Low Attack Rate of Summertime Influenza: Could It Be the Host?

Abstract

Sir—I read with interest the report of an outbreak of summertime influenza caused by virus that was imported into a military barracks [1]. The authors offered several potential explanations for the unusually low attack rate, but left out one of the most obvious: the fact that the outbreak occurred during the summer. A recent hypothesis proposes that human physiology may change in response to summertime lighting conditions and that we periodically become relatively resistant to agents that cause infections in the winter [2]. Whether this would explain the low secondary attack rate in this military population, which was living in close quarters, is worth considering. More importantly, it might help explain why influenza does not spread in epidemic form in the United States, except during the winter, despite what must be hundreds of similar virus importations by travelers returning from the Southern Hemisphere each summer. Because the barracks housed >1100 language students from around the world, the authors may have the means at hand to test this hypothesis. If it is true, one would expect higher attack rates among those students who recently arrived from countries in the Southern Hemisphere.