Outer-membrane protein subtypes of Haemophilus influenzae type b and spread of disease in day-care centers.

Isolates of Haemophilus influenzae type b with a distinctive outer-membrane protein subtype, designated 1H, were responsible for 11 of 13 cases of invasive infection in children attending six day-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 secondary attack rate in contacts younger than four years of age exposed to disease due to isolates of the 1H subtype was 16 per 1,000, compared with 1.6 per 1,000 after exposure to disease caused by subtypes other than 1H (P less than 0.008). Carriage rates 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 1H and non-1H strains were capable of colonizing contacts, but 1H strains may be more pathogenic than other subtypes.