CDC reports switch to 2-dose regimen of Merck's chickenpox vaccine a success

In the decade after Merck ($MRK) began selling its chickenpox vaccine in the U.S. the proportion of infants immunized against the virus rose to almost 90%. Yet immunized kids continued to catch chickenpox and the virus even killed two people who had received the vaccine. This week the Centers for Disease Control and Prevention (CDC) reported on the success of the strategy it adopted to prevent such cases.

The Advisory Committee on Immunization Practices (ACIP) initiated the new strategy when it recommended switching to a two-dose regimen. ACIP also encouraged health agencies to develop and enforce policies that make proof of immunity against chickenpox, also known as varicella, a requirement for starting school. In 2007 just four states required all kids entering elementary school to have received two doses of a varicella vaccine. By 2012 such policies were in place in 36 states, as shown in the map above (courtesy of the CDC).

In its Morbidity and Mortality Weekly Report CDC digs into data from 6 monitoring sites to assess the effect the policies have had on uptake of the two-dose regimen. The proportion of 7-year-old kids who had received two doses across the sites in 2006 ranged from 3.6% to 8.9%. By 2012 the range had increased to 79.9% to 92%. The current vaccination rate is comparable to the proportion of kids who receive two doses of the measles, mumps and rubella (MMR) vaccine and is nearing the Healthy People 2020 goal of 95%.

However, it appears the trend is being driven by ACIP's recommendation of a two-dose regimen, not school entry requirements. Uptake of the two-dose regimen was higher at the four sites in states with school entry requirements, but the difference wasn't statistically significant. Nonetheless, CDC thinks adoption of entry requirements in more states--and for school grades beyond kindergarten--could help increase vaccination coverage to the level targeted by Healthy People 2020.

The latest CDC report didn't look at the policy's effect on cases of chickenpox, but previous studies found incidence and outbreaks have declined 67% to 76% since ACIP recommended a two-dose regimen in 2007.

- read the CDC report