Cost-effectiveness of Prophylactic Zika Virus Vaccine in the Americas

Zika virus remains a major public health concern because of its association with microcephaly and other neurologic disorders in newborns. A prophylactic vaccine has the potential to reduce disease incidence and eliminate birth defects resulting from prenatal Zika virus infection in future outbreaks. We evaluated the cost-effectiveness of a Zika vaccine candidate, assuming a protection efficacy of 60%–90%, for 18 countries in the Americas affected by the 2015–2017 Zika virus outbreaks. Encapsulating the demographics of these countries in an agent-based model, our results show that vaccinating women of reproductive age would be very cost-effective for sufficiently low (<$16) vaccination costs per recipient, depending on the country-specific Zika attack rate. In all countries studied, the median reduction of microcephaly was >75% with vaccination. These findings indicate that targeted vaccination of women of reproductive age is a noteworthy preventive measure for mitigating the effects of Zika virus infection in future outbreaks.

1 bite per day (3). Sexual transmission of Zika virus was included in the model for persons >15 years of age and in a monogamous context. The frequency of sexual encounters for partnered persons was sampled from age-dependent distributions (Appendix Tables 2 and 3). For an individual in the age group , the partner was selected from the age group ± 5 years of age.
Upon successful Zika virus transmission, susceptible persons entered an intrinsic incubation period (IIP), sampled for each person from the associated distribution (6,7). After the IIP elapsed, a fraction (sampled between 40% to 80%) of infected persons entered asymptomatic infection without developing clinical symptoms (8,9). In our previous studies (3,10), Zika virus transmission from asymptomatic infection was modeled by a relative transmissibility factor compared with symptomatic infection, which ranged from 0.1 to 0.9. Here we assumed the same transmissibility for both asymptomatic and symptomatic infection, with any transmission reduction in asymptomatic infection accounted for in the calibration process.
Persons who recovered from either asymptomatic or symptomatic infection were assumed to be immune to reinfection for the remainder of the simulation time. The total number of pregnant women was calculated based on the country-specific fertility rate of population in each simulation (Appendix Table 4). Ignoring fatal complications, the number of pregnant women at any point in time for each simulation was calculated by the following (14):

Additional Scenarios
Future Zika virus outbreaks may occur with different attack rates from those estimated for the 2015-2017 outbreaks. Therefore, we conducted cost-effectiveness analysis for 2 additional scenarios. In the first scenario, we calibrated the model to an increase of 4% in the estimated attack rate for each country. In the second scenario, the model was calibrated to a 4% decrease in the estimated attack rates, with a lower bound of 1%, for each country. The levels of preexisting herd immunity at the onset of simulations remained the same as those in the Table in the main article.
In the scenario with increased attack rates, the results show that the vaccine is very costeffective (using per-capita GDP as the threshold) for a VCPI up to $20 in Nicaragua and up to $50 in French Guiana (Appendix Figure 6). The upper VCPI for other countries ranged between these values. Similarly, using 3 times the per capita GDP as the threshold, the vaccine is still cost-effective for a VCPI up to $26 in Nicaragua and up to $98 in French Guiana (Appendix Figure 6). In the scenario with decreased attack rates, the vaccine is (very) cost-effective for a VCPI up to ($4) $9 in Mexico and up to ($41 We also calculated the percentage reduction of microcephaly during pregnancy for both scenarios of increased and decreased attack rates. We found that the median percentage reduction in both scenarios was >75% in all countries (Appendix Figure 8).  $3,516 $144-$4,575 $34 $12,092 $7,379-$15,050 Bolivia $27 $1,669 $36 $7,038 $4,745 Brazil $21 $6,356 $1,223 $38 $21,725 $14,441 Colombia $23 $4,184 $1,349 $35 $14,086 $9,736 Costa Rica $16 $7,352 $1,234 $29 $29,061 $15,