df <-
 STUDY   COV   FLAG   Freq
 1       SEX   1       12
 1       SEX   2       15
 2       RACE  1       5
 2       RACE  2       10
 2       RACE  3       15

dfout <-
 STUDY   COV   RATIO
 1       SEX   12:15
 2       RACE  5:10:15

library(dplyr)
  dfout<- df%>%
    group_by(STUDY,COV) %>%
    summarise(RATIO=paste(Freq,":",lag(Freq)))