The Food and Drug Administration (FDA) announced on October 30th that Dengvaxia’s, Sanofi Pasteur’s dengue vaccine, file has been accepted for priority review within the regulatory agency. With this announcement, the FDA will ensure that a decision will be declared on approval in the United States within six months for the world’s first licensed vaccine protecting against this flavivirus. While this declaration by the FDA displays an improved pragmatic approach to addressing neglected tropical diseases (NTDs), this vaccine has created controversy throughout the global health community. This vaccine is licensed in twenty countries to date and implemented into country wide vaccination programs. However, the concerns accompanying this recombinant, live, attenuated, tetravalent dengue vaccination have led to a discontinuation of this technology with a loss of confidence in several nation states. The Philippines, the first country to complement their vaccination program with this vaccine, has even instructed Sanofi to reimburse the $70 million the country spent to vaccinate 830,000 children. This has caused many global health experts to doubt the impact this vaccine can have throughout the world – causing many to wonder if the global health community is settling on a dengue vaccine.
The dengue virus is estimated to cause 400 million infections each year spanning each of the World Health Organization’s (WHO) regions. This arbovirus belongs to Flaviviridae family and is spread to humans through the bite of an infected female Aedes aegypti mosquito and to a lesser extent from the Aedes albopictus species. The dengue virus has four unique serotypes, DEN-1, DEN-2, DEN-3, and DEN-4, which has caused an effective vaccine to be eluded for centuries. When a person is infected with one certain serotype, the person gains life-long immunity to that serotype. However, if that person contracts a different serotype, it increases the risk of the person developing severe dengue. This phenomenon is called antibody-dependent enhancement (ADE) which allows the different serotype to enter cells more efficiently due to the previously created antibodies from the initial serotype. The symptoms that dengue causes depend on primary or secondary infection. Primary infection results in an acute febrile illness that is typically cleared by the immune system within seven days, while secondary infection can lead to dengue hemorrhagic fever or dengue shock syndrome causing serious morbidity and mortality. The dengue virus currently has no approved treatments – highlighting the importance of an effective and safe vaccine for children and adults alike.
The significant setbacks for Dengvaxia first arose when Sanofi Pasteur released interim studies concerning children aged 2 to 16 receiving the vaccine who were seronegative. This information was released on November 29, 2017 and revealed that among dengue-seronegative participants, recipients had increased rates of hospitalization for virologically confirmed dengue (VCD) and severe VCD in the vaccine group than in the group not administered the vaccine. These risks were significantly elevated in patients who were aged 2 to 8 years of age and became evident earlier than those aged 9 to 16 years of age. When this data became available, it led to the Strategic Advisory Group of Experts on Immunization (SAGE) of the WHO to reconvene and update their guidance on Dengvaxia. On April 18, 2018, SAGE recommended that for countries considering implementing Dengvaxia, every individual should be screened to determine their serological status with only seropositive persons receiving the vaccine.
The flavivirus genus includes other NTDs including the Zika virus, Japanese Encephalitis, West Nile Fever and Yellow Fever in addition to the Dengue Virus. The symptoms of each these ailments can present almost identically, especially in their milder forms, seeming almost flu-like in nature. When considering these identical disease presentations and the WHO’s recommendation to prescreen individuals for Dengvaxia, health care professionals must turn to dengue serological testing to ensure best practice – if the vaccine is accidentally given to a person with, for example, the Zika virus with no previous case of dengue due to a misdiagnosis from medical history, this would increase the risk of morbidity and mortality if dengue was contracted subsequently. The gold standard for serological testing is isolation and characterization of the virus, like PCR; however, this typically takes six or more days to receive the results and can be burdensome with it’s cost on a public health care system. A more common approach is enzyme immunoassay (ELISA) which is cost effective and less time consuming. However, in areas where two or more of the aforementioned flaviviruses exist, there is IgG cross reactivity between the viruses causing false positives for the dengue virus when ELISA is used. This often rules out the use of ELISA due to a common vector, Aedes aegypti, being able to spread two or more of these viruses within the same zone. Since the dengue virus is endemic throughout the developing world, dedicated health care professionals in these areas often don’t have funds, technology, or training in order to utilize the gold standard, PCR, in dengue testing — further highlighting the health disparities that exist on this earth. This leaves a major barrier to giving proper care to a large portion of humanity including administering this vaccine safely.
With the addition of rapid, accurate dengue test for the serological status of individuals that is in the pipelines (although no estimate of how soon it will be developed has been released yet), this vaccine will certainly find its niche in the global health society. However, this niche will exclude an enormous percentage of humans that would benefit from a safe and effective dengue vaccine. Those individuals that are currently seronegative and those who don’t have access to well-funded public health care system will continue to be at risk for developing the fatal consequences of the dengue virus. Global health leaders need to continue to promote and demand a vaccine that will ensure protection for a greater majority of people. Although this vaccine will serve some well, health care professionals must not settle until the dengue virus and each neglected tropical disease is properly addressed.