Invasive pneumococcal isolates from Danish infants (0-90 days) during the years 1943 to 2013

Background: The seven-valent pneumococcal conjugate vaccine (PCV-7) was introduced in the Danish childhood immunization program (at 3, 5 and 12 months of age) in 2007 and was replaced with PCV-13 in 2010 without changes to the schedule. After the introduction of these vaccines the incidence of invasive pneumococcal disease (IPD) due to vaccine types (VTs) declined markedly in children aged 0-2 years; however, cases among infants too young to be protected by vaccination have not been studied in detail. We present data on IPD in infants less than 90 days from 1943 until 2013. Study design: The study included all infants younger than 90 days born from 1943 through 2013, who had not been PCV vaccinated and from whom a pneumococcus isolate from blood or cerebrospinal fluid had been submitted to the Danish national reference laboratory. All isolates were serotyped using Pneumotest Latex and Quellung reaction. Results: A total of 216 IPD cases were identified. The age group specific incidence (total number of IPD cases per 100,000 live births) varied from 0 to 16 in the period 1943 to 2007 and from 1.7 to 9.2 in the period 2008 to 2013. IPD cases due to PCV-7 serotypes were not observed later than 2009. Conclusion: In Danish infants younger than 90 days, IPD due to PCV-7 serotypes has decreased and has not been observed since 2009, but the total incidence of IPD has not changed.