Herd immunity and pneumococcal conjugate vaccine: a quantitative model

Area of Science:

• Epidemiology
• Vaccinology
• Public Health

Background:

• Invasive pneumococcal disease (IPD) significantly impacts older children and adults.
• The introduction of pneumococcal conjugate vaccine (PCV7) in 2000 targeted young children.

Purpose of the Study:

• To estimate the herd (indirect) effects of PCV7 vaccination in young children on IPD incidence in unvaccinated populations aged 5 years and older.
• To quantify the impact of varying PCV7 doses (1, 2, or >=3) on indirect protection.

Main Methods:

• Utilized a quantitative empirical model based on prospective surveillance data (Active Bacterial Core surveillance) from 1994-2003 in metropolitan Atlanta.
• Estimated PCV7 vaccine coverage in children (19-35 months) using the National Immunization Survey.
• Employed Poisson regression to analyze trends in IPD incidence among unvaccinated groups before and after PCV7 introduction.

Main Results:

• Significant declines in PCV7 serotypes were observed across all unvaccinated age groups (5-19, 20-39, 40-64, >64 years) from 2000-2003.
• Modeled incidence of PCV7 serotypes dropped by 38.4%, 62.0%, and 76.6% in unvaccinated persons 5+ years with 1, 2, and 3+ PCV7 doses, respectively.
• Greater indirect impact of PCV7 was observed in African-Americans compared to whites. Increasing trends of non-vaccine serotypes (e.g., 19A) were noted but were smaller than decreases in vaccine serotypes.

Conclusions:

• PCV7 vaccination in children induces significant herd immunity, reducing IPD incidence in unvaccinated older individuals.
• Effective herd effects can be achieved even with incomplete vaccine coverage or fewer than 3-4 doses.
• The findings highlight the broad public health impact of conjugate vaccines and suggest potential ethnic disparities in indirect protection.