<p>　To combat the increase in the cases of refractory acute otitis media (AOM) caused by drug-resistant Streptococcus pneumoniae (DRSP) since 2000, penicillin antibiotics (PCs) were proposed as first-choice drugs for the treatment of childhood acute upper respiratory tract infections. The 7-valent pneumococcal conjugate vaccine (PCV7) was subsequently introduced in 2010, and it was followed by the 13-valent pneumococcal conjugate vaccine (PCV13) in 2013. Since then there have been reports of decreases in the number of AOM cases, in detections of S. pneumoniae and DRSP involvement in AOM, and in severe AOM cases requiring myringotomy. Attending group child care and/or having a sibling attending group care (group care factor) and age below 2 years (age factor) are regarded as background factors for DRSP detection, but there has been only a handful of reports on the change of these background factors. We therefore targeted the 794 AOM cases from which S. pneumoniae was isolated during 2005-2009 (period I) before the introduction of PCVs and the 382 AOM cases from which it was isolated during 2016-2020 (period II) after the introduction of PCVs, and retrospectively investigated the frequency of DRSP detection and background factors. PCs were the first-choice drugs for the treatment of childhood upper respiratory tract infections in both my hospital and surrounding medical institutions throughout periods I and II. The results showed that the frequency detection of DRSP was significantly lower in period II than in period I, and there were no significant differences between the frequency of DRSP detection according to whether the group care factor or age factor was present or absent in period II. Similar results have been reported in a pediatric survey, suggesting the efficacy of introducing the PCVs.</p>