Methods A stratified multistage random sampling method was used to select 844 children born in 1997 from 120 villages in 89 county-level cities,counties and districts.

• 方法采用分层多阶段随机抽样方法,调查了广东省89个市(县、区)的120个村(居委会)的844名1997年出生儿童的免疫状况。