Double trouble

 作者:庆锸     |      日期:2019-03-08 04:02:02
By Michael Day SOME vaccines now in the pipeline could do more harm than good, say biologists who have been studying the effects of the strange ability of dengue fever virus to cause more severe disease the second time it infects someone. When a virus or bacterium exists in several strains, infection by one strain usually confers some immunity to the others. This is not true for the dengue fever virus, carried by mosquitoes, which has four main strains. Infection by any strain causes more serious disease if it has been preceded by another. Immunologists think the initial infection must somehow alter the immune system in a way that weakens the ability of immune cells to resist a subsequent attack. As a result, infection by a second strain of dengue causes more severe illness, and leads to higher levels of the virus in the blood. This in turn probably makes the strain spread more quickly. To find out how this effect alters the spread of the disease in humans, Neil Ferguson and his colleagues at the University of Oxford created a mathematical model of these infection characteristics (Proceedings of the National Academy of Sciences, vol 96, p 790). Such characteristics make for extremely complicated disease patterns, Ferguson says. “Sometimes they may be truly chaotic.” The model also explains the mystery of why outbreaks of dengue fever can crop up suddenly and unexpectedly. Ferguson also warns that vaccines against dengue, currently under development, could do more harm than good unless they protect against all strains of the disease. Immunising people against just one or two of the strains using killed viruses would probably increase the virulence of the other strains, he says. “If we don’t look out for this, a vaccine could have a negative effect,