This study is a prospective cohort study, and the main exposure factor is BVHS. BVHS is an ordered classification variable, taking the median of BVHS as the median, we regard the higher BVHS as the exposure group and the lower as the control group. The sample size of the cohort study was calculated by using the sample size formula of the cohort study when the sample size of the exposure group was equal to that of the control group: , α indicates the significant level, set to 0.05; 1-β indicates the test efficacy, set to 0.9; p0 indicates the incidence of the control group; ; p1 indicates the incidence for the exposure group, ; ,; The incidence of natural population events was taken as the estimation of the incidence of the control group, that is , and the risk ratio (RR) value of the exposure group was used to estimate the incidence of the exposure group. According to the previous research results of our group10, the RR value was estimated to be 1.94, . Under the above parameters, the minimum sample size is 1471.