Zika virus (ZIKV) is a flavivirus similar to Dengue virus (DENV) in terms of transmission and clinical manifestations, and usually both viruses are found to co-circulate. ZIKV is usually transmitted by mosquitoes bites, but may also be transmitted by blood transfusion, via the maternal-foetal route, and sexually. After 2015, when the most extensive outbreak of ZIKV had occurred in Brazil and subsequently spread throughout the rest of South America, it became evident that ZIKV infection during the first trimester of pregnancy was associated with microcephaly and other neurological complications in newborns. As a result, the development of a vaccine against ZIKV became an urgent goal. A major issue with DENV vaccines, and therefore likely also with ZIKV vaccines, is the induction of antibodies that fail to neutralize the virus properly and cause antibody-dependent enhancement (ADE) of the infection instead. It has previously been shown that antibodies against the third domain of the envelope protein (EDIII) induces optimally neutralizing antibodies with no evidence for ADE for other viral strains. Therefore, we generated a ZIKV vaccine based on the EDIII domain displayed on the immunologically optimized Cucumber mosaic virus (CuMVtt) derived virus-like particles (VLPs) formulated in dioleoyl phosphatidylserine (DOPS) as adjuvant. The vaccine induced high levels of specific IgG after a single injection. The antibodies were able to neutralise ZIKV without enhancing infection by DENV in vitro. Thus, the here described vaccine based on EDIII displayed on VLPs was able to stimulate production of antibodies specifically neutralizing ZIKV without potentially enhancing disease caused by DENV.

Tick-borne encephalitis (TBE) is a serious neurological disease caused by TBE virus (TBEV). Because antiviral treatment options are not available, vaccination is the key prophylactic measure against TBEV infections. Despite the availability of effective vaccines, cases of vaccination breakthrough infections have been reported. The multienzymatic non-structural protein 3 (NS3) of orthoflaviviruses plays an important role in polyprotein processing and virus replication. In the present study, we evaluated NS3 of TBEV as a potential vaccine target for the induction of protective immunity. To this end, a recombinant modified vaccinia virus Ankara that drives the expression of the TBEV NS3 gene (MVA-NS3) was constructed. MVA-NS3 was used to immunize C57BL/6 mice. It induced NS3-specific immune responses, in particular T cell responses, especially against the helicase domain of NS3. However, MVA-NS3-immunized mice were not protected from subsequent challenge infection with a lethal dose of the TBEV strain Neudoerfl, indicating that in contrast to immunity to prME and NS1, NS3-specific immunity is not an independent correlate of protection against TBEV in this mouse model.