Zika epidemic has peaked and may run its course within 18 months, say experts

The Zika virus epidemic in Latin America is likely to run its course within the next 18 months, according to a study by researchers in the UK and US.

Zika is currently circulating in more than 35 countries in the Americas and is primarily spread by mosquitoes. It is believed to cause the birth defect microcephaly, that results in babies being born with and unusually small head and possible damage to the brain, as well as the autoimmune disorder Guillain-Barré syndrome that can result in temporary paralysis.

But scientists say that the epidemic might have reached its peak.

Writing in the journal Science, researchers from Imperial College, London and the Johns Hopkins Bloomberg School of Public Health in Baltimore, describe how they have created a model of the disease to explore how the epidemic will play out. The model is based on surveillance data currently available for the Zika epidemic in Latin America, together with information on mosquito populations, plus data regarding similar, mosquito-borne viruses such as dengue.

The researchers say the findings suggest the current epidemic is likely to last for three years in total. “The three years counts for the transmission we have seen so far, and we have seen at least a year of very high levels of transmission,” said Neil Ferguson, lead author of the research from Imperial College, London. “So we are probably coming to roughly about the halfway point.”

After that, says Ferguson, further large-scale epidemics are unlikely to occur for at least decade because a large proportion of the population will be left immune to the virus in the wake of the current epidemic. That, he says, offers protection by “herd immunity” whereby there are too few people susceptible to the Zika virus for the disease to spread.

“The only way that can change is by new people who are not immune entering the population - and the way that has to happen is through births,” said Ferguson. “Basically an entire generation need to be born before you get a high enough fraction of the population now not immune [to Zika] to sustain transmission at epidemic levels.”

That, the researchers explain, means that although smaller outbreaks are possible, there is likely to be a long gap between epidemics.

“It is a good thing because we have time to develop new effective counter measures like vaccines and novel ways to control mosquito populations, but it poses some challenges as well,” said Ferguson. Among them, he says, is that by the time vaccines currently in development are ready to trial, there may not be enough cases of Zika in Latin America to test them against.

The researchers also warn that Zika is unlikely to be contained by current attempts to control mosquito populations. Moreover, they say, such measures could potentially extend the length of the current epidemic and shrink the time until the next one by slowing down, but failing to stop, the spread of the disease.

Eventually, the researchers suggest, the Zika virus could become endemic to Latin America.

“When it finally comes back the next [Zika] epidemic won’t be anywhere near as big as the one Latin America is experiencing now, but that also means there is a greater chance that it won’t burn itself out, which could lead to the virus being able to circulate in the human population year,” said Ferguson. But, he adds, “Whether Zika is capable of doing that we do not know at the current time.”

As well as the positive message that the virus should soon burn out, the authors also point out that local information is valuable if, for example, women are considering delaying pregnancy.

Rosanna Peeling, professor and chair of diagnostics research at the London School of Hygiene and Tropical Medicine, who was not involved in the work, said such models of disease outbreaks can be extremely useful. Not only, she said, can they help scientists understand how a virus spreads through a population it has not previously infected, but they can also be used to explore the impact of various potential public health interventions.

But, she adds, while the new model predicts the current Zika virus epidemic will die down in the coming year or so “it will be still incredibly important that we continue to act now to develop better diagnostic tests, potential vaccines, and other tools that will help us bring the outbreak under control as quickly as possible.”

Ian Clarke, Zika Incident Manager at the World Health Organisation said, “Vector control is currently the most effective way to stop the transmission of Zika virus, chikungunya, dengue and yellow fever, which are also transmitted by Aedes mosquitoes.”

But, he adds, “much is still unknown about the circulation of the virus, how immunity might be developing, and other issues related to when and how the current Zika outbreak might end. The [new research] adds to the scientific community’s understanding of these issues.”