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Sanofi Pasteur\u2019s Dengvaxia was the first dengue vaccine to be licensed. Originally licensed in Mexico in December 2015, it is now licenced in 20 countries, <\/em>according to the World Health Organization<\/em><\/a> (WHO), for use in people aged 9 and 45 living in dengue-endemic areas. While safe for people who had previously been infected with dengue, the vaccine appears to put people who have never been infected at higher risk of more severe dengue.<\/em> So, what was behind the vaccine\u2019s strange behaviour? And what does it mean for dengue vaccination programs?<\/em><\/p>\n Essentially it comes down to a phenomenon known as antibody-dependent enhancement<\/a> (ADE). The dengue virus has four serotypes<\/a>, all of which can cause disease. Anyone catching dengue for the first time will develop antibodies with long-lasting protection against the infection\u2019s serotype. They also develop antibodies that protect against the other serotypes \u2013 but that protection only lasts a short time. Once it wanes, that person will only have protection against the first (or primary) serotype.<\/p>\n Dengue and ADE<\/em><\/p>\n Imagine they then catch the virus a second time. Their pre-existing dengue antibodies would only stop the virus if the second infection were of the same serotype as the first. If not, ADE occurs. With ADE, these antibodies \u2013 being of a different serotype to the infecting virus \u2013 don\u2019t neutralise it. Scientists believe<\/a> that they instead bind to the virus and help it enter and infect the person\u2019s cells.<\/p>\n What would most likely have otherwise been a mild viral infection becomes severe. In other words, the impact of the dengue virus can be more severe if you contract one dengue serotype, then a different dengue serotype.<\/p>\n An ideal dengue vaccine has to provoke the body to produce antibodies for neutralising all four dengue serotypes to avoid ADE. But research seems to show<\/a> that, in people who have never had dengue, Dengvaxia may predominantly provoke the production of antibodies that protect against once specific serotype: DENV-4.<\/p>\n Dengue pre-vaccination screening<\/em><\/p>\n The WHO currently recommends<\/a> pre-vaccination screening for dengue so the vaccine is only given to people who have previously been infected. It notes that \u201cDecisions about implementing a pre-vaccination screening strategy will require careful assessment\u201d and \u201cVaccination should be considered as part of an integrated dengue prevention and control strategy.\u201d<\/p>\n Laboratory-based tests would be the most likely approach for dengue pre-vaccination screening; but processes can be time-consuming and these tests require a fair amount of laboratory resources. Researchers believe<\/a> there may be delays of several days between sampling and receiving the result. Patients unwilling or unable to attend the clinic more than once (initially, for sampling and, secondly, for vaccination) would not get the protection they need.<\/p>\n