THE dengue fever virus can mutate much more rapidly than anyone thought, which could thwart the efforts of the teams rushing to create a vaccine.
Over the past few decades, the mosquito-borne virus has spread dramatically and now infects about 50 million people each year in tropical areas, killing at least 12,000. Two-fifths of the world鈥檚 population are at risk, and global warming will allow the virus to increase its range.
Developing an effective vaccine that protects against each of the four main types of the dengue virus has proved difficult. Now work by John Aaskov鈥檚 team at the Queensland University of Technology in Australia suggests that a successful vaccine might not remain effective for long.
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The team has found that two mutated strains of dengue-1 caused 95 per cent of 15,000 cases in Myanmar in 2001 鈥� the country鈥檚 largest ever outbreak. Comparisons of viruses from patients suggest that these strains evolved locally within a year, Aaskov says. 鈥淲e don鈥檛 have the genotype of every dengue-1 virus, so we can鈥檛 be sure they didn鈥檛 come from somewhere else. But we think that鈥檚 very unlikely.鈥�
In April, the team reported that two different dengue-2 strains had recombined in a mosquito to create a third strain 鈥� the first direct evidence that new strains can emerge this way (Journal of Virology, vol 77, p 4463). And last year, the team found that two new strains of a dengue-3 virus in Thailand had evolved in less than a year, replacing the dominant local strain and causing the biggest dengue epidemic in Thai history (Virology, vol 301, p 148).
Every virus mutates to some extent. 鈥淏ut these results make us feel that the dengue virus is changing far more rapidly and dramatically than was thought previously,鈥� Aaskov says. 鈥淢y concern is that these new recombinant vaccines are genetically very, very narrow, and if the virus is able to change quickly, we might not get the sort of efficacy that we would like.鈥�
That might result in groups abandoning their attempts to create a vaccine. Aaskov thinks one solution might be to persuade licensing authorities to permit annual changes to the make-up of any dengue vaccine, as happens for flu vaccines.
Other researchers are cautious about Aaskov鈥檚 conclusions. The yellow fever vaccine has been on the market since the 1940s and remains effective despite the evolution of the virus, points out John Mackenzie of the University of Queensland in Brisbane. But Aaskov says that the dengue viruses have greater opportunity for change than the yellow fever ones because they are far more widely transmitted.