Mixed messages about risk of Zika

Two new studies bring mixed messages about the risk of becoming infected by the Zika virus.

A Yale University report downplays the risk of visitors to the Olympic Games, while an English study suggests that that nearly tens of millions of people, including two million childbearing-age women, in Central and South America could become infected by the virus by the end of the first wave of the epidemic.

Researchers at Yale School of Public Health (YSPH) say that in a worst-case scenario, fewer than 40 visitors to Rio de Janeiro will return to their home countries carrying the mosquito-borne virus.

“It’s important to understand the low degree of risk posed by the Olympics in the scheme of many other factors contributing to international Zika virus spread,” lead author Joseph Lewnard said in a media release.

Up to 500,000 athletes, support staff and visitors are expected in Rio for the Games. But, as it is winter in the southern hemisphere, mosquito activity has subsided from its peak earlier this year when a higher than usual number of babies born with microcephaly.

Zika has been associated with the congenital disorder marked by a smaller-than-average head, as well as the autoimmune condition, Guillain-Barre syndrome.

“The possibility that travellers returning from the Olympics may spread Zika has become a polemic issue that has led to athletes dropping out of the event, and without evidence, undue stigmatisation of Brazil,” says Albert Ko who was part of the team that constructed a mathematical model of the potential for infection.

“This study provides data, which together with initial findings from Brazilian scientists, show that these concerns may be largely exaggerated.” 

That model shows between three and 37 individuals were likely to catch the disease. The findings were published online in Annals of Internal Medicine.

The news is not so good for childbearing age women in Central and South America.

Researchers from demographic organisation the WorldPop Project and the universities of Southampton, Oxford and Notre Dame, say that 1.65 million childbearing women in the region could become infected.

In all, more than 90 million infections could result from the initial spread of Zika.

The projections are detailed in a paper published in Nature Microbiology.

That model shows Brazil is expected to have three times the number of infections than anywhere else, due to the size of the country and the conditions there that are favourable to it spreading.

The University of Southampton’s Andrew Tatem said it was hard to predict accurately the number of childbearing women may because many cases show no symptoms.

“This largely invalidates methods based on case data and presents a formidable challenge for scientists trying to understand the likely impact of the disease on populations,” he says.

To get around this, the study models the spread of the disease by examining its likely impact at very local levels, per five square kilometre areas.

This local data was then brought together to model infection rates region-wide.

“These projections are an important early contribution to global efforts to understand the scale of the Zika epidemic, and provide information about its possible magnitude to help allow for better planning for surveillance and outbreak response, both internationally and locally,” Tatem said in a statement.