There is no national immunization registry. It is only a statewide thing, some states do it and some don't. And even within the states that do it, some doctors don't participate in it. Until they develop a national registry, it is impossible for them to know for sure whether you got the vaccine or not. Also, HIPAA laws prevent private companies from being able to have access to your medical records. Unless something changes soon, you will have to give them permission to see your health records.