years, although there is a possibility of smaller outbreaks during this period.
Lead author Professor Neil Ferguson explained: “This study uses all available data to provide an understanding of how the disease will unfold and allows us to gauge the threat in the imminent future. Our analysis suggests that Zika spread is not containable, but that the epidemic will burn itself out within two to three years.”
Ferguson and his colleagues collated all existing data for Zika transmission across Latin America. The team then used this information, along with data on similar viruses, such as dengue, to build a mathematical model to represent the current epidemic and future waves of transmission. Using this model, the team calculated that the current epidemic would end within two to three years because people are unlikely to be infected with Zika virus twice.
“The current explosive epidemic will burn itself out due to a phenomenon called herd immunity,” Ferguson said. “Because the virus is unable to infect the same person twice––thanks to the immune system generating antibodies to kill it––the epidemic reaches a stage where there are too few people left to infect for transmission to be sustained.
“Using our model, we predict large-scale transmission will not restart for at least another 10 years, until there is a new generation in the population who have not been exposed to the Zika virus. This mirrors other epidemics, such as chikungunya––a similar virus to Zika––where we have seen explosive epidemics followed by long periods with few new cases.”
The Zika virus is carried by the Aedes aegypti mosquito, but the team cautioned that any large-scale government programs to target the mosquitoes may have a limited effect.
“The virus is very similar to the dengue virus, and transmitted by the same mosquito,” Ferguson said. “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 that any efforts to slow the 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.”
The team’s findings raise issues for vaccine development, Ferguson said. “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.”
Ferguson emphasized that there are still many questions to answer about the Zika virus––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,” he remarked. “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 Niño 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 that prior dengue exposure may amplify Zika infection in a person.
“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,” he said.