Vijay Yewale and Panna Choudhury,

As far as Hib and other conjugate polysaccharide vaccines are concerned, the dose schedule depends on the maturity of the immune system with increasing age and not sub-clinical infection leading to some immunity, as is thought of. That is why one needs lesser number of doses as age advances. This is also proved by the fact that children with baseline antibodies are not protected against disease unless the levels are more than protective levels (which we typically achieved very early in life with conjugate vaccines).

For rotavirus infection, first natural infection is the most severe and often leads to hospitalization. However, it also then protects against severe disease with subsequent exposure to any other rotavirus. Whereas the rotavirus vaccine mimicks this natural process, but without the vaccine suffering from any rotavirus vaccine virus. Hence to say that only one dose should be enough at 4-6 months is not correct as (i) that particular child may not have been exposed to any rotavirus by that age as is evident by the epidemiology in India; (ii) there is no mild first disease leading to sub-clinical immunity; and (iii) waiting for 4-6 months may lead to first natural infection in that child before the vaccine is given and may lead to severe disease. The second dose is given to ensure seroconversion/protection in those who may not have seroconverted/protected with the first dose.