Passive immunisation of convalescent human anti-Zika plasma protects against challenge with New World Zika virus in cynomolgus macaques

Zika virus (ZIKV) causes neurological complications in susceptible individuals, highlighted in the recent South American epidemic. Natural ZIKV infection elicits host responses capable of preventing subsequent re-infection, raising expectations for effective vaccination. Defining protective immune correlates will inform viral intervention strategies, particularly vaccine development. Non-human primate (NHP) species are susceptible to ZIKV and represent models for vaccine development. The protective efficacy of a human anti-ZIKV convalescent plasma pool (16/320-14) developed as a candidate reference material for a WHO International Standard was evaluated in macaques. Convalescent plasma administered to four cynomolgus macaques (Macaca fascicularis) intra-peritoneally 24 hrs prior to sub-cutaneous challenge with 10³ pfu ZIKV_PRVABC59 protected against detectable infection, with absence of detectable ZIKV RNA in blood and lymphoid tissues. Passively immunised anti-ZIKV immunoglobulin administered prior to time of challenge remained present only at very low levels 42 days post-challenge. Absence of de novo antibody responses in passively immunised macaques indicate sterilising immunity compared with naïve challenge controls that exhibited active ZIKV-specific IgM and IgG responses post-challenge. Demonstration that the presence of convalescent anti-ZIKV at levels of 400 IU/mL neutralising antibody protects against virus challenge provides a scientific framework for development of anti-ZIKV vaccines and facilitates regulatory approval.