Serotypes and Antimicrobial Resistance of Isolates Causing Invasive Pneumococcal Disease in Adult Patients in Portugal After the 13-Valent Conjugate Vaccine Received an Adult Indication, 2012–2014

Abstract

Background. In 2010, the 13-valent pneumococcal conjugate vaccine (PCV13) replaced the 7-valent pneumococcal conjugate vaccine (PCV7) that had been given to children through the private sector since 2001. PCV13 received an adult indication in 2012 and in 2015 was included in the national immunization program for children. Methods. Cases of invasive pneumococcal disease (IPD) were defined by recovering pneumococci from normally sterile sites. All IPD isolates recovered from adults (≥18 years) in 2012-2014 were included. Isolates were serotyped by the Quellung reaction and antimicrobial susceptibility was evaluated by disk diffusion and Etest. Results. A total of 1186 isolates, mostly from blood (92%), were analyzed. Among the 54 different serotypes detected, those most frequent were as follows: 3 (14%), 8 (10%), 19A (7 %), 22F (7%), 14 (6%) and 7F (5%). PCV13 serotypes declined from 52% (2012) to 38% (2014, p < 0.001), mainly due to decreases in serotypes 1, 7F and 19A. Serotype 3 remained the most frequent cause of adult IPD presenting only minor fluctuations. The proportion of IPD caused by PCV7 serotypes remained stable during the study period (14%), but was smaller than in the previous period (19% in 2009-2011, p = 0.004). Non-PCV13 serotypes continued the increase initiated in the late post-PCV7 period, with serotypes 8 and 22F being the most important emerging serotypes. Serotype 15A increased from 2012 to 2014 and was strongly associated with antimicrobial resistance. In spite of this, decreases in the resistance associated serotypes 14 and 19A led to an overall decrease of penicillin non-susceptibility (17% to 13%, p = 0.086) and erythromycin resistance (19% to 13%, p = 0.014). Conclusion. The proportion of isolates causing adult IPD expressing PCV13 serotypes fell below 40% for the first time since 2000. Although this indicates a strong herd protection effect from children vaccination, the persistence of PCV13 serotypes in adults warrants continued surveillance to monitor the potential benefits of direct adult vaccination.