Measured by the population at risk and economic burden, dengue is the most globally important mosquito-transmitted viral infection. Between 1990 and 2013, the number of cases more than doubled every ten years. One recent estimate indicates 390 million dengue infections per year. And, like its cousins West Nile, yellow fever, Japanese encephalitis, and Zika, the vector-borne flavivirus dengue has spread widely in recent decades.
Before 1970, only nine countries had experienced severe dengue epidemics. Spurred by growing urbanization, mobile populations, and increases in the range of the Aedes aegypti vector due to climate change, an estimated 3.9 billion people in 128 countries are at risk of infection today. The virus is endemic in parts of Africa, the Americas, the Eastern Mediterranean, South-East Asia, and the Western Pacific. WHO reports that in 2015, 2.35 million cases of dengue were reported in the Americas alone, of which 10,200 cases were diagnosed as severe dengue, defined by WHO as exhibiting severe plasma leakage, severe hemorrhage, and/or severe organ impairment. Severe dengue caused 1,181 deaths.
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The danger of dengue spreading further is high: among travelers returning from low- and middle-income countries, dengue is the second most diagnosed cause of fever after malaria. In 2012, dengue on Madeira, a popular year-round resort with about one million tourists annually, resulted in more than 2,000 cases on the island and additional cases imported to mainland Portugal and ten other countries. Local transmission was reported for the first time in France and Croatia in 2010. An outbreak of dengue in Europe is clearly possible.
Developing a vaccine against dengue has been challenging. With four closely related viruses that can cause the disease, the goal for a vaccine has been to provide protection against all. This task was complicated by limited understanding of how severe dengue occurs, how the virus interacts with the immune system, and how to measure if a person is immune. Nevertheless, the first dengue vaccine was registered in December, 2015 and the WHO Strategic Advisory Group of Experts (SAGE) on Immunization has recommended countries consider introduction of the vaccine in geographic settings with high endemicity. Additional vaccine candidates are under evaluation in clinical trials and preclinical studies. Dengue can be prevented by vaccination, but vaccine production is limited - full scale production capacity will reach 100 million vaccine doses annually, but the population at risk approaches 4 billion and global vaccination programs are expensive.
While WHO expects vaccines to be an integrated part of a global dengue prevention and control strategy, the international infectious disease community needs to introduce additional measures to address the epidemic including improved systems for sharing data from entomologic surveillance conducted by vector control units and human disease surveillance conducted by epidemiologists. Information is the strongest defense against infectious disease. Information is what is needed to stop the spread of dengue. Information from surveillance is what's needed to provide the early detection of disease needed to prevent or control outbreaks.
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Larry Madoff, MD
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