A nested case-control study measuring pertussis vaccine effectiveness and duration of protection in Manitoba, Canada, 1992-2015: A Canadian Immunization Research Network Study.

BACKGROUND

Pertussis persists in Manitoba despite the universal availability of pertussis vaccines. Recent cases have included previously vaccinated individuals, raising concerns about declining vaccine effectiveness (VE). We measured pertussis VE and duration of protection using Manitoba's provincial immunization and communicable disease registries.

METHODS

Using a nested case-control design, individuals with laboratory-confirmed pertussis in Manitoba diagnosed between April 1, 1992, and March 31, 2015, were matched to up to five population-based controls on age, gender, geography, and case physician or number of physician visits. Conditional logistic regression was used to estimate VE against pertussis for both the whole-cell (wP) and acellular (aP) pertussis vaccines. Duration of protection was assessed using time since last dose.

RESULTS

Data on 534 eligible cases and 2614 controls were available for analysis. The adjusted VE estimate for aP-containing vaccines was 85% (95%CI: 74-91%); VE was 89% (66-96%) one to three years after the last vaccination. The adjusted VE of wP-containing vaccines was -15% (-91-31%) during a large outbreak in 1994 and 1995 compared to 35% (-26-66%) during non-outbreak years.

CONCLUSIONS

Our estimates suggest that the aP vaccine was effective in preventing pertussis since its introduction in Manitoba. VE was lower during a large outbreak, highlighting the importance of separately analyzing outbreak periods when estimating pertussis VE over time.