Gubler D J, Reed D, Rosen L, Hitchcock J R
Am J Trop Med Hyg. 1978 May;27(3):581-9. doi: 10.4269/ajtmh.1978.27.581.
An outbreak of dengue type 2 infection occurred in the Pacific island Kingdom of Tonga in 1974 and an outbreak of dengue type 1 occurred there in 1975. The 1974 outbreak was characterized by relatively mild clinical disease with few hemorrhagic manifestations, a low attack rate, and relatively low viremia levels. The 1975 outbreak was characterized by relatively severe disease with frequent hemorrhagic manifestations and a high attack rate. The differences between the outbreaks could not be attributed to differences in abudance of, or susceptibility to infection of, mosquito vectors or to the prior immune status or other characteristics of the human population. It appeared that a difference in viral virulence was the most likely explanation.
1974年，太平洋岛国汤加爆发了2型登革热感染疫情，1975年又在那里爆发了1型登革热疫情。1974年的疫情特点是临床疾病相对较轻，出血表现较少，发病率较低，病毒血症水平也相对较低。1975年的疫情特点是疾病相对严重，出血表现频繁，发病率较高。两次疫情的差异不能归因于蚊媒数量或对感染的易感性差异，也不能归因于人群的既往免疫状态或其他特征。看来病毒毒力的差异是最有可能的解释。
