Effectiveness of acellular pertussis vaccine and evolution of pertussis incidence in the community of Madrid from 1998 to 2015.

INTRODUCTION: Pertussis is a communicable disease that primarily affects infants. Vaccination has led to an important reduction in the incidence of the disease, however, resurgence of the disease has been observed. This study aimed to analyze the incidence of pertussis and assess the vaccination effectiveness (VE) of different schedules of acellular pertussis vaccination in the community of Madrid.

METHODS: Pertussis cases notified to the Mandatory Disease Reporting System from 1998 to 2015 were analyzed. Five comparison periods were created: 1998-2001 (reference), 2002-2005, 2006-2009, 2010-2012 and 2013-2015. The incidence ratio (IR) between inter-epidemic periods was analyzed using a Poisson regression. VE was calculated using the screening method. Vaccine status data were collected from the vaccine registry.

RESULTS: In total, 3855 cases were notified. Inter-epidemic periods were observed every 3-4 years. The incidence increased (IR: 5.99, p < 0.05) in the 2013-2015 period, particularly among infants younger than 1 month (IR: 32.41, p < 0.05). Vaccination data were available in 89% of cases. For those receiving the last dose at ≤6-month VE was 89.9% (95% confidence interval (CI): 87.3-92.0) after one year of follow-up, and 85.5% (95% CI: 82.4-88.1) after 11 years of follow-up. For those receiving the last dose at 18-months VE decreased from 98.8% (95% CI: 98.3-99.1) to 85.1% (95% CI: 81.9-87.7) in the same period, and for those receiving the last dose at 4-year VE decreased from 99.6% (95% CI: 99.3-99.7) to 79.3% (95% CI: 74.6-83.1).

CONCLUSIONS: B. pertussis is circulating in our population, as shown by the epidemic peaks and increased incidence of pertussis in recent years. VE increased with the number of doses and decreased with the follow-up period. The effect of this and other vaccination strategies must be monitored to control the disease.