Background In the United States, the 13-valent pneumococcal conjugate vaccine has been recommended for children since 2010 and for adults aged ≥65 years since 2014. We assessed S. pneumoniae antimicrobial nonsusceptibility among adults with suspected pneumonia from hospital settings.

Methods Isolates were collected from 105 US sites between 2009 and 2017 in the SENTRY Antimicrobial Surveillance Program. Clinical and Laboratory Standards Institute methods were used for susceptibility testing. Serotypes were determined by cpsB sequence obtained by PCR or whole genome sequencing, plus multiplex PCR and/or Neufeld Quellung reactions as needed.

Findings Of 7254 S. pneumoniae isolates analyzed, 63.6% and 36.4% were from patients aged 18‒64 and ≥65 years, respectively. Among all isolates, penicillin and ceftriaxone nonsusceptibility declined by 72.3% and 73.8%, respectively, with smaller changes observed for other antibiotics. Nonsusceptibility patterns were serotype-specific; for example, nonsusceptibility was relatively stable for serotype 19A but declined for 19F. Simultaneously, the percentage of serotype 19A isolates decreased from 17.4% to 3.9%, whereas for serotype 19F this percentage increased from 2.8% to 5.0%. The percentage of serotype 3 isolates that were nonsusceptible increased for select antibiotic classes, and the percentage of serotype 3 among all isolates increased minimally from 10.2% to 11.8%.

Interpretation Overall pneumococcal nonsusceptibility patterns were influenced by distinct patterns within serotypes, indicating the likelihood of serotype-specific resistance mechanisms. Serotype 19A observations were consistent with vaccine-induced reductions in circulation with no change in the organism susceptibility, whereas the nonsusceptibility increases for serotypes 3 and 19F may indicate circulation of more antibiotic-resistant clones.