More than 1.6 (1.45-2.06) million childbearing women and 93.4 (81.6-117.1) million people in total in the Americas could become infected during the current wave of the Zika virus epidemic, according to a model-based projection published online in Nature Microbiology this week. The study suggests that, based on current rates of adverse fetal outcomes in infected women, tens of thousands of pregnancies could be affected.
Zika virus is a mosquito-borne pathogen that has been associated with a range of fetal developmental conditions, including congenital microcephaly, during its rapid spread across the Americas. As of 30 June there have been 1,674 confirmed cases of microcephaly associated with Zika virus infection in five countries. At present there is no vaccine available for preventing Zika virus infection.
Alex Perkins and colleagues formulated an improved method for estimating the portion of the population in the Americas that is at risk of Zika virus infection. The method is an improvement of previous estimates because it takes into account ecological theories such as herd immunity - when immunity to Zika infection in the community surpasses a critical mass and confers indirect immunity on the rest of the population - and basic reproduction number (the estimated number of new people that one infected person can infect in a completely susceptible population). They combined these theories with actual levels of Zika infection in the population - as measured by blood serum (seroprevalence data) - and highly spatially resolved data about drivers of transmission to make location-specific projections of Zika virus attack rates. As Zika-specific transmission parameters are still largely unknown, the authors used information from epidemics of dengue virus and Chikungunya virus to produce a more realistic projection. The authors note that their projections should be interpreted as either a plausible worse-case scenario or an expectation of local epidemic size, conditional on there being a local epidemic in the first place.