The epidemiology of invasive pneumococcal disease in older adults in the post-PCV era. Has there been a herd effect?

Abstract

The 7 and 13-valent pneumococcal conjugate vaccines (PCVs) have reduced the incidence of invasive pneumococcal disease (IPD) in children in many countries. The objective of this work was to assess the impact of PCVs and potential herd-protection in older adults in Ireland. IPD notification and typing data from adults ⩾65 years of age from July 2007 to June 2016 was assessed using national surveillance data. There was a 94% reduction in PCV7 serotypes from 2007-2008 to 2015-2016, incidence rate ratio (IRR 0·05, P < 0·0001). However, there was no decline in the additional PCV13 (PCV13-7) serotypes over the same period (IRR 0·90) nor in comparison with the pre-PCV13 period 2009-2010 (IRR 0·92). The incidence of serotypes in the 23-valent pneumococcal polysaccharide vaccine only (PPV23-PCV13) and non-vaccine types (NVTs) increased significantly (IRR 2·17, P = 0·0002 and IRR 3·43, P = 0·0001 respectively). Consequently, the overall IPD incidence rate in adults has remained relatively unchanged (from 28·66/100 000 to 28·88/100 000, IRR 1·01, P = 0·9477). Serotype 19A and NVTs were mainly responsible for penicillin resistance in recent years. The decline of PCV7 serotypes indicate that the introduction of PCV7 resulted in herd-protection for adults. However, increases in PPV23-PCV13 and NVTs suggest that changes in vaccination strategy amongst older adults are needed to build on the success of PCVs in children.

Keywords: Streptococcus pneumoniae (pneumococcus); IPD in adults; Pneumococcal infection; Surveillance; typing; vaccine preventable diseases.

Fig. 1.

The IPD incidence rate (IR) per 100 000 population in Ireland by age group, from 2007–08 to 2015–16.

Fig. 2.

The IPD incidence rate (IR) in adults aged ⩾65 years of age in Ireland, per serotype group based on epidemiological years (July 2007–June 2016). PCV7 denotes the serotypes in the 7-valent conjugate vaccine. PCV13-7 denotes the serotypes in the 13-valent conjugate vaccine not included in PCV7. PPV23-PCV13 denotes the serotypes in the 23-valent polysaccharide vaccine not included in PCV13. Non-vaccine types (NV1) denote all other serotypes not included in either the PCV7/13 or PPV23 vaccines and non-typable isolates. ^aPCV7 was introduced to the childhood immunisation schedule in September 2008. ^bPCV13 was introduced to the childhood immunisation schedule in December 2010.