The effect of pneumococcal conjugate vaccines on incidence and microbiology associated with complicated acute otitis media.

Objectives: The objectives of this study were to investigate the incidence of complicated acute otitis media (cAOM) as well as the associated microbiology before and after introduction of the 7- and 13-valent pneumococcal conjugate vaccines (PCV-7 and -13), respectively. CAOM comprises "heavy" AOM (AOM demanding hospitalization), mastodismus (M) and acute mastoiditis (AM).

Methods: A retrospective cohort study of the incidence and microbiology associated with cAOM during the non-PCV era, the PCV-7 and 13 eras, respectively. Clinical and microbiological data were prospectively registered in a local database. The incidences of cAOM as well as the distribution of various bacterial strains in the three eras were compared.

Results: A total of 246 cases of cAOM (125 in the pre-vaccine period (2001-2006), 50 in the PCV-7 period (2007-2010) and 71 in the PCV-13 period (2011-2015)) were identified. The incidence of hAOM decreased by 62% in the PCV7-era but increased to almost pre-vaccine levels in the PCV-13 era. In the M + AM group, a decrease by almost 21% in the PCV7-era was found compared to the pre vaccine era, whereas the decrease was only 12% in the PCV13-era. The three most common findings in both hAOM and M + AM were Streptococcus pneumonia (SP), group A streptococcus (GAS) and "no growth". In the hAOM group, SP decreased from 38% in the pre-vaccine era to 31% in the PCV7-era and further to 16% in the PCV13-era. GAS decreased from 17% in the pre-vaccine era to 0% in the PCV7-era and 16% in the PCV13-era. The percentage of "no growth" increased from 12% to 38% and 44%, respectively. In the M + AM group, SP decreased to 10% in the PCV13-era compared with 44% in the pre-vaccine era and 41% in the PCV7-era. An increase in GAS from 15% in the pre-vaccine era and PCV7-era to 30% in the PCV13-era was observed. The "no growth" percentage increased from 13% in the pre-vaccine era to 26% in the PCV7-era and 33% in the PCV13-era.

Conclusion: Introduction of PCV7 and PCV13 has been associated with an overall reduction of cAOM in Central Region Denmark. Pneumococci were still one of the two most common bacteria species related to cAOM though a decrease in pneumococci positive cases was observed. We found an increase in M + AM induced by GAS and a relatively large increase in "no growth", which might be caused by a more aggressive pre-hospital approach to treatment with antibiotics. Consequently, it is not evident whether the reduction of incidences is caused by the vaccines or a more aggressive antimicrobial attitude to manage AOM. The shift to GAS from SP is worrisome, and therefore continuous surveillance of the microbiology associated with AOM is warranted.