Emergence of Streptococcus pneumoniae serotype 15A after the introduction of the conjugate vaccine in Victoria

To the Editor: Streptococcus pneumoniae (the pneumococcus) is a major cause of severe bacterial disease worldwide. Since the introduction of the pneumococcal conjugate vaccine (PCV) there have been changes in the epidemiology of invasive pneumococcal disease (IPD), with an increase in infections with pneumococcal serotypes not included in the conjugate vaccines.1 The Microbiological Diagnostic Unit Public Health Laboratory at the University of Melbourne commenced pneumococcal serotyping in 1994. Before the PCV was included in the national immunisation schedule in 2005, a single isolate of S. pneumoniae serotype 15A, a non-vaccine serotype, had been detected in 1999 among Victorian invasive infections. The next serotype 15A invasive isolate was not seen until April 2009. The number of serotype 15A infections began increasing thereafter and accounted for 4% (13/295) of Victorian IPD from 1 January until 30 September 2012.

Increases in serotype 15A infections have been noted elsewhere,2,3 including in outbreak situations.4 No links have been found between the 30 Victorian patients notified to the Department of Health with serotype 15A infection from 1 January 2009 until 30 September 2012. No geographical clustering was noted. Most infections have been among patients over 40 years of age (29/30). Indigenous status was known for 26 patients, of whom none identified as Aboriginal or Torres Strait Islander. Seven patients (23%) died. Underlying medical conditions were known for 25 patients: 14 were immuno-compromised and 11 had chronic conditions. Clinical manifestation was known for 27 patients, with pneumonia being the most common condition (21/27). Seven patients had previous hospitalisations for pneumonia.

Twenty-two patients infected with serotype 15A were aged 65 years and over. Of these, 15 had been vaccinated with the 23-valent polysaccharide vaccine (PPV23), three had not, and the vaccine status of the remaining four was unknown.

Most of the patients with serotype 15A (27/30) had infections susceptible to penicillin and ceftriaxone, but as has been found elsewhere, high-level resistance (> 256 μg/mL) to macrolides was common (29/30).5 Pulsed-field gel electrophoresis (PFGE) was used to identify a predominant pattern. Multi-locus sequence typing (MLST) was performed on the 1999 isolate, and five recent isolates with differing PFGE patterns.6 All were identified as MLST sequence type 63 (ST63). Increases in serotype 15A worldwide have been shown to result from the expansion of an ST63 clone present in small numbers in the community before PCV introduction.7

Serotype 15A is not included in any of the conjugate vaccines (7-valent, 10-valent or 13-valent PCV) or the PPV23. Its abrupt appearance in the Victorian population, along with its high-level antimicrobial resistance and increased mortality rate, shows the importance of continued enhanced surveillance of IPD.