Prolonged Pertussis Outbreak in Ontario Originating in an Under-immunized Religious Community

Area of Science:

• Epidemiology
• Infectious Disease Control
• Public Health

Background:

• A significant pertussis outbreak occurred in Ontario starting in November 2011.
• The outbreak originated in an under-immunized religious community and spread to the general population and a second religious community.

Purpose of the Study:

• To compare pertussis epidemiology between religious communities and the general population.
• To understand transmission dynamics during a prolonged outbreak.

Main Methods:

• Analysis of pertussis cases reported between November 2011 and April 2013.
• Utilized the integrated Public Health Information System (iPHIS) for case data.
• Assessed community membership, immunization status, treatment, and outcomes through case investigations.

Main Results:

• 443 confirmed and probable cases were reported across 7 health units.
• The outbreak began in one religious community (138 cases), spreading to the general population (273 cases) and a second community (32 cases).
• Cases in the religious community were younger, higher risk, and less immunized; the general population had cases primarily aged 10-14 years with waning immunity.

Conclusions:

• Pertussis epidemiology differs significantly between under-immunized religious communities and the general population.
• Outbreak transmission to the general community is expected.
• The high proportion of immunized cases in the general population requires further investigation.