7-Valent pneumococcal conjugate vaccine and lower respiratory tract infections: effectiveness of a 2-dose versus 3-dose primary series.

Background: Immunogenicity studies suggest antibody responses from a 7-valent pneumococcal conjugate vaccine (PCV7) regimen consisting of 2 doses in the primary series are less immunogenic, for at least several vaccine serotypes, compared with a regimen consisting of 3 doses; evidence of effectiveness for prevention of invasive pneumococcal disease for both regimens is available but comparative data are lacking for prevention of lower respiratory tract diseases (LRTD).

Methods: We compared rates of LRTD between children who were born in 2002 and received 2 versus 3 PCV7 doses in the primary series, both before and after receipt of the booster dose, using a retrospective matched-cohort design and health insurance claims data. Two-dose and 3-dose children were matched (1:1) using propensity scoring. Cumulative rates of hospital admissions and outpatient visits for LRTD were tallied during the post-primary/pre-booster period and the post-booster period (to age 3 years), respectively.

Results: During the post-primary/pre-booster period, 3-dose children (n=3293) had 7.8 (95% CI: 0.8 to 14.8) fewer LRTD-related hospital admissions (per 1000 children) and 57 (95% CI: -6 to 128) fewer LRTD-related outpatient visits (per 1000 children) than matched 2-dose subjects (n=3293). During the post-booster period, the numbers of LRTD-related hospital admissions and outpatient visits did not differ significantly between 3-dose and 2-dose children.

Conclusions: Our findings suggest that a 2-dose PCV7 primary series, while conferring savings from reduced vaccine costs in comparison with a 3-dose primary series, also may confer less protection against LRTD in the first year of life, at least during the period soon after the vaccine is introduced.