Pandemic lessons from Iceland

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Abstract
Pandemic influenza arrived in Iceland on October 19, 1918, and spread across the country with a speed and virulence that was typical of the so-called Spanish flu. The epidemiologic features of this pandemic are described by Gottfredsson et al. (1) in this issue of PNAS. These features, only now being described with precision some 90 years after the events, help to explain the unusual lethality of the historical epidemic and also highlight vulnerabilities that might be exploited for modern control measures. However, the most important contribution of the Gottfredsson et al. article is the detailed analysis of genetic susceptibility and their conclusion that family exposures outweighed genetics as risk factors for fatal influenza during this pandemic. This finding was made possible both because of the unique genealogical database that exists in Iceland and because the rapid spread of the disease across the island enabled the authors to identify pandemic deaths with relative precision. The 1918 pandemic of influenza moved across Iceland in only 42 days, a phenomenon more influenced by the very short serial interval of 4.1 days between generations of cases than by the transmissibility of the virus. Transmissibility, measured as the average number of individuals infected by each case, was modest at 2.2 for the Iceland epidemic, and mortality was 2.8% with a W-shaped age distribution curve. This is consistent with estimates from other countries (2–5) that have prompted speculation that similar viruses might be vulnerable to early containment (2, 6). During those 42 days, 521 Icelanders died, an increase of >500% from the comparable period of the year before. This unprecedented clustering of deaths in a short period also allowed the investigators to identify influenza-associated deaths with unusual precision, despite the lack of a specific diagnostic test in the previral era. Specificity in the …

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