Effective immunization with live attenuated influenza A virus can be achieved in early infancy

The immunogenicity of vaccines can be modified in infancy by maternal antibodies and other immunizations. Hence, the safety and immunogenicity of two doses of 10⁷ TCID₅₀ of live, attenuated cold-adapted (ca) influenza A/Kawasaki/86 (H1N1) reassortant virus vaccine given with or apart from childhood immunizations were evaluated in infants, starting at 2, 4, or >6 months of age, in randomized, double-blind trials. The ca vaccine was safe and did not significantly reduce the immunogenicity of the childhood vaccines in these infants. Infectivity of ca virus (virus shedding, ≥4-fold rise in serum hemagglutination inhibition antibody titer, or both) was affected by age, quantity of ca virus given, and prior ca virus infection. Two doses of 10⁷ TCID₅₀ of ca influenza virus infected all infants, indicating that both doses are probably needed to achieve immunity against influenza in infants <6 months of age.