Explaining rapid reinfections in multiple-wave influenza outbreaks: Tristan da Cunha 1971 epidemic as a case study

Camacho, Anton; Ballesteros, Sébastien; Graham, Andrea L.; Carrat, Fabrice; Ratmann, Oliver; Cazelles, Bernard

On August 13, a ship returning from Cape Town landed five islanders on TdC. Three of them developed acute respiratory disease during the 8 day voyage and the other two presented similar symptoms immediately after landing. Various family gatherings welcomed their disembarkation and in the ensuing days, an epidemic started to spread rapidly throughout the whole island population. After three weeks of propagation, while the epidemic was declining, some islanders developed second attacks and a second peak of cases was recorded. The epidemic faded out after this second wave and lasted a total of 59 days (figure 1a). Among the 284 islanders, 273 (96%) experienced at least one attack and 92 (32%, mainly adults) experienced two attacks. Only a few individuals experienced their single attack during the second epidemic wave. Unfortunately, only 312 of the 365 attacks (85%) are known to within a single day of accuracy and constitute the dataset [13] . A precise description of the clinical features of the illness as well as a review of the secondary infections were provided by Mantle & Tyrrell [13] . The authors reported that 85 per cent of the first attacks were moderate or severe, and this proportion decreased to 50 per cent for the second attacks. Serological analyses of 11 infected individuals demonstrated a high level of antibody against A/H3N2, a subtype to which the population had never previously been exposed. Moreover, seroconversion of individuals infected for the first time during either the first or the second epidemic wave attests that the virus was circulating throughout the epidemic. Unfortunately, no virological analysis was conducted to show whether first and second attacks were due to antigenically differing strains of A/H3N2.