Investigation of an Influenza A (H3N2) outbreak in evacuation centres following the Great East Japan earthquake, 2011

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.

An outbreak of influenza A (H3N2) occurred in the ECs after the Great East Japan Earthquake of 2011. Natural disasters, including earthquakes, may lead to a surge of evacuees, population displacement, and a breakdown of the healthcare system in affected areas. Consequently, the risk of an infectious disease outbreak after a natural disaster is increased because of various factors such as poor hygiene and overcrowding of emergency shelters [20]. A variety of infectious diseases can potentially cause outbreaks following natural disasters [6]; however, the source of infection is more likely to be a microbial agent circulating in the community rather than one introduced from the outside [8]. The Great East Japan Earthquake occurred on 11 March 2011 when influenza was still circulating in Japan; therefore, the possibility of an influenza outbreak at that time was increased as reported in a previous study [15]. The influenza outbreak began more than 1 week after the earthquake and simultaneously spread to 5 ECs and accounted for 105 evacuees in Yamamoto. ECs where higher AR was recorded had the adult male case as an index case and 51.7% and 71.0% of cases were aged between 15 and 64 years in EC (B) and (C) respectively. Although an exact transmission route was not identified, our findings suggested that the influenza virus was introduced into the ECs through search and rescue activities, particularly in the early phase of this outbreak, in which a large number of working age adults were engaged together with others arriving from affected areas. Previous molecular study observed different clades of influenza A (H3N2) before and after the earthquake [14]. Similar to the cholera outbreak after the Haitian earthquake in 2010, molecular evidence indicated that Vibrio cholerae was introduced from outside of the affected area [21–23]. Taken together, these findings indicated a potential risk of the introduction of infectious diseases into affected areas by relief workers. Thus, it is important to ensure the health status and vaccination history among relief workers before they are permitted to enter affected areas.

The AR in children <15 years old was estimated to be less than the average AR in both EC (B) and (C), but this inding did not directly reflect a low proportion of children in the ECs because the proportions were compatible with those reported in the 2010 National Census. In fact, children are more frequently in contact with their parents and relatives than schoolmates or playmates as indicated by a study regarding social mixing during school holidays [24]. In our study, the majority of childhood cases had either a room link or a familial link [80% in EC (B) and 67% in EC (C)]. Other than these two ECs, there was one influenza case each in EC (A) and EC (E), in which the patient was <15 years. The latter case was the index case, but the route of infection remained unclear.

There were no deaths reported in this outbreak; however, the population of ≥65 years accounted for 21.9% cases. Taken together with the fact that influenza A (H3N2) cases are more severe, particularly in the elderly [25], this outbreak had the potential to cause significant morbidity and mortality, which motivated healthcare officials in Japan to prioritize measures to prevent the spread of influenza in ECs and to promote early treatment, particularly in the elderly. Finally, the mean AR was 5.3% (range, 0.8%–11.1%), which was relatively low, indicating successful interruption of influenza transmission by implementing several preventive measures such as active case identification, case cohorting in designated rooms, and wearing masks as previously described [15].

This study had some limitations. First, we could only obtain 27 POC residues from the 105 cases and 22 were positive for influenza A (H3N2). However, a surveillance study revealed a predominant circulation of influenza A(H3N2) in ECs in Miyagi Prefecture [11]. We believe that the observed outbreaks were caused by influenza A(H3N2). Second, we could only obtain background information of the evacuees in two ECs; therefore, we may have missed some characteristics of influenza transmission, particularly in the ECs with only two cases. Third, information regarding control measures such as wearing masks and vaccination history was insufficient; therefore, it was somewhat difficult to evaluate the effectiveness of control measures in the ECs. In spite of these limitations, a combination of preventive measures consisting of case cohorting, personal hygiene, wearing masks and early detection and treatment led to a low influenza AR.

In conclusion, influenza can cause outbreaks in a disaster setting when the disaster occurs during an epidemic influenza season. The transmission route is more likely to be associated with same-room and familial links. The importance of monitoring influenza in ECs for implementing preventive control measures should be emphasized.