Background: We report 2-year persistence of immune response to Takeda's prophylactic purified formalin-inactivated whole Zika virus vaccine candidate (TAK-426) compared with that observed after natural infection. Methods: A randomized, observer-blind, placebo-controlled, dose-selection, phase 1 trial was conducted in 18–49-year-old adults at 9 centers (7 in the United States, 2 in Puerto Rico) from 13 November 2017 to 24 November 2020. Primary objectives were safety, tolerability, and immunogenicity of 3 increasing doses of TAK-426 administered as 2 doses 28 days apart to flavivirus (FV)–naive and FV-primed adults. Here, we report on safety and persistence of immunity up to 2 years after primary vaccination with 10-μg TAK-426, the highest dose, and compare neutralizing antibody responses with those observed after natural infection. Results: TAK-426 at 10-μg had an acceptable safety profile in FV-naive and FV-primed adults up to 24 months after dose 2. Seropositivity for neutralizing antibodies was 100% at 1 year, and 93.8% and 76.2% at 2 years in FV-naive and FV-primed groups, respectively. TAK-426 responses were comparable in magnitude and kinetics with those elicited by natural Zika virus infection. Conclusions: These results support the further clinical development of TAK-426 for both FV-naive and FV-primed populations. Clinical Trials Registration: NCT03343626

A traditional phase 3 clinical efficacy study for a Zika vaccine may be unfeasible because of the current low transmission of Zika virus (ZIKV). An alternative clinical development approach to evaluate Zika vaccine efficacy (VE) is therefore required, delineated in the US FDA’s Accelerated Approval Program for licensure, which utilizes an anti-Zika neutralizing antibody (Zika NAb) titer correlated with non-human primate (NHP) protection as a surrogate endpoint. In this accelerated approval approach, the estimation of VE would be inferred from the percentage of phase 3 trial participants achieving the established surrogate endpoint. We provide a statistical framework to predict the probability of protection for human participants vaccinated with a purified inactivated ZIKV vaccine (TAK-426), in the absence of VE measurements, using NHP data under a single-correlate model. Based on a logistic regression (LR) with bias-reduction model, a probability of 90% protection in humans is expected with a ZIKV NAb geometric mean titer (GMT) ≥ 3.38 log₁₀ half-maximal effective concentration (EC₅₀). The predicted probability of protection of TAK-426 against ZIKV infection was determined using the two-parameter LR model that fit the calculated VE in rhesus macaques and the flavivirus-naïve phase 1 trial participants’ ZIKV NAb GMTs log₁₀ EC₅₀, measured by a ZIKV reporter virus particle assay, at 1 month post dose 2. The TAK-426 10 µg dose predicted a probability of protection from infection of 98% among flavivirus-naïve phase 1 trial participants.

We previously reported the first-in-human assessment of three doses (2, 5, and 10 µg) of purified inactivated Zika virus vaccine (PIZV or TAK-426) in the Phase 1 ZIK-101 study (NCT03343626). Here, we report dose selection based on extended safety and immunogenicity data (6 months post-vaccination) and discuss considerations (e.g., immunological, historic, flavivirus immunological cross-reactions) for selecting a Zika virus (ZIKV) vaccine dose formulation. TAK-426 dose selection was conducted at the first interim analysis, and was based on cumulative safety data from both flavivirus-naïve (up to ≥28 days post-dose PD2) and flavivirus-primed participants (up to ≥28 days PD1), and on immunogenicity data from flavivirus-naïve participants only (at 28 days PD1 and 28 days PD2). The safety profile from TAK-426 recipients was compared to placebo recipients. Immunogenicity was assessed by geometric mean titer ratios of neutralizing anti-ZIKV antibodies and differences in seroconversion rates. There was no significant difference in safety between the three TAK-426 doses. The 10 μg dose provided the earliest and strongest immune response (with close to 100% seroconversion and higher antibody titers PD1 in flavivirus-naïve participants), and was well tolerated with acceptable safety profiles in both flavivirus-naïve and flavivirus-primed participants; this dose was selected for further development.

Background: Zika virus (ZIKV) emergence in 2015–2016 was characterized by high attack rates and a wave of Congenital Zika Syndrome cases that affected several countries in the Americas. The sudden drop in virus transmission in the following years and the lack of a reliable correlate of protection have hampered the development of vaccines. ZIKV neutralizing antibodies (nAbs) responses to natural ZIKV infection provide insights into the potential efficacy of vaccine candidates. Methods: In this study, we compared anti-ZIKV nAb responses generated by a ZIKV vaccine (TAK-426), to those elicited by natural ZIKV infection in participants from diverse geographic areas using the same neutralizing antibody assay. Results: Those with a ZIKV infection (inapparent or symptomatic) exhibited higher levels of ZIKV nAbs, than TAK-426 vaccine recipients at all time points. The differences were less pronounced 1 month after TAK-426 dose 2. ZIKV nAb titers in vaccinated recipients were above the calculated threshold of protection at 1 month post-dose 2 for flavivirus (FV)-naive participants and at 1 and 6 months post-dose 2 for FV-primed participants. The kinetics of ZIKV nAbs were similar for both the natural infection and vaccination groups, exhibiting a peak, decline, and stabilization pattern; however, vaccine non-inferiority was not demonstrated. Conclusions: Our findings suggest that nAbs evoked by the current phase 1 formulation and dosage of TAK-426 may not protect against a ZIKV infection in endemic countries and that a booster dose should be further evaluated.