Zika Virus Overview and Impact

Zika Virus (ZIKV) and its Impact

• ZIKV is a member of the Flaviviridae family, first identified in Uganda in 1954.
• Prior to 2015, ZIKV was associated with mild, flu-like symptoms.
• In late 2015, a ZIKV epidemic in Brazil led to a surge in cases of fetal microcephaly, prompting global efforts to develop a vaccine.
• ZIKV is primarily spread through Aedes mosquitos, but can also be transmitted transplacentally, through blood transfusions, and sexually.
• Genetic polymorphisms have been linked to the development of congenital Zika syndrome (CZS).
• The World Health Organization (WHO) declared the end of the ZIKV epidemic in late 2016, but vaccine development remains a priority due to the potential for resurgence.
• ZIKV is a concern for individuals traveling to or living in endemic areas, particularly pregnant women.
• While ZIKV transmission is currently at low levels, vaccine development is challenging due to the difficulty in establishing clinical trial sites and enrolling participants.

Vaccine Development and Research

• Vaccination prevents ZIKV distribution to fetal tissues, including the brain.
• ZIKV-associated neuropathology was absent in offspring of Ad26.M.Env vaccinated dams, while pathology was limited in fetuses from unvaccinated, challenged dams.
• Vaccine efficacy is linked to the induction of ZIKV neutralizing antibodies in pregnant rhesus macaques.
• Clinical trials for ZIKV vaccines are underway, with several candidates completing Phase I and II safety studies.
• The rhesus macaque model has proven useful in studying the dynamics of viral replication and shedding during ZIKV infection.
• Studies in mice and non-human primates have demonstrated that inducing neutralizing antibodies through various vaccine platforms effectively prevents ZIKV acquisition.

Future Directions

• Ideally, a ZIKV vaccine could be deployed quickly in the event of a resurgence and administered safely to pregnant women.
• Continued research and development are crucial for creating an effective and readily available ZIKV vaccine to protect vulnerable populations from the devastating effects of CZS.

Study Background

• The study investigated the effectiveness of an investigational Ad26.M.Env vaccine against Zika virus (ZIKV) in pregnant macaques.
• The vaccine used a modified adenovirus type 26 (Ad26) vector expressing the ZIKV envelope (Env) protein as a potential ZIKV immunogen.
• The study compared the vaccine's effect on vaccinated pregnant macaques versus unvaccinated pregnant macaques challenged with ZIKV.

Vaccine Efficacy

• Following ZIKV challenge, both vaccinated and unvaccinated macaques exhibited peak viral RNA (vRNA) levels on day 7 post-challenge.
• Importantly, all vaccinated macaques remained negative for detectable vRNA in body fluids (colorectal, vaginal, saliva) throughout the study.
• In contrast, unvaccinated macaques showed detectable viral loads for an average of 42 days.

Immune Responses

• The Ad26.M.Env vaccine induced low-level cellular immune responses, primarily against the Env protein, in vaccinated pregnant macaques.
• The study measured interferon-gamma (IFNγ) production by peripheral blood mononuclear cells (PBMCs) as an indicator of cellular immunity.
• Although responses were generally lower than expected, they indicated some degree of immune priming by the vaccine.

Fetal Development

• The researchers closely monitored fetal development using ultrasound throughout the pregnancies.
• No significant differences were observed in fetal biometric parameters or brain development between vaccinated and unvaccinated macaques.
• This suggests that the vaccine did not adversely affect fetal growth or neurological development in pregnant macaques.

Protection from ZIKV Infection

• At term, all fetuses born to vaccinated dams were negative for ZIKV vRNA, indicating complete protection from ZIKV infection.
• Histopathological evaluation of fetal tissues revealed no evidence of viral replication in any of the vaccinated fetuses.
• In contrast, two out of five unvaccinated dams gave birth to fetuses with detectable ZIKV vRNA in their tissues, suggesting transmission from the infected mothers.

Summary

• These results demonstrate that the Ad26.M.Env vaccine effectively protected pregnant macaques from ZIKV infection and prevented vertical transmission to their offspring.
• While the vaccine induced lower than expected immune responses, it proved sufficient to confer complete protection against ZIKV infection in this animal model.
• Further research is warranted to optimize the vaccine and explore its potential application in human pregnancies.

Adenovirus-based Vaccines for Zika Virus

• An Ad26.ZEBOV Ebola vaccine is licensed and recommended for childhood use in Ebola endemic regions.
• The Ad26.M.Env (Ad26.ZIKV.001) vaccine for Zika Virus has been tested in a Phase I clinical trial.
• All tested vaccine regimens were well-tolerated, with no safety concerns identified.
• Vaccination with Ad26.M.Env induced robust Zika Virus neutralizing titers in humans similar to those achieved in macaques.
• Immune sera from vaccinated humans transferred to mice could protect mice against Zika Virus.
• Adenoviral-based vaccines combatting COVID-19 were associated with immune-mediated thrombocytopenia and thrombosis.
• The mechanisms for these adverse effects remain unclear.
• Further research is required to determine if these adverse effects are linked to the interaction between the Adenovirus-based vaccine and the COVID-19 spike protein, or the Adenovirus vaccine platform itself.
• Adenoviral constructs are still widely studied and will likely remain an important platform for global use.

Zika Virus Pandemic Risk

• Previous Zika Virus outbreaks in Malaysia have shown a cyclical re-emergence due to waning immunity over time.
• The United States faces a risk of future Zika Virus pandemics due to the presence of the Aedes aegypti mosquito vector in the Southern United States.
• The lack of exposure to Zika Virus during the 2016 epidemic makes the US population vulnerable to future outbreaks.
• A well-tolerated and safe vaccine is needed for pre- and perinatal use to protect susceptible individuals.

mRNA-based Zika Virus Vaccines

• mRNA-based vaccines against Zika Virus have demonstrated immunogenicity and protective effects in non-human primate challenge studies.
• These vaccines have shown variable efficacy in inducing neutralizing antibody responses in humans.

Ad26.M.Env Vaccine and Protection Against Zika Virus in Pregnant Macaques

• A study investigated the safety and efficacy of an Ad26.M.Env vaccine against Zika virus (ZIKV) in pregnant rhesus macaques.
• Pregnant macaques were divided into two groups: vaccinated with Ad26.M.Env and non-vaccinated control.
• Both groups were challenged with ZIKV at 6 weeks post-conception (~10-30 weeks post-vaccination).
• Key finding: Vaccinated dams remained completely protected from ZIKV infection in blood and tissues.
• Key finding: All fetuses born to vaccinated dams were also negative for ZIKV, and had no ZIKV-associated histopathological abnormalities.
• Key finding: Non-vaccinated dams and their fetuses exhibited ZIKV infection.
• The study suggests that the Ad26.M.Env vaccine is safe and efficacious in protecting pregnant macaques and their fetuses from ZIKV infection.
• The Ad26.M.Env vaccine elicited strong cellular immune responses in vaccinated animals.
• The vaccine also induced robust ZIKV neutralizing antibody titers, comparable to human volunteers in a Phase 1 clinical trial.

Neutralization Assays

• The study measured ZIKV-specific neutralizing antibodies using two methods: fold reduction neutralization (FRNT) and micro-neutralization (MN) assays.
• FRNT assay is based on the principle of measuring the reduction in virus infectivity after exposure to serum containing neutralizing antibodies
• MN assay involves serially diluting serum samples in microplates, adding ZIKV, and then observing the effect of the serum on viral replication in Vero cells.
• Both FRNT and MN assays confirmed the presence of neutralizing antibodies in vaccinated animals.

Zika Virus Challenge

• ZIKV challenge stock was prepared from the 2015 Brazilian epidemic strain.
• Pregnant macaques were infected with 1x103 PFU (1×106 vp) of ZIKV via subcutaneous route at 6 weeks post-conception.

Clinical Monitoring

• All animals were monitored for clinical signs, body weight, and complete blood cell count (CBC) throughout the study period.
• Ultrasonography was performed bi-weekly to assess pregnancy status and fetal development.
• No serious adverse events were reported in relation to vaccination or ZIKV challenge.

Fetal Evaluation

• Dam were euthanized at term and fetuses underwent C-section.
• Fetal and placental tissues were collected for histopathology, viral RNA analysis, and gross examination.