Invasive pneumococcal diseases in children in Hokkaido, Japan from April 2000, to March 2015

In Japan, the 7-valent pneumococcal conjugate vaccine (PCV-7) became commercially available as a voluntary vaccine in March 2010. It was included in the routine immunization schedule in April 2013 and was replaced by PCV-13 in November 2013. We evaluated 146 cases of invasive pneumococcal disease (IPD) in 142 children (2 developed the disease twice, and 1 developed it three times) treated in the northern district of Hokkaido, Japan from April 2000 to March 2015, before and after the introduction of PCV-7. The incidence rate per 100,000 people aged <5 years showed an increasing trend between April 2000 and March 2010, and reached 87.5 per 100,000 people per year between April 2009 and March 2010, which was immediately before the introduction of PCV-7. Subsequently, the incidence rate started to show a decreasing trend and reached as low as 9.5 per 100,000 people per year between April 2013 and March 2014. However, the incidence rate showed an increasing trend again between April 2014 and March 2015, reaching 33.4 per 100,000 people per year. Serotyping was performed for the 77 strains collected between April 2000 and March 2010. The most frequently isolated serotype was 6B (31.2%), followed by 23F (14.3%) and 19F (13.0%). Among them, 55 strains were covered by PCV-7 (71.4%), and 64 strains were covered by PCV-13 (83.1%). Of the 33 strains collected between April 2010 and March 2015, 14 were covered by PCV-7 (42.4%) and 16 were covered by PCV-13 (48.4%), showing a significant decrease (p < 0.01).