Potential carrier priming effect in Australian infants after 7-valent pneumococcal conjugate vaccine introduction

AIM:To investigate evidence of clinical protection in infants after one dose of 7-valent pneumococcal conjugate vaccine(7vPCV) owing to carrier priming.METHODS:Using Australian National Notifiable Diseases Surveillance System data,we conducted a descriptive analysis of cases of vaccine type invasive pneumococcal disease(VT-IPD) during "catch-up" years,when 7vPCV was carrier primed by prior administration of DTPa vaccine.We compared the number of VT-IPD cases occurring 2-9 wk after a single dose of 7vPCV(carrier primed),with those < 2 wk post vaccination,when no protection from 7vPCV was expected yet.Further comparison was conducted to compare the occurrence of VT-IPD cases vs non-VT-IPD cases after a single carrier-primed dose of 7vPCV.RESULTS:We found four VT-IPD cases occurring <2 wk after one carrier primed dose of 7vPCV while only one case occurred 2-9 wk later.Upon further comparison with the non-VT-IPD cases that occurred after one carrier primed dose of 7vPCV,two cases were detected within 2 wk,whereas seven occurred within2-9 wk later;suggesting a substantial level of protection from VT-IPD occurring from 2 wk after carrier-primed dose of 7vPCV.CONCLUSION:This data suggest that infants may benefit from just one dose of 7vPCV,likely through enhanced immunity from carrier priming effect.If this is proven,an adjusted 2-dose schedule(where the first dose of PCV is not given until after DTPa) may be sufficient and more cost-effective.