Whittle H C, Aaby P, Samb B, Jensen H, Bennett J, Simondon F
MRC Laboratories, Fajara, Banjul, The Gambia.
Lancet. 1999 Jan 9;353(9147):98-102. doi: 10.1016/S0140-6736(98)02364-2.
Despite a high coverage with measles vaccines in parts of west Africa, epidemics of measles occur with reduced severity in an increasing proportion of older children who have been vaccinated. We examined the effect of exposure to natural measles on immunity in vaccinated children.
Our study was carried out in 1992 during an epidemic of measles in Niakhar, a rural area of Senegal with about 27,000 inhabitants who mostly live in compounds that include several households; within each household people live in different huts. Vaccine coverage in Niakhar was 81% at the time of our study. We measured haemagglutinin-inhibiting antibody at exposure and twice thereafter (after 4-5 weeks and at 6 months) in 36 vaccinated and 87 unvaccinated children. The frequency of measles and subclinical measles--defined as a four-fold or greater rise in antibody titre without clinical signs or symptoms--was related to intensity of exposure according to whether the index case was in the same hut, household, or compound.
Clinical measles occurred in 20 (56%) of 36 unvaccinated children and in one (1%) of 87 vaccinated children. Subclinical measles occurred in 39 (45%) of 86 vaccinated children who were exposed to measles and in four (25%) of 16 unvaccinated children. The frequency was inversely related to pre-exposure antibody concentration (p<0.001 for trend) and directly related to intensity of exposure (p=0.002 for trend). Antibody concentrations in subclinical cases increased on average by 45-fold and remained raised for at least 6 months.
Increased antibody titre after subclinical measles may be common in vaccinated children in West Africa where the intensity of exposure is high. As measles vaccination coverage increases, the circulation of wild measles will decrease, and vaccine-induced antibody is less likely to be boosted. Thus, new epidemics, albeit milder in form, may occur in vaccinated areas which should be recognised in campaigns to eradicate measles.
尽管西非部分地区麻疹疫苗接种率很高，但在接种过疫苗的大龄儿童中，麻疹疫情的严重程度有所降低，且此类情况日益增多。我们研究了自然感染麻疹对已接种疫苗儿童免疫力的影响。
1992年，在塞内加尔农村地区尼亚喀尔爆发麻疹疫情期间开展了我们的研究。尼亚喀尔约有27000名居民，大多居住在包含多户家庭的大院中；每户人家住在不同的小屋里。我们研究时，尼亚喀尔的疫苗接种率为81%。我们对36名接种疫苗儿童和87名未接种疫苗儿童在接触麻疹时以及之后两次（4 - 5周后和6个月时）测量了血凝抑制抗体。根据指示病例是否在同一小屋、同一户或同一大院，将麻疹和亚临床麻疹（定义为抗体滴度升高四倍或更多且无临床症状）的发生频率与接触强度相关联。
36名未接种疫苗儿童中有20名（56%）出现临床麻疹，87名接种疫苗儿童中有1名（1%）出现临床麻疹。86名接触麻疹的接种疫苗儿童中有39名（45%）出现亚临床麻疹，16名未接种疫苗儿童中有4名（25%）出现亚临床麻疹。该频率与接触前抗体浓度呈负相关（趋势p<0.001），与接触强度呈正相关（趋势p = 0.002）。亚临床病例的抗体浓度平均增加45倍，并至少持续升高6个月。
在西非接触强度高的地区，已接种疫苗儿童在亚临床麻疹后抗体滴度升高可能很常见。随着麻疹疫苗接种率的提高，野生麻疹的传播将减少，疫苗诱导的抗体不太可能得到增强。因此，在接种疫苗地区可能会出现新的疫情，尽管形式较为温和，这在麻疹根除运动中应予以认识。
