Subclass distribution of IgA antibodies in saliva and serum after immunization with Haemophilus influenzae type b conjugate vaccines

IgA subclass distribution of antibodies against capsular polysaccharide (PS) of Haemophilus influenzae type b (Hib) was studied in saliva and serum samples of children vaccinated with two (n = 58) or three doses (n = 53) of Hib vaccine. One month after the second dose of Hib conjugate vaccine, at 7 months old, 40% of the children had IgA1 and 41% had IgA2 anti-Hib PS antibodies in saliva. One month after the third dose, at 15–25 months old, IgA1 was the predominating subclass; 72% of the children had IgA1, 26% had IgA2 anti-Hib PS in saliva. The mean concentration of IgA1 anti-Hib PS, expressed as optical density (OD) values, was significantly higher after three doses (OD 80.7) than after two doses (OD 18.9). The mean concentration of IgA2 did not change significantly after the third dose (OD 23.8 after two doses, OD 18.1 after three doses). In serum, IgA1 anti-Hib PS predominated both after two (17% had IgA1, none had IgA2) and three doses (72% had IgA1, 4% had IgA2) of Hib vaccine. In conclusion, both IgA1 and IgA2 anti-Hib PS were found in saliva of immunized children after two doses of Hib conjugate vaccine, whereas the third vaccine dose induced a shift towards IgA1 anti-Hib PS dominance in saliva.