Assessing the population at risk of Zika virus in Asia – is the emergency really over?

Abstract

On November 18, 2016, the WHO ended its designation of the Zika virus (ZIKV) epidemic as a Public Health Emergency of International Concern (PHEIC). At the same time, ZIKV transmission continues in Asia, with the number of Asian countries reporting Zika cases increasing over the last 2 years. Applying a method that combines epidemiological theory with data on epidemic size and drivers of transmission, we characterised the population at risk of ZIKV infection from Aedes aegypti mosquitoes in 15 countries in Asia. Projections made under the assumption of no pre-existing immunity suggest that up to 785 (range: 730–992) million people in Asia would be at risk of ZIKV infection under that scenario. Assuming that 20% of ZIKV infections are symptomatic, this implies an upper limit of 146–198 million for the population at risk of a clinical episode of Zika. Due to limited information about pre-existing immunity to ZIKV in the region, we were unable to make specific numerical projections under a more realistic assumption about pre-existing immunity. Even so, combining numerical projections under an assumption of no pre-existing immunity together with theoretical insights about the extent to which pre-existing immunity may lower epidemic size, our results suggest that the population at risk of ZIKV infection in Asia could be even larger than in the Americas. As a result, we conclude that the WHO’s removal of the PHEIC designation should not be interpreted as an indication that the threat posed by ZIKV has subsided.

Serologic surveillance of maternal Zika infection in a prospective cohort in Leon, Nicaragua during the peak of the Zika epidemic

“…[2] Incident ZIKV infections have markedly declined, but there is concern that ZIKV will continue to circulate and spread to susceptible populations. [3,4] Unlike other flaviviruses, ZIKV can be transmitted from pregnant mother to fetus, causing fetal loss, growth restriction, and congenital Zika syndrome (CZS), which consists of a constellation of findings including microcephaly, fetal brain and ocular anomalies, and contractures [5][6][7]. The ZIKV epidemic was declared an international emergency, largely due to the threat to the developing fetus.…”

Global expansion and redistribution of Aedes-borne virus transmission risk with climate change

“…Aedes-borne virus expansion into regions that lack previous exposure is particularly concerning, given the potential for explosive outbreaks when arboviruses are first introduced into naïve populations, like chikungunya and Zika in the Americas [57]. The emergence of a Zika pandemic in the Old World [58], the establishment of chikungunya in Europe beyond small outbreaks [29], or introduction of dengue anywhere a particular serotype has not recently been found, is a critical concern for global health preparedness. However, because effects of climate on vector-borne disease transmission are nonlinear [6,14,35,[59][60][61][62], climate change may increase transmission potential in some settings and decrease it in others, yet the potential for climate-driven shifts and decreases in disease burden is less well-understood (but see Ryan et al [13]).…”

“…Climate change is likely to change the relationship between transmission risk and disease burden at fine scales within those zones of transmission nonlinearly, such that areas with shorter seasons of transmission could still experience increased overall disease burdens, or vice versa. Combining broad spatial models with finer-scale models of attack rates or outbreak size is a critical step towards bridging scales [58,79], but more broadly, building consensus and revealing similarities and differences between all available models via transparency, is of paramount importance [80].…”

Downgrading disease transmission risk estimates using terminal importations

“…South America (26)(27)(28). Differentiating regions that can sustain a ZIKV epidemic (R 0 >1) from 336 those that cannot is vital to effective planning and resource allocation for future preparedness 337 plans.…”