Outer-Membrane Protein Subtypes of Haemophilus influenzae Type band Spread of Disease in Day-Care Centers

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Abstract
Isolates of Haemophilus influenzae type b with a distinctive outer-membrane protein subtype, designated IH, were responsible for 11 of 13 cases of invasive infection in children attending sixday-care centers with secondary cases of disease, in comparison to three of 14 cases in children attending 10 centers with single cases and two centers with unrelated cases. The other 11 cases were caused by type b isolates of six different subtypes. The secondaryattack rate in contacts youngerthan four years of age exposed to disease due to isolates of the IH subtype was 16 per 1,000, comparedwith 1.6 per 1,000 after exposure to disease caused by subtypes other than IH (P < 0.008). Carriagerates were increased in day-care-center contacts exposed to patients with disease, and most of the carriers were colonized by isolates with subtypes homologous to those responsible for the respective index cases. Both IH and non-IH strains were capable of colonizing contacts, but IH strains may be more pathogenic than other subtypes.
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