Upsurge of conjugate vaccine serotype 4 invasive pneumococcal disease clusters among adults experiencing homelessness in California, Colorado, and New Mexico.

After 7-valent pneumococcal conjugate vaccine introduction in the US in 2000, invasive pneumococcal disease (IPD) due to serotype 4 greatly decreased in children and adults. Starting in 2013, serotype 4 IPD incidence increased among adults >18 years old within three of ten Active Bacterial Core surveillance sites. Of 325 serotype 4 cases among adults in 2010-2018, 36% were from persons experiencing homelessness (PEH); incidence of serotype 4 IPD among PEH was 100-300 times higher than in the general population within these 3 areas. Genome sequencing for isolates recovered during 2015-2018 (n=246), revealed that increases in serotype 4 IPD were driven by lineages, ST10172, ST244, and ST695. Within each lineage, clusters of near-identical isolates indicated close temporal relatedness. Increases in serotype 4 IPD were limited to Colorado, California, and New Mexico, with highest increases observed among PEH, who were at increased risk for exposure to and infections caused by these strains.