Sentinel surveillance of influenza-like illness in the Central African Republic, 2010–2015

Influenza-like-illness (ILI) is a major public health problem worldwide [1]. Annual epidemics of influenza infection are estimated to result in 3–5 million cases of severe illness and 250,000–500,000 deaths. In developed countries, annual ILI epidemics infect about 10–20% of the population each season and cause febrile illnesses that range in severity from mild to debilitating and can lead in some instances to hospitalisation and even death [2]. Influenza viruses cause illness in individuals of all ages [3, 4]. Until recently, the epidemiology of ILI in developed countries was poorly understood; however, the few investigations of seasonal outbreaks in Africa reported alarmingly high case fatality rates [5‐7]. A review of the literature between 1980 and 2009 showed limited data on influenza for most countries in sub-Saharan Africa [8]. In 2006, the African Network for Influenza Surveillance and Epidemiology (ANISE) was set up to generate data on the burden and epidemiology of influenza in Africa [7, 9]. The results show a substantial burden of influenza infection in Africa [10‐17].

In the Central African Republic, acute respiratory infection is the commonest reason for paediatric consultation. In 2008, the Ministry of Health established a national influenza surveillance system to monitor the circulation of seasonal influenza and also to detect the emergence and spread of novel influenza strains with pandemic potential. The Institut Pasteur of Bangui was mandated to conduct surveillance and to identify influenza viruses in the laboratory. The surveillance system is financially supported by the United States Department of Health and Human Services. This paper reports the results of ILI surveillance in the Central African Republic between January 2010 and December 2015.

This first report of the results of sentinel surveillance for ILI in the Central African Republic shows that the influenza virus circulates, with peak activity during the rainy season (July–December). Although the seasonality of influenza viruses in African countries is not yet understood, we observed trends similar to those in other studies in tropical countries. This finding corroborates that of other countries that report increasing activity of influenza during the rainy season [11, 14, 21‐23]. It has been argued that transmission of influenza viruses is influenced by increased contact due to promiscuity during the rainy season, when people mainly stay indoors [11]; however, in some tropical zones, seasonal patterns are less pronounced, with year-round detection of virus [13]. Conflicting results have been reported on the association between climate and influenza seasonality, and increased influenza transmission in the dry season may be due to enhanced viral transmission with low humidity [24]. It is surprising that the positivity rate for influenza virus found in our study was lower than those reported in Africa before 2010 [7, 11, 13, 14, 25] and during the same period as this study [12]. The main reason for the differences in percentage positive between our findings and those of other countries is the temporal distribution of influenza viruses [14].

We also found a significant discrepancy from other studies in the influenza virus-positive rates by age. Our study shows relatively high percentages of influenza cases in infants aged 0–6 months (8.8%) and in people aged between 15 and 50 years (11.0%), while other studies reported the highest rates of influenza viral infection in children aged 5–14 years and the lowest proportion among those aged 0–4 years [7, 14]. It has been proposed that the proportion of influenza viral disease in children <6 months of age is lower because of active transport of influenza-specific maternal antibodies across the placenta [26], although this distribution pattern is not universal [27].

The distribution of pandemic influenza A/H1N1pdm09 was highest in children <5 years, as seen in other studies [14, 16, 28]. We found that influenza B virus infected mainly children <5 years, although people aged 15–50 years generally had high proportions. Maman et al. in Togo also reported that influenza B infections occurred more frequently among older children and young adults [14].

The study has some limitations. First, the data were collected only at sentinel sites in Bangui and surrounding districts and cannot be generalised to the entire population of the country. Moreover, children were over-represented, particularly in Bangui, where the two main sentinel sites provide health care mainly to children. Another limitation was that surveillance was conducted only on outpatients, whereas it will be essential to report the proportion of severe acute respiratory infection associated with influenza viruses in hospitalised cases.

Our findings contribute to understanding of the epidemiology of influenza in Africa. Influenza viruses circulate mainly during rainy season. We found that pandemic influenza A/H1N1pdm09 is circulating in the Central African Republic. Sentinel surveillance should be extended to other settings in the country, and the 91.0% of samples that test influenza-negative should be tested to identify other viral and bacterial etiologies. Furthermore, sentinel surveillance of severe acute respiratory illness should be conducted.