Introduction

Humans are threatened by a very small species on this Earth. The predator enters the premises quietly, stalks its prey and bites to cause that ouch moment. The lingering itch is not the problem; it’s the saliva transmitting thousands of virus particles in the human body. These mosquito-borne diseases are causing over 1 million deaths and pain and suffering in over hundreds of millions of people every year [1]. Common illnesses caused by mosquitoes include malaria, dengue, chikungunya, Japanese encephalitis, yellow fever and Zika. Others like West Nile encephalitis, Eastern Equine encephalitis, La Crosse encephalitis and St. Louis encephalitis are less prevalent in Indian subcontinent. Exposure to mosquitoes is practically unavoidable. Preventative medicines cannot be a long-term sustainable treatment. So far, yellow fever is one success story for vaccination against mosquito-borne diseases. The deadly dengue, malaria and chikungunya are yet to create milestones in vaccination history.

Japanese Encephalitis (JE)

JE virus is the most common vaccine-preventable cause of encephalitis in Asia, occurring in most of Asia and parts of the western Pacific. Symptomatic JE is rare, but the case-fatality rate among those with encephalitis is around 30% with fair number having permanent neurologic or psychiatric sequelae [2].

It is primarily a disease of children in endemic countries, where adults have acquired immunity through natural infection. The overall incidence in travelers from non-endemic countries traveling to Asia is <1 case per 1 million travelers. However, expatriates and travelers who have prolonged stay in rural areas with active JE virus transmission are at similar risk as the susceptible resident population (5–50 cases per 100,000 children per year) [3].

Vaccination against JE virus has resulted in reduced mortality and morbidity associated with the disease [4]. WHO recommends that JE vaccination should be integrated into national immunization schedules in all areas where JE disease is recognized as a public health problem. Advisory Committee on Vaccines and Immunization Practices of Indian Academy of Pediatrics (ACVIP of IAP) recommends routine vaccination for all individuals living in endemic areas (180 districts of West Bengal, Bihar, Karnataka, Tamil Nadu, Andhra Pradesh, Assam, Uttar Pradesh, Manipur and Goa) till 18 y. Recommendations for use of vaccine in travelers are as follows [4]:

  • Travelers who plan to spend a month or longer in endemic areas during the JE virus transmission season.

  • Short-term (<1 mo) travelers to endemic areas during the JE virus transmission season if they plan to travel outside of an urban area and have an increased risk for JE virus exposure.

  • Travelers to an area with an ongoing JE outbreak.

  • Travelers to endemic areas who are uncertain of specific destinations, activities, or duration of travel.

  • Not recommended for short-term travelers whose visit will be restricted to urban areas or periods outside of a well-defined JE virus transmission season.

Three types of vaccine are available –

  1. 1)

    Inactivated vaccine

    1. a.

      Inactivated Mouse Brain Vaccine (JE – VAX)

    2. b.

      Inactivated Primary Hamster Kidney Cells - P3 – China

    3. c.

      Inactivated Vero-cell culture derived (SA14-14-2 vaccine – IXIARO and 821564XY, JE vaccine JENVAC)

  2. 2)

    Live Attenuated Primary Hamster Kidney Cells – SA14-14-2

  3. 3)

    ChimeriVax-JE (IMOJEV)

Inactivated Vaccine

  1. a.

    Inactivated Mouse Brain Vaccine (JE – VAX) is being produced in Russia and Japan since 1930’s. It was recommended for use in adults and children aged ≥1 y but is no longer being produced due to development of cell culture-based vaccines [4].

  2. b.

    Inactivated Primary Hamster Kidney Cells - P3–China vaccine was used in China but after the introduction of live attenuated vaccine this vaccine is hardly ever used.

  3. c.

    Inactivated Vero-cell culture derived (IXIARO and JENEVAC)

A Vero-cell derived, inactivated and alum-adjuvanted JE vaccine based on the SA14-14-2 strain was approved in 2009 in North America, Australia and various European countries [2]. In 2010 Advisory Committee on Immunization Practices (ACIP) licensed this vaccine for >17-y-old but later in 2013 recommendations were changed [4, 5]. Since then it can be used from 2 mo of age. In India, ACVIP of IAP recommends immunization at 1 y of age by JE vaccine based on the SA14-14-2 strain [6]. Another inactivated Vero-cell culture derived Kolar strain, 821564XY, JE vaccine (JENVAC) is recommended by ACVIP of IAP to be given at minimum 1 y of age [6]. Table 1 enlists dosage and schedule of Inactivated Vero-cell derived JE vaccines.

Table 1 Schedule and doses of inactivated Vero-cell derived JE vaccines
Full size table

A phase III trial demonstrated that co-administration of IXIARO with Hepatitis A vaccine has no adverse effect on immunogenicity of either vaccine [7].

Live Attenuated Primary Hamster Kidney Cells – SA14-14-2

Since 1988, more than 300 million doses of this live attenuated vaccine have been administered in China for its high efficacy and safety profile [8]. It is manufactured in China and caters more than 50% of the global production of all JE vaccines in 2005. It is licensed in western pacific region and Asia (Nepal, Sri Lanka, India, and Korea). It’s 2-dose regime has reported 91.1% seroprotection after first dose and 97% after the second dose with self-limiting adverse events [8, 9]. ACVIP of IAP recommends minimum age of 8 mo for vaccination. Two dose schedule, first dose at 9 mo along with measles vaccine and second at 16 to 18 mo along with DTP booster is followed [6]. Wang et al. successfully evaluated the immunogenicity of live attenuated SA14-14-2 virus vaccine and measles vaccine (MV) administrated concomitantly [7].

ChimeriVax-JE

ChimeriVax-JE is a live, attenuated vaccine against Japanese encephalitis, using yellow fever (YF) 17D vaccine as a vector [10]. Single dose of then ChimeriVax-JE (now IMOJEV) was found to be safe, highly immunogenic and capable of inducing long-lasting immunity in both preclinical and clinical trials [10]. It has been licensed in Australia and Thailand. Each 0.5 ml reconstituted dose contains 4.0–5.8 log plaque-forming units (PFU) of live attenuated Japanese recombinant encephalitis virus [11]. Table 2 shows schedule of this vaccination. Recently, immunogenicity of this vaccine was studied in Thai children, as a booster dose after primary vaccination with live attenuated SA14-14-2 vaccine and it showed strong amnestic response [12].

Table 2 Schedule of ChimeriVax-JE vaccine
Full size table

Studies have proved successful co-administration or sequential administration of a live attenuated Japanese encephalitis chimeric virus vaccine (JE-CV) with live attenuated yellow fever vaccine [13] and measles, mumps, rubella vaccine (MMR) [7].

Yellow Fever

Yellow fever, a serious disease caused by the yellow fever virus of flavivirus family is prevalent in certain parts of Africa and South America. The 17D vaccine, a live, attenuated viral strain, is the only commercially available yellow fever vaccine (YF-VAX) [14]. It is prepared by culturing the 17D-204 strain of yellow fever virus and contains not less than 4.74 log10 plaque forming units (PFU) per 0.5 ml dose throughout the life of the product [15]. A single subcutaneous (or intramuscular) injection of the vaccine is highly effective (approaching 100%). All persons 9 mo – 59 y of age traveling to or living in an area where risk of yellow fever is known to exist, or traveling to a country with an entry requirement for the vaccination must be immunized [16]. Travelers after vaccination receive a stamped and signed "International Certificate of Vaccination or Prophylaxis" (yellow card) which becomes valid 10 d after vaccination and holds good till 10 y.

Malaria

Malaria, the parasitic, mosquito-borne disease is a leading cause of death and illness, hitting hardest in resource-poor tropical and subtropical regions. Based on World Malaria Report 2015, deployment of various tools to control malaria have been able to decrease malaria cases and deaths by 37% and 60% since 2000; yet, there were 214 million cases of malaria globally in 2015 (uncertainty range 149–303 million) and 4 lakhs malaria deaths [17]. The parasite had been difficult to control and has survived for millennia. Malaria control is threatened by resistance to artemisinins and pyrethroids, so a safe, effective and affordable vaccination can be an easy tool to close the gap left by other interventions. A feasible malaria vaccination came to the knowledge in late 1960s when repeated immunization of rodents with irradiated sporozoites (IrSp) demonstrated complete protection against sporozoite challenge and circumsporozoite protein (CSP) was discovered [18]. Immunization of human volunteers with the bite of P. falciparum nfected and radiation-attenuated Anopheles mosquitoes showed effective response but the process was neither cost-effective nor practical. Since then there had been a dozen of approaches for malaria vaccine [19] in advanced preclinical or clinical stages of evaluation.

Approach to Malaria Vaccine

  1. 1.

    Sporozoite subunit vaccination, especially with the CS protein: e.g., RTS,S in adjuvant.

  2. 2.

    Irradiated sporozoite or genetically attenuated sporozoite immunization either by mosquito bite or using injected purified sporozoites.

  3. 3.

    Immunization with DNA and/or viral vectors to induce T cells against the liver-stage parasites, or to target other life cycle stages.

  4. 4.

    Use of whole blood-stage malaria parasites as immunogens.

  5. 5.

    Use of protein in adjuvant vaccines to reduce the growth rate of blood-stage parasites.

  6. 6.

    Use of protein (or long peptide) in adjuvant vaccines to induce antibody-dependent cellular inhibition (ADCI) of blood-stage parasites.

  7. 7.

    Use of peptide-based vaccines, mainly against blood-stage parasites—e.g., SPf66, PEV3a.

  8. 8.

    Development of anti-disease vaccines based on parasite toxins—e.g., GPI-based.

  9. 9.

    Immunization with parasite adhesion ligands such as PfEMP1.

  10. 10.

    Use of parasite antigens, such as the Var2 protein, preferentially expressed in the placenta to prevent malaria in pregnancy.

  11. 11.

    Immunization with sexual stage parasite antigens as transmission-blocking vaccines.

  12. 12.

    Use of mosquito antigens as transmission-blocking vaccines.

RTS,S/AS01 Vaccine (Mosquirix)

This vaccine has completed Phase 3 evaluation and received a positive regulatory assessment [20]. It has been developed through a partnership between GlaxoSmithKline Biologicals (GSK) and the PATH Malaria Vaccine Initiative (MVI), with support from the Bill & Melinda Gates Foundation and from a network of African research centres. It acts against Plasmodium falciparum and offers no protection against P. vivax malaria.

In October 2015, WHO jointly convened the Strategic Advisory Group of Experts (SAGE) 2 on Immunization and the Malaria Policy Advisory Committee (MPAC) 3 to review all evidence regarding RTS,S relevant for global policy. SAGE/MPAC recommended pilot implementation of RTS,S in parts of 3–5 sub-Saharan African countries. SAGE and MPAC refused trials among infants aged 6–14 wk due to inferior efficacy seen in this age group [21].

Dose and Schedule

Three-dose initial series with a minimum interval of 4 wk between doses, followed by a 4th dose at 15–18 mo after the 3rd dose is recommended. The 1st dose should be administered as close as possible to age 5 mo and the 3rd dose should be completed by 9 mo of age. It can be co-administered with other vaccines in the National Immunization Programme with no serious adverse effect.

Efficacy

Over the full duration of the trial, vaccine efficacy against clinical malaria in infants was 27% in the group that received four doses of RTS,S (3 doses at 6, 10 and 14 wk of age, and a fourth dose 18 mo later); and 18% in the group that did not receive the fourth dose of the vaccine. In these infants, no significant efficacy was noted against severe malaria, with or without a fourth dose. Among children aged 5–17 mo who received four doses on a 0, 1, 2, 20 mo schedule, vaccine efficacy against clinical malaria was 39% and against severe malaria was 31.5%, with reductions in severe anemia, malaria hospitalizations and all-cause hospitalizations [21]. Among children aged 5–17 mo who did not receive a fourth dose of the vaccine, no protection was seen against severe malaria, as cases prevented in the first 18 mo occurred later. These results highlight the importance of a fourth dose with this vaccine, as efficacy is short-lived.

Side-effects

Febrile convulsions occurred within 7 d after vaccination and there was an increase in the number of cases of meningitis and cerebral malaria in the group receiving the malaria vaccine compared to the control group. It is now clarified that the majority of cases had a bacterial cause but the incidence of meningitis will be monitored throughout the coming trials [22].

On 17th November 2016, WHO announced that the RTS,S vaccine would be rolled out in pilot projects in 3 countries in sub-Saharan Africa and that vaccinations would begin in 2018.

Dengue

Dengue is the most rapidly spreading mosquito-borne viral disease in the world. Past 50 y have witnessed 30-fold increase in its incidence with increasing geographic expansion to new countries and, in the present decade, from urban to rural settings. The incidence of dengue increased greatly between 1990 and 2013, with the number of cases more than doubling every decade, from 8·3 million (3.3 million–17.2 million) apparent cases in 1990, to 58·4 million (23.6 million–121.9 million) apparent cases in 2013 [23].

Dengue, a mosquito-borne flavivirus disease is caused by four closely related viruses, the Dengue viruses (DV) 1–4. Massive urbanization, overcrowding and poor living conditions; increased human migration; failure of vector control programs; and the global emergence of more virulent genotypes of DV have made eradication of dengue difficult. A dengue vaccine would therefore, be a major tool in controlling the disease. The first successful dengue vaccine was reported in 1945 by Sabin and Schlesinger, who attenuated the “Hawaiian” strain (serotype DEN-1) of DV in mouse brain by serial passage and then used this mouse brain vaccine to protect 16 volunteers against the bites of infected A. aegypti mosquitoes [24]. The most advanced approaches are live attenuated vaccines (LAVs) followed by inactivated/recombinant adjuvanted vaccines and DNA vaccines [25]. Various dengue vaccines are in clinical and preclinical stage of development [25].

The first licensed dengue vaccine, Dengvaxia (CYD-TDV) by Sanofi Pasteur, was first registered in Mexico in December, 2015. The vaccine is also licensed in the Philippines, Brazil, El Salvador and Paraguay [26]. The Philippines has included this vaccine in its National Immunization Program (NIP) but India is yet to do so as Phase III trial has not started in India so far.

WHO has recommended introduction of the dengue vaccine CYD-TDV in countries with epidemiologically high burden of disease i.e., seroprevalence of approximately 70% or greater in the age group targeted for vaccination to maximize public health impact and cost-effectiveness.

Populations with seroprevalence between 50% and 70% can be vaccinated but the impact of the vaccination programme may be lower. WHO does not recommend vaccination when seroprevalence is below 50% [27].

It is a live recombinant vaccine containing 4 live attenuated recombinant viruses representing serotypes 1, 2, 3, and 4. Each monovalent CYD recombinant is obtained separately by replacing the genes encoding the prM and E proteins of the attenuated yellow fever (YF) 17D virus genome with the corresponding genes of the 4 wild-type dengue viruses. The final formulation contains 4.5–6.0 log10 median cell-culture infectious doses (CCID50) of each of the live attenuated dengue serotype 1, 2, 3 and 4 vaccine viruses [27].

Dose and Schedule

It is administered as a 3-dose series given as a 0, 6, and 12-mo schedule. It has been registered for use in individuals 9–45 y of age living in endemic areas. Co-administration with other vaccines is permissible. Recent study in Mexico showed that co-administration of the DTaP-IPV/Hib booster vaccine with CYD-TDV does not affect immunogenicity or safety profile of either vaccine [28]. Co-administration with YF vaccine and MMR also did not identify any safety concerns. Trials with human papillomavirus (HPV) and tetanus toxoid and reduced-dose diphtheria (TdaP) vaccines are underway.

Efficacy

Godói et al. in their systemic review and meta-analysis have derived vaccine efficacy of 60% in participants aged 2–16 y old, with DENV4 and DENV2 presenting the best and worst results, respectively [29].

Side-effects

Erythema and swelling were frequently associated but systemic adverse events were few.

Other Candidate Vaccines

There are at least five more candidates in clinical development but not yet licensed. TV003 and TV005 are most advanced vaccines developed by the U.S National Institutes of Health (US NIH) that are licensed. They are attenuated wild-type virus strains containing serotypes 1, 3, and 4 as complete viruses, and serotype 2 as recombinant virus. TV003 or TV005 has been licensed to Butantan, VaBiotech, Panacea, Serum Institute of India and Merck pharmaceuticals. A single dose of either TV003 or TV005 induced upto 90% seroconversion in flavivirus seronegative adults and elicited near-sterilizing immunity to a second dose of vaccine administered 6–12 mo later. Another live recombinant tetravalent vaccine, TDV (formerly DENVax) using a whole attenuated DEN2 virus and recombinant DEN1, DEN3, and DEN4 having DEN2 backbone is being developed by Takeda. Many Phase 1 and Phase 2 trials are undergoing and multicenter Phase 3 study is being planned. A tetravalent purified inactivated vaccine (GSK/Walter Reed Army Institute of Research), a tetravalent recombinant subunit vaccine based on the dengue wild-type pre-membrane and truncated envelope protein (Merck), a monovalent plasmid DNA vaccine (US Navy Medical Research Center), and an inactivated vaccine/live attenuated vaccine heterologous prime boost (Walter Reed Army Institute of Research) are in Phase 1 trial [30].

West Nile Virus

West Nile virus (WNV) is a mosquito-borne flavivirus and member of the Japanese encephalitis (JE) serocomplex. It was responsible for a serious U.S. epidemic recently in 2012 [31]. Till now no human WNV vaccine is available, and treatment options are also limited, causing high mortality and morbidity from human infection. An approved human JE vaccine (JE-ADVAX), which contains an inactivated cell culture JE virus antigen formulated with Advax delta inulin adjuvant, was studied against WNV and it proved heterologous protection against WNV infection in wild-type and β2-microglobulin-deficient (β2m−he) murine models [32]. A chimeric live attenuated West Nile virus vaccine, rWN/DEN4Δ30, was studied by Pierce et al. in flavivirus-naive adults aged 50–65 y. It was well tolerated and found to be immunogenic [33]. A hydrogen peroxide-inactivated whole virion WNV vaccine (HydroVax-001WNV) in a preclinical trial in mice resulted in robust virus-specific neutralizing antibody responses and protection against WNV-associated mortality and an excellent safety profile [34]. Apart from these, various clinical and preclinical trials are underway.

Zika

The recent rapid spread of Zika virus in America had devastating consequences on pregnant women and infants. Mosquito control, protection of the blood supply, contraception by barrier methods and others are current preventive measures but a safe and effective vaccine is must. Present strategies for Zika virus vaccine development are recombinant live attenuated vaccines, purified inactivated vaccines (PIVs), DNA vaccines, and viral vectored vaccines [35]. The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, has launched a clinical trial of a vaccine candidate intended to prevent Zika virus infection. The inactivated virus vaccine has already been tested in monkeys, where it proved effective against Zika [36].

Chikungunya

This mosquito-borne viral disease was identified in southern Tanzania in 1952 during an outbreak. It reached Indian Ocean in twenty-first century and since 2005, India, Indonesia, Maldives, Myanmar and Thailand have reported over 1.9 million cases [37]. Past year, even the newborns were not left unaffected by the deadly virus. It will be remembered for years to come for devastating epidemics and debilitating and chronic joint pains it caused. Prevention and control relies heavily on reducing breeding sites of the mosquitoes. During outbreaks, insecticides may be sprayed to kill mosquitoes and immature larvae. There is no specific treatment available. Various preclinical and clinical trials are in process using inactivated virus, live-attenuated viruses, virus-like particle (VLP), alphavirus chimeras, recombinant and consensus-based DNA technology to make an effective vaccine but none is licensed for commercial use till date [38]. Erasmus et al. used insect-specific alphavirus, Eilat virus (EILV), as a vaccine platform. Using EILV cDNA clone, a chimeric virus containing the chikungunya virus (CHIKV) structural proteins was designed. A single dose of EILV/CHIKV produced in mosquito cells elicited rapid (within 4 d) and long-lasting (>290 d) neutralizing antibodies in two different mouse models and non-human primates [39]. Researchers from Themis Bioscience and collaborators from the University of Vienna and from France and the United States studied live recombinant measles-virus-based chikungunya vaccine. The two dose vaccination resulted in a 100% sero-conversion for all participants [40]. The vaccine had an overall good safety profile, and none had vaccination-related serious adverse events. It is a promising measles-virus-based vaccine for use in human beings in future.

Conclusions

We have come long way in research trials for vaccines. Vaccines for malaria and dengue have been licensed in certain areas. Vaccines against Yellow fever and Japanese encephalitis have already dragged the mortality and morbidity to a lower level. Scientists are finally on the brink of introducing promising vaccines that might help control newer mosquito-borne viral risks that now dominate public attention: Zika and chikungunya. Wait for few more years may witness newer vaccines in the vaccination schedule.