Vaccine impact and effectiveness of meningococcal serogroup ACWY conjugate vaccine implementation in the Netherlands: a nationwide surveillance study.

BACKGROUND In response to the recent serogroup W invasive meningococcal disease (IMD-W) epidemic in the Netherlands, meningococcal serogroup C (MenC) conjugate vaccination for 14-month-olds was replaced with a MenACWY conjugate vaccination, and a mass campaign targeting 14-18 year-olds was executed. We investigated the impact of MenACWY vaccination implementation in 2018-2020 on incidence rates and estimated vaccine effectiveness (VE). METHODS We extracted all IMD cases diagnosed between July 2014 and December 2020 from the national surveillance system. We calculated age group-specific incidence rate ratios by comparing incidence rates before (July 2017-March 2018) and after (July 2019-March 2020) MenACWY vaccination implementation. We estimated VE in vaccine-eligible cases using the screening method. RESULTS Overall, IMD-W incidence rate lowered by 61% (95%CI 40-74). It declined by 82% (95%CI 18-96) in vaccine-eligible age group (15-36 month-olds and 14-18 year-olds) and by 57% (95%CI 34-72) in vaccine non-eligible age groups. VE was 92% (95%CI -20-99.5) against IMD-W vaccine-eligible toddlers. No IMD-W cases were reported in vaccine-eligible teenagers after the campaign. CONCLUSIONS The MenACWY vaccination programme was effective in preventing IMD-W in the target population. The IMD-W incidence reduction in vaccine non-eligible age groups may be caused by indirect effects of the vaccination programme. However, disentangling natural fluctuation from vaccine-effect was not possible. Our findings encourage the use of toddler- and teenager MenACWY vaccination in national immunization programmes especially when implemented together with a teenager mass campaign during an epidemic.