An early wave of influenza in northern England during the 1951 influenza season was documented with even higher mortality than that of the 1918 pandemic, but the situation never evolved into a pandemic. Similarly, the public health community acted swiftly to develop a vaccine in response to human cases of swine influenza in 1976, and a pandemic did not occur. During the early phase of this epidemic, the rapid identification of persons who are likely to have severe disease, as compared with those who are likely to have mild disease, can guide epidemic or pandemic response strategies. Our outline of the age-stratification profile of risk provides a possible foundation for control strategies on the basis of the biologic plausibility of partial protection from earlier exposure. Further studies are under way in Mexico to elucidate other potential risk factors for severity of S-OIV infection to guide targeted control efforts.


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