Inactivated vaccines. 2. Laboratory indices of protectionPresented by the senior author before the symposium on Influenza Vaccines, London, 27 April 1972.

Abstract

Summary 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%.