Background and Objective:
Streptococcus Pneumoniae is a leading cause of mild to severe respiratory infections worldwide, especially among children below 5 years of age. In Sweden, a pneumococcal conjugated vaccine (PCV) covering 7 serotypes of S. Pneumoniae (PCV7) was introduced in 2007, and was replaced by a thirteen-valent vaccine (PCV13) in 2011. A previous study, conducted between 2003 and 2012 reported a decrease in pneumonia and sinusitis among young children after the introduction of PCV7, in addition to the known effect of the vaccine on invasive pulmonary disease (IPD) burden. The aim of this study is to follow-up on the incidence of pneumonia, sinusitis and IPD in Stockholm between 2012 and 2016.

Methods:
This is a population-based study of hospital registry data on hospitalization of children (aged 0-18 years) with a diagnosis of sinusitis, pneumonia, mastoiditis, bacterial meningitis, empyema, viral pneumonia and pyelonephritis. The setting is Stockholm County, Sweden. The analysis was carried out comparing three time periods: 2003-2007 (before PCV7), 2008-2012 (after PCV7) and 2012-2016 (follow up, with PCV13).

Results:
Hospitalizations for bacterial pneumonia decreased in all age groups from 2003 to 2016 (from 449.7 to 339.7 /100.000 among 0-1 year old; from 250 to 170.8/100.000 among 2-4 year old; from 51.3 to 41.4 /100.000 among 5-17 year old). The incidence of sinusitis decreased significantly both among children aged 0-1 years (from 69.9 to 23/100.000) and among those aged 2-4 years (from 25 .5 to 14.3/100.000). The incidence of mastoiditis and bacterial meningitis also decreased significantly among children younger than 2 years of age (mastoiditis: from 41 to 15/100.000; bacterial meningitis: from 29 to 7/100.000).

Conclusion:
The effect of vaccination among children is therefore sustained even 8 years after the introduction of PCV7 and 5 years after the introduction of PCV13.