Monitoring of influenza viruses in the aftermath of the Great East Japan earthquake.

Influenza has a significant impact on public health when a natural disaster occurs during the influenza season. However, the epidemiological characteristics of influenza following natural disasters have not been well documented due to the difficulty of implementing laboratory-based influenza surveillance in such situations. The Great East Japan Earthquake occurred on March 11, 2011, when influenza was already circulating. Since routine influenza surveillance was not performed in Miyagi Prefecture, we initiated an ad hoc laboratory-based monitoring system immediately after the earthquake. From March 15 to May 19, we tested 277 samples for influenza virus collected around Sendai City and from evacuation centers in Miyagi Prefecture. Influenza A (H3N2) was detected in 112 cases, influenza A (H1N1) 2009 in one case, and influenza B in 92 cases. The H3N2 virus was dominant until the 14th week. However, a sudden increase in the number of influenza B cases occurred after schools were reopened. According to phylogenetic analysis, a major clade switch of the H3N2 virus took place after the earthquake. The Yamagata lineage of influenza B was detected in one patient from western Japan, indicating the importing of viruses into the affected area.