Vaccines can be broadly classified as live attenuated (replicating) and non-replicating. When live attenuated viral vaccines are inoculated the host should get infected before the immune system can respond adequately. Infection results in amplification of the virus and its antigens; the inoculated dose by itself does not contain sufficient antigenic mass for stimulating the immune system. Occasionally an inoculated child may not get infected; failure of infection results in failure of immunity (primary vaccine failure). Immunity induced by vaccine virus will have similar breadth to that induced by the wild virus, in terms of the range of epitopes covered by immune response and the balance of Th-1 and Th-2 responses to important epitopes. The resultant immunity provides protection from disease when exposed to (or even if infected by) the target wild virus. But, it will always be weaker (lower in magnitude and antibody titre) than that induced by the wild virus. On the contrary, immunity induced by non-replicating viral vaccine is narrower – as the antigenic determinants are selectively limited during vaccine production. Immune response is predominantly Th-2 with humoral antibodies. But, since high antigenic mass is inoculated, with priming and boosting, the immune response is usually much greater in magnitude than that of natural infection, as illustrated by killed virus vaccines against rabies, hepatitis A and poliomyelitis. Such unnaturally high antibody levels offer advantages such as mucosal immunity due to IgG ‘spill over’ as in the case of hepatitis A and poliovirus vaccines. Thus, there are pluses and minuses for both kinds of vaccines from the efficacy angle. Safety is another matter.