The Great East Japan Earthquake of magnitude 9.0 that struck on 11 March 2011 resulted in more than 18000 deaths or cases of missing persons [
1]. The large-scale tsunami that followed the earthquake devastated many coastal areas of the Tohoku region, including Miyagi Prefecture, and many residents of the tsunami-affected areas were compelled to reside in evacuation centres (ECs). Reportedly, more than 1300 ECs were established to accommodate more than 315000 evacuees in Miyagi Prefecture [
2]. These ECs included officially designated centres and other buildings such as community centres, city halls, nursing homes, and schools to manage the huge surge of evacuees.
Japan has one of the most rapidly aging populations and the 2010 National Census found that 23% of the Japanese population was 65 years or older [
3]. The coastal area of the Tohoku region, which was most severely affected by the tsunami, has a higher percentage of elderly residents than the national average and, consequently, an increase in the incidence of some diseases that affect the elderly, such as pneumonia [
4], occurred after the earthquake.
After a natural disaster, the risk of infectious disease outbreaks often becomes a major concern. Infectious diseases that may cause outbreaks in post-disaster settings can be categorized into four groups: waterborne diseases, acute respiratory infections, vector-borne diseases, and infections as a result of wounds or injuries [
5]. Of these diseases, acute respiratory infections, including influenza, are among the most common that occur after natural disasters [
6,
7]. However, there is limited information regarding influenza outbreaks after natural disasters [
8] partly because of the limited ability to confirm suspected cases [
7]. The influenza virus is one of the most common causes of acute respiratory diseases that are usually self-limited but can sometimes lead to severe complications such as pneumonia and influenza associated encephalopathy [
9]. In Japan, influenza has been monitored under national surveillance with approximately 5,000 sentinel sites [
10] and the estimated number of influenza outpatients was higher among children less than 10 years while approximately 45% of reported influenza inpatients were the elderly aged over 70 years in 2011/2012 season [
11]. Residents aged ≥65 years are prioritized for influenza vaccination; 51% of the elderly of that age was vaccinated in 2006, which has been increased from 17% in 2000 [
12]. In Japan, seasonal influenza epidemics usually occur between December and March. At the time of the Great East Japan Earthquake on 11 March 2011, influenza A (H3N2) was still circulating, and there was an increasing trend of influenza B toward April 2011, both nationwide [
13] and in the affected areas [
14,
15]. Studies regarding influenza-associated mortality revealed that a higher impact was associated with influenza A (H3N2) compared with other seasonal influenza viruses [
16,
17] and influenza A(H1N1)pdm09 during the pandemic period [
18], particularly among the elderly. Because the majority of evacuees in the ECs were elderly, there was a heightened concern regarding severe outbreaks due to influenza A (H3N2). To respond to a potential influenza outbreak in the ECs, we conducted an outbreak investigation in ECs located in Yamamoto, Miyagi Prefecture, Japan. In this study, we describe the epidemiological characteristics of influenza in an EC setting.