Jaydeep Choudhury*
Academic Registrar, Institute of Child Health, Kolkata. *
Vaccines in general may be classified into two groups, non-replicating and replicating vaccines. The non-replicating vaccines are inactivated organisms, subunits or toxoids. The components of these vaccines do not replicate in the body and act by inducing antibody reaction. The doses of these protein antigens required for immune stimulation is lower in children than in adults. The replicating vaccines, such as live virus vaccines act by establishing infection, which are mild and sub-clinical. The ultimate antigenic mass in these vaccines should be huge and it results from viral multiplication in the body. As these vaccines are meant to establish infection, there is no reason to alter the dose according to body mass. These replicating vaccines have to be given in 'median cell culture infectious doses'. In case of chicken pox vaccines 1000 to 5000 median cell culture infectious dose is enough, whereas in case of oral polio vaccine, 100 times this dose is necessary as it is poorly infectious.

In case of non-replicating vaccines, the repeated doses act as boosters. The principle of a second dose of chicken pox vaccine does not have a booster effect as it acts by causing infection. The response is similar to multiple doses of oral polio vaccine. Above 12 years, there is a fair chance of missing on the first occasion, a second dose often achieves the goal.

It has been observed that the seroconversion rate of chicken pox vaccine is 97% or more in children 1 to 12 years. Above 13 years, the seroconversion rates are only 78% to 82%. In the age group 1 to 12 years, the first dose of chicken pox vaccine acts as a subdued natural infection. There is development of herd immunity. It leads to the development of IgG and memory cells with provision for development of anamnestic reaction. When the child is exposed to chicken pox viruses from the surrounding like friends in school, the natural infections act as natural booster. Even if there is breakthrough infection, only 10-15 vesicles erupt. Thirteen years or above is vulnerable age, chicken pox infection may be more severe. If there is breakthrough infection, 100-200 vesicles erupt. Infection in such children is associated with significant morbidity. If the child is not vaccinated till 13 years, there are no memory cells and no provision of anamnestic reason, as he has not suffered from any chicken pox 'disease'. Mere contact will not produce memory cells. These children will not get enough boosting effect from natural infections and the second dose will act as catch up immunization. When a second dose of chicken pox vaccine is given after 4 to 8 weeks, the cumulative response increases to 99%. Hence the number of doses are based on the response rate and this is the reason behind administration of two doses of chicken pox vaccine above 12 years of age. Thus, the recommendation of two doses of chicken pox vaccine above 12 years of age is evidence based.

Apparently a reasonable alternative seems to be doubling the dose of chicken pox vaccine and give it in one shot. But from immunological standpoint, if the potency of the single dose for those above 12 years of age has to be increased, perhaps 10 times higher dose should be given in order to achieve near 99% response. This also is not an acceptable solution as chicken pox vaccine is very expensive. Two doses of chicken pox vaccine will double the cost, but ten times dose will cost ten times more.

Considering all these factors chicken pox vaccine should be administered 0.5 ml single dose in children 12 months to 12 years. In children above 12 years and adults two doses of 0.5 ml each should be administered at an interval of 4-8 weeks. Along with the international organizations like WHO, UNICEF and American Academy of Pediatrics, IAP Committee on Immunization also endorses this schedule, though it is not recommended for universal immunization. The current schedule is well established and has its own merit and there is no need to change.
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