How effective is the vaccine?
• The effectiveness of the vaccine varies a little from year to year, depending on how well the vaccine matches the virus for that year • The effectiveness also depends on the age and the strength of the immune system of the child • Despite these variations the vaccine has been shown to have an efficacy of 59% in children aged 2-16 years in one study (5) and CDC in the USA has just announced a 75% efficacy in their childhood vaccination program for influenza where children received two doses of vaccine. These data were for the age group 6 months to 2 years and were collected across two US winters.
Varicella zoster virus vaccine is highly effective in children and adults. A single dose completely protects 85% of immunised children against developing clinical chickenpox. Immunised children who are not fully protected will almost always develop only mild disease if exposed to varicella zoster virus; vaccine effectiveness against moderate or severe disease is 97%.9 Recently there have been some varicella outbreaks where the effectiveness of the vaccine was lower than expected.10,11,12,13 Children immunised more than four years previously appeared at increased risk of breakthrough disease in these outbreaks, although almost all cases had mild disease. Despite having mild disease, immunised children with breakthrough disease are contagious and should be subject to the same school exclusion criteria used for other cases of chickenpox.
