A study of influenza in residential schools provided the opportunity to assess the significance of antibody as a predictor of immunity. Five hundred and fifty-six pupils from 8 schools were included in the investigations, and the outcome for these children in 27 naturally occurring outbreaks of influenza was analysed. The outbreaks comprised 5 caused by strains of influenza A H3N2, 10 caused by strains of influenza A H1N1, and 12 caused by strains of influenza B. On 8 occasions a second outbreak of the same serotype occurred in a school.
There was a general correlation between the presence of antibody to the outbreak strain and protection from infection. For each of the three influenza virus serotypes the infection rate in those with no detectable antibody was approximately 80%. Those with past experience of the virus but no antibody to the outbreak strain experienced lower infection rates (62% overall) but the infection rates were lowest in those with intermediate and high level antibody to the challenge strain (18% overall).
Vaccine was used by three of the schools. The effect of antibody derived from recent experience, either natural or vaccine-induced, on subsequent challenge with a drifted strain i.e. one showing antigenic drift away from the previous strain, was compared. Intermediate or high level antibody to the challenge strain in those who had experienced a recent natural infection was associated with a low infection rate (9%). A similar level of antibody produced in response to vaccination was associated with a significantly higher infection rate (23%:P 0-025). Among the vaccinees who had produced such antibody the infection rate was highest (32%) in those who had responded to vaccine in the presence of antibody to the vaccine strain.
The evidence from this study indicates that whilst antibody surveys of populations may provide some information about susceptibility to challenge with new strains of influenza viruses, the cirucumstances of the induction of the antibody affect its value as a predictor of immunity.
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