The response to the Zika virus is fairly typical of both our strengths and weaknesses in dealing with infectious diseases, especially when they emerge. Existing drugs against other viruses were identified as potential therapeutics for Zika, efforts to develop a vaccine are underway and nearing completion, and the concern about Zika spurred some needed and valuable research. Those are encouraging signs, but these efforts were somewhat slow to get started and preceded by the usual complacency. We tend to close the barn door only after most of the horses have escaped.
We look to vaccines to provide an important protection against many infectious diseases. The two main problems are the availability of the vaccine, and the time it takes to develop a new vaccine. Yellow fever is a relative of Zika. It is an infection of historic importance, legendary for its lethality. The effectiveness and safety of the yellow fever vaccine is a real success story. Unfortunately, because of limited availability and cost, those who need the vaccine the most are not able to get it. After the eruption of Zika in South America, work urgently began on new vaccines for Zika. But, the necessary pace of vaccine development meant that there would be vaccine candidates to test only when the original epidemic was waning and there were no longer enough patients to do rigorous trials. The same happened with the Ebola vaccine, which is being clinically tested now, and hopefully will be available for future epidemics, but was too late to help in the massive 2014 outbreak. Recent unexpected problems with a new vaccine for dengue—a fairly close relative of Zika—may also cause further concerns in using a Zika vaccine. We really need to be more agile and proactive in our response.
We neglect public health measures at our peril. Vaccines are good public health measures, but public health is not just vaccines. Both dengue and Zika were introduced to the world in the last few decades. Aedes aegypti, the chief mosquito vector for yellow fever, dengue and Zika (among others) came to the western hemisphere with the slave trade, over 400 years ago. So when Zika arrived recently, it had an efficient transmission mechanism already in place. Because of yellow fever concerns, the mosquito had been largely controlled in South America in the mid-20th century, but these successful efforts were discontinued later in the century. The mosquito populations came back in force, allowing the introduction and explosive spread of dengue in the 1980s and Zika more recently. Ironically, if the mosquito control efforts had been kept up, we probably would not be talking about either of them today. The Indian subcontinent, fortunately, has never had yellow fever but has plenty of Aedes aegypti mosquitoes. Only our vigilance and good fortune can prevent yellow fever from becoming established the same way dengue and Zika have in South America. We really cannot afford to remain complacent until the crisis hits.
Dr Stephen S. Morse is professor of epidemiology and director, Infectious Disease Epidemiology Certificate, Mailman School of Public Health, Columbia University.