Primary schoolchildren in Grade 3 and 4 in the selected
schools were recruited at the start of the study. Only
children who lived in the same district as their schools for
a continuous period not less than 12 months were included
in the final analysis. The baseline study was carried out
from September to November 2006, and follow-up was done
6 months later, from April to June 2007. Informed consent
was obtained from the parents prior to the survey. This
study has been approved by the Medical Research Ethics
Committee of Sun Yat-sen University