The data from our 1968-69 influenza vaccine field trials are anlaysed and pre-challenge haemagglutinin and neuraminidase serum antibodies are evaluated as indices of protection. Prevention of flu-like disease, fever, confinement to bed, and/or seroconversion to Hong Kong was significantly related to post-vaccine A/Hong Kong/68(H3N2) haemagglutination-inhibition (HI) titres. Prevention of disease was also related, although not significantly statistically in every category, to pre-challenge A/Hong Kong/68 neuraminidase inhibition (NI) titres. The trend was the same regardless of whether the origin of the NI antibody was through A/Aichi/68 or A/Japan/62 vaccines or through pre-Hong Kong influenza infections. In summarizing the data using fever as an index of disease, the attack rate (AR) among volunteers without Hong Kong NI or HI antibody was 45%. Presence of NI antibody, in the absence of HI antibody, significantly reduced the AR to 24%. Those with both NI and HI titres experienced a still lower AR of 14%. Those with HI and NI titres both > 1: 160 ran little risk of disease, with an AR of 7%.
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