Effectiveness of a vaccination programme for an epidemic of meningococcal B in New Zealand.

New Zealand has experienced a prolonged epidemic of meningococcal B disease since 1991. The epidemic has waned significantly since its most recent peak in 2001. A strain-specific vaccine, MeNZB, was introduced to control the epidemic in 2004, achieving 81% coverage of people under the age of 20. The vaccine was rolled out in a staged manner allowing the comparison of disease rates in vaccinated and unvaccinated individuals in each year. Vaccine effectiveness in people aged under 20 years is estimated using a Poisson regression model in the years 2001-2008, including adjustments for year, season, age, ethnicity, region and socioeconomic status. Further analyses investigate the dose response relationship, waning of the vaccine effect after one year, and cross-protection against other strains of meningococcal disease. The primary analysis estimates MeNZB vaccine effectiveness to be 77% (95% CI 62-85) after 3 doses and a mean follow-up time of 3.2 years. There is evidence for a protective effect after 2 doses 47% (95% CI 16-67), and no evidence for a waning of effectiveness after one year. Simultaneous modelling of invasive pneumococcal disease and epidemic strain meningococcal B suggests a degree of residual confounding that reduces the effectiveness estimate to 68%. There is evidence for some cross-protection of MeNZB against non-epidemic strains. The MeNZB vaccine was effective against the New Zealand epidemic strain of meningococcal B disease. Between July 2004 and December 2008 an estimated 210 epidemic strain cases (95% CI 100-380), six deaths and 15-30 cases of severe sequelae were avoided in New Zealand due to the introduction of the MeNZB vaccine.