Study design and population
uncooperative behavior, hypersensitivity
HYPERSENSITIVITY, TEETH ENAMEL HYPOPLASIA
Children in this study were recruited from a larger randomized controlled clinical trial conducted in the kindergartens of Hong Kong, with an optimal fluoride drinking water concentration of 0.5 ppm. The kindergartens were selected by purposive sampling from a list of non-profit-making kindergartens obtained from the Hong Kong Education Bureau. To ensure generalizability of the findings and for the control of confounders, stratified random sampling generated a pool of 135 participants from the clinical trial, with 45 in each group. The trial obtained ethical approval from Institutional Review Board of the University of Hong Kong (reference number: UW 18–054) and was conducted in accordance with the best clinical and research practices and in accord with the declaration of Helsinki (1964). The protocol was registered at Children were 3–4 years of age with no prior history of any severe medical condition, special health care needs or long-term medication. Children were excluded if they had antibiotic intake within the past month, primary teeth enamel hypoplasia, professional fluoride treatment in the past six months, hypersensitivity to any constituents in the study varnishes, or uncooperative behavior during examination or sample collection.