Observed Pneumococcal Carriage Among Alaska Native Children Who Received Reduced-Dose Schedules of 13-Valent Pneumococcal Conjugate Vaccine Between 2010 and 2012

To the Editor—In 2010, the World Health Organization (WHO) and the US Advisory Committee on Immunization Practices (ACIP) recommended the use of the 13-valent pneumococcal conjugate vaccine (PCV13) for children aged 6 weeks to 71 months in schedules of 3 primary infant doses (3 + 0), 3 doses with a booster after 1 year of age (3 + 1), or 2 primary infant doses with booster (2 + 1) [1, 2]. In countries where PCV13 use has eliminated vaccine-type disease, experts question whether a 1or 2-dose PCV13 schedule is sufficient to maintain indirect protection [3]. In immunogenicity studies, 81%–100% of children demonstrated sufficient antipolysaccharide immunoglobulin after 1 infant dose [4]. However, studies of clinical outcomes have been inconclusive. In a case-control study in the United States, low numbers of cases prevented investigators from calculating relative odds of invasive disease between partial dose groups [5]. Carriage studies can provide another marker of vaccine effectiveness, although comparisons of full and reduced PCV13 schedules and carriage have not been reported. From 2008 through 2013, we conducted annual cross-sectional pneumococcal carriage surveys among Alaskan children aged <5 years [6]. In 2010, PCV13 replaced 7-valent pneumococcal conjugate vaccine (PCV7) in Alaska’s childhood immunization schedule. During this time, children who started the vaccination schedule with PCV7 completed it with PCV13. For example, infants who received 2 doses of PCV7 could receive a final primary and booster dose of PCV13, yielding a PCV13 schedule of 1 + 1. Children enrolled during this period provided the opportunity to compare carriage of the 6 additional PCV13 serotypes (ie, 1, 3, 5, 6A, 7F, or 19A) across partial PCV13 dose groups. For each study participant, we obtained a nasopharyngeal swab specimen for pneumococcal identification and serotype determination [7]. Children were included if they received their first dose of PCV before 6 months of age. Primary doses were any dose before 12 months of age; booster doses were any dose after 12 months. Children receiving only PCV7 were included in the category of “no PCV13 doses.” We compared the prevalence of carriage of the 6 additional PCV13 serotypes between dose groups using generalized estimating equations. Models were adjusted for the year of study enrollment. From 2010 to 2012, 2762 children were enrolled; 51% were colonized with pneumococcus. Receipt of any PCV13 vaccine increased from 8% to 94%, while carriage of the additional 6 PCV13 serotypes declined from 9% to 3% (Table 1). We did not detect any statistical differences in carriage of the additional PCV13 vaccine types comparing reducedor fulldose groups to children who received no PCV13. The statistical power for these comparisons ranged from 4% to 25%. Our results indicate that we had insufficient power to detect a difference between dose groups, despite a large sample size. To evaluate the effect C O R R E S P O N D E N C E