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 United States 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 within 3 of 10 Active Bacterial Core surveillance sites. Of 325 serotype 4 cases among adults in 2010-2018, 36% were 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 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 among PEH, who were at increased risk for exposure to and infections caused by these strains.

Automated summary

Following the introduction of the 7-valent pneumococcal conjugate vaccine in the United States in 2000, cases of invasive pneumococcal disease due to serotype 4 significantly decreased in children and adults. However, starting in 2013, there was an increase in serotype 4 IPD cases among adults, with a higher incidence seen among persons experiencing homelessness. Genomic sequencing identified specific lineages driving the increase in serotype 4 IPD cases, primarily affecting individuals in Colorado, California, and New Mexico.