英国政府は反ワクチンなのか？水痘ワクチンの定期接種化に反対している

Why isn't the chickenpox vaccination part of the routine childhood immunisation schedule?

There's a worry that introducing chickenpox vaccination for all children could increase the risk of chickenpox and shingles in older people.

Whilst chickenpox during childhood is unpleasant, the vast majority of children recover quickly and easily. In adults, chickenpox is more severe and the risk of complications increases with age. 

If a childhood chickenpox vaccination programme was introduced people would not catch chickenpox as children (as the infection would no longer circulate in areas where the majority of children had been vaccinated). This would leave unvaccinated children (there will always be a few who are unable or choose not to have the vaccine) susceptible to contracting chickenpox as adults when they are more likely to develop a more severe infection or a secondary complication, or in pregnancy when there is a risk of the infection harming the baby.

We could also see a significant increase in cases of shingles in adults. Adults who are naturally exposed to chickenpox (such as through contact with infected children) receive a natural boosting of their chickenpox antibodies which prevents the chickenpox virus (which remains dormant in the body after chickenpox infection) from reactivating in their bodies in the majority of cases and causing shingles.

If you vaccinate children against chickenpox, you lose this natural boosting so current levels of immunity in adults will drop and more shingles will occur.