Alum tends to result in Th2 response, which is not very useful against viral infection as the T cells required to clear infected cells are not sufficiently activated. A lot of antibodies will be made but an RNA virus could mutate very rapidly in response, eventually leading to a new serotype that would require a different vaccine altogether. Or worse, result in antibody enhanced disease.

Most modern vaccines made against viral diseases use specialised adjuvants to avoid this known problem.