Background: Pneumococcal polysaccharide vaccine (PPV) to prevent invasive pneumococcal disease (IPD) has been recommended for all persons aged ≥65 and for younger persons with comorbidities.  Routine vaccination of younger adults and scheduled revaccinations at 10-15 year intervals are being considered.  Recent changes in the epidemiology of pneumococcal serotypes due to the childhood conjugate vaccine complicate vaccine policy decision making.

Methods: We used a Markov model to examine seven strategies: no vaccination, one vaccination (age 50 or 65), two vaccinations (50/65 or 65/80), three vaccinations (50/65/80) or four vaccinations (50/60/70/80) in US population cohorts.  We used NHIS data to segment cohorts into comorbid illness groups to model differential vaccine effectiveness and IPD rates based on age and comorbidity.  CDC data (Active Bacterial Core surveillance) were used to model IPD rates and diminishing IPD likelihood from PPV contained serotypes.  A Delphi panel supplied estimates of PPV efficacy based on age of and time since vaccination, number of previous PPV, and comorbidities present.  Vaccination side effect data were from the literature.  We report preliminary results here, with effectiveness measured as the relative proportion of IPD cases prevented.

Results: With no vaccination, IPD risk from age 50 onward was 0.79%.  The table below gives the number of IPD cases, relative risk (RR), and the incremental relative risk reduction (IRRR) associated with each PPV strategy.

Strategy	IPD/105	RR	IRRR
No PPV	790	1.0	-
65 only	736	0.93	6.9%
50 only	716	0.91	2.7%
65/80	703	0.89	1.8%
50/65	662	0.84	6.0%
50/65/80	629	0.80	4.9%
50/60/70/80	593	0.75	5.8%

Results were sensitive to the experts' low end estimate of vaccine effectiveness, significantly decreasing incremental RRR's, particularly for revaccination at age 80.

Conclusion: Routine vaccination of 50 year olds and revaccination at 10 or 15 year intervals could substantially reduce IPD burden, despite changes in IPD serotype epidemiology.  Further analyses will examine other effectiveness endpoints and the cost-effectiveness of vaccination strategies.