Why, if the vaccines work, is South Korea experiencing year after year increases in chickenpox (varicella) outbreaks as their mandated vaccine uptake (97%) reaches close to universal compliance?
In 2005, the varicella (chickenpox) vaccine was mandated in South Korean for infants at 12 to 15 months of age. Despite near universal vaccine uptake (97% by 2011), rather than see a reduced burden of chickenpox in South Korea as would be expected, no substantial decreases have been observed there nationwide. Quite to the contrary, the number of varicella patients reported to the Korea Centers for Disease Control and Prevention (KCDC) increased from 22.6 cases per 100,000 in 2006 to 71.6 cases per 100,000 in 2011.
The obvious question emerges: If the chickenpox vaccine is effective, why is a population whose vaccination rates have been steadily increasing year after year seeing increasing rates of breakthrough infection?