A new study in Nature Microbiology suggests that 1.65 million childbearing women in Central and South America are potentially at risk from being infected with Zika during the current epidemic. Based on the current observable rates of problematic pregnancies linked to the virus, this model-based study is also the first to give a comprehensive estimate of how many future pregnancies could experience adverse neurological disorders.

“Other researchers are still trying to get a handle on this, but it currently looks like this could be somewhere between 1 and 13 percent [of pregnancies],” Alex Perkins, coordinating researcher of the study and the Eck Family assistant professor of epidemiology at the University of Notre Dame, told IFLScience. Pregnancies in the first trimester are thought to be the most vulnerable in this regard.

This team’s model is one of the most advanced to date. Among other factors, it takes into account how similar diseases spread by the same vector (such as dengue and Chikungunya), the immunological resistance (or lack thereof) of certain populations to Zika, the ability of the virus to spread within communities, and the current levels of Zika infections in the region.

In total, over 94 million people of all kinds are at risk of being infected with Zika during the current epidemic. It’s important to note, though, that these are upper limits – a worst case scenario projection.

In any case, Zika will continue its microbial march across the world, and health officials remain particularly concerned about how it will spread in the coming months. With the Rio Olympics approaching, there’s a chance that infection rates could spike when hundreds of thousands of visitors to Brazil put themselves at risk by traveling to the dark heart of the epidemic.

Although anyone can contract Zika via its primary vector – the Aedes aegypti mosquito – those most at risk are expectant mothers, whose fetuses can experience severe neuronal damage, which can take the form of microcephaly or something even worse.

^{Fumigation deters mosquitos, and is constantly being used in communities all over South America. Will Rodrigues/Shutterstock}

As of June 30, there have been 1,674 cases of Zika-related microcephaly as it has spread across South and Central America. Although there is no current vaccine for the disease, experimental ones have just begun early-stage clinical trials. However, the epidemic will probably end before these vaccines are available for public use due to a concept known as herd immunity.

This refers to a feature of any such viral or bacterial epidemic, wherein enough people will develop a natural resistance to the disease after being previously infected. Without a “fresh” population to infect, the number of new infections drops down to sub-epidemic levels, and one study suggests this will cause the current epidemic to cease within the next three years, with no resurgence for at least a decade after that.

“It is interesting and important to note that both our paper and theirs rely heavily on the concept of herd immunity,” Perkins added. “The result of our work is a projection of how strong or how weak herd immunity is likely to be in different areas. Overall, I would say that these two studies… are actually quite complementary.”

This means that although herd immunity will likely be the method by which Zika meets its perhaps temporary end, it will not come into effect quick enough to prevent up to as many as 1.65 million childbearing women from contracting the disease – once again highlighting how the next few years pose a huge risk to expectant mothers. 

^{Mosquitos are the primary vector for the virus. tavizta/Shutterstock}