Researchers modeling the rampant spread of the Zika virus say that, like a wildfire consuming a parched landscape, the epidemic that has caused a plague of birth defects in Brazil is already showing signs of slowing and is likely to largely burn itself out in three years.
Peak Zika spread may already have passed, said researchers writing in the journal Science. At a slower rate and with seasonal ebb and flow, the Zika virus’ march across the Americas, they reckon, will likely grind to a halt.
After Latin America’s current Zika epidemic goes to ground, the group’s epidemiological model suggests that another epidemic spread of Zika in the Americas is unlikely for “at least a decade.”
“The current epidemic is not containable; at best, interventions can mitigate its health impacts,” wrote a group of researchers led by Imperial College London epidemiologist Neil M. Ferguson. “More optimistically,” they added, “the natural dynamics of the epidemic are now likely to give a multiyear window to develop new interventions before further large-scale outbreaks occur.”
Discovered in 1947 and introduced into Brazil in 2013, the Zika virus has brought mild illness to the populations of 65 countries, and has become epidemic throughout South and Central America. Since 2014, more than 1,400 babies with grievous birth defects have been born to women in Brazil who were infected with Zika during their pregnancies. And Zika, which has been determined to cause brain abnormalities in gestating fetuses, has been blamed.
Until at least 2026, the researchers said, the virus’ prodigious spread will be its undoing: having infected and thus inoculated most in its path, Zika would be deprived of the kindling that allows a viral outbreak to gain a foothold and march across a human landscape it has trodden before.
With much of Latin and South America having already been exposed to Zika virus and thus immune to further onslaughts, researchers said Thursday that “herd immunity” is likely to take over by 2019. Herd immunity takes hold when a dwindling slice of the population remains vulnerable to becoming ill and sustaining the transmission of virus to others who have not yet been exposed.
Under those circumstances, small, localized outbreaks of Zika will occur, the group acknowledged. But the viral epidemic’s opening volley in the Americas will have ended.
The conditions for an epidemic’s resurgence do not return until a new generation of never-infected children repopulates the landscape with potential victims, and elderly people’s immunity to Zika begins to wane.
In Zika’s case, that process will take a decade or more, the authors of the new report project.
Future epidemics of Zika infection, the group said, will likely affect a younger and younger population, as the probability rises that most older people in the virus’ path have been exposed before and developed immunity. Even so, they wrote, future outbreaks will continue to take hold in areas where women of childbearing age have not yet been exposed to the virus. As a result, “substantial risk to pregnant women in future Zika epidemics” is highly likely.
Those projections came in two articles published Thursday in the journal Science. To make them, a respected team of epidemiologists collected data on the rate at which the Zika virus, which incubates in the bellies of biting mosquitoes, spread through Brazil and marched northward through Latin America.
They measured Zika’s rate of spread as it waxed and waned with the seasons and different mosquito populations. And they gauged the deceleration of its spread as large swaths of the populations in Zika’s path were infected and gained immunity against the bug.
Finally, in a bid to sketch the likely future of the epidemic, they projected those observed patterns onto a map of the areas into which Zika is now spreading, and allowed those patterns to play themselves out.
The results are subject to many caveats, and to assumptions that could be unique to the circumstances from which the model’s data were collected. But modeling results like these allows researchers to make informed estimates of epidemic patterns, and often to highlight factors that could accelerate or delay a viral epidemic’s resolution.
Mosquito-control efforts were one such factor flagged by the authors of the latest study.
Evidence from other outbreaks of mosquito-borne illness has suggested that battling Zika by reducing mosquito populations could slow the population’s path to herd immunity, wrote Ferguson and his colleagues.
By protecting more of the population in Zika’s path from becoming uninfected, aggressive mosquito control could also leave a larger share of people uninoculated. That, in turn, could shorten the time window during which there exist too few potential victims for a Zika epidemic to stage a resurgence.
A delay in reaching the threshold for herd immunity, wrote the researchers, could make it particularly hard for women who have delayed pregnancy to remain patient.
The new analyses are not without detractors.
“I think this model is a real overreach of the data we have,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy. Osterholm pointed to the four-year spread across the United States of the West Nile virus — which is in the same family of viruses as Zika. Because only mosquitoes and not birds are implicated in its spread, Zika, he said, likely spreads far less fast. Yet Zika is thought by the researchers to have covered far more ground in much less time than West Nile took to spread.
“I really believe the worst is yet to come with Zika throughout the Americas,” said Osterholm. “It has not peaked.”