An outbreak of influenza A2 occurred in 1963 in a children's home five months after half the population had been given two injections of commercial polyvalent vaccine. About 90% of the children who had received the influenza vaccine before the outbreak had an antibody titer of 1:80 or higher against the 1962 strain of influenza A2. The frequency of clinical illnesses for immunized children was slightly but significantly less than for the nonimmunized children. The 1963 outbreak of influenza A2 had a lower infectivity rate, occurred over a longer period of time, and was associated with fewer severe illnesses than the outbreak of influenza B which occurred in the same home in 1962, when no child had received influenza vaccine.
Moffet HL, Cramblett HG, Dobbins J, Salem W. Outbreak of Influenza A2 Among Immunized Children. JAMA. 1964;190(9):806–810. doi:10.1001/jama.1964.03070220012002