OF 28330 individual's CD4+ count at HIV diagnosis, there are 17469 missing values of CD4+ count. The variables I have are State, Sex, Exposure route, Year of HIV Diagnosis, Year of Last Negative, Year of Primary Infection. There are large number of missing years in the last two variables.

How should I deal with the missing values without delete those? I want to do weighting variables by each stratum in the variable, but don't know how. And I cannot find relevant information about weighting online as well. Is there anyone here can tell me? Thanks in advance.