Epidemiological, Clinical and Virological Profiles of Influenza Infection in the Democratic Republic of the Congo, from 2009 to 2018 ()
1. Introduction
Following the emergence in 1997, of a Highly Pathogenic Avian Virus A/H5N1, in Asia that spread in Europe in 2005 and Africa in 2006 (Egypt, Nigeria and Cameroon), infecting both poultry and humans [1] , some African countries like Kenya [2] , Tanzania [3] , Rwanda [4] , Nigeria [5] , Senegal [6] and Zambia [7] , had set up influenza surveillance systems with WHO and CDC/Atlanta [8] support.
Since 2007, the Democratic Republic of Congo (DRCongo) has implemented an influenza sentinel surveillance network with the following objectives: 1) to detect new influenza strains with a potential pandemic; 2) to determine epidemiological traits of influenza and other viral respiratory diseases; 3) to monitor the trend of deaths imputable to severe acute respiratory illness (SARI) patients; 4) to determine the proportion of positive influenza cases amid inpatients and outpatients; 5) to collect meaningful epidemiological, virological and socio-economic data for estimating the influenza burden in the DRCongo.
Based on sentinel surveillance data, the prevalence of influenza infection was respectively estimated to be 1205 and 48.5 for ILI (Influenza-like Illness) and SARI confirmed patients per 100,000 inhabitants (national and provincial burden) in 2016 [9] . About mortality, the case-fatality proportion was estimated to be 1.2% of SARI positive cases [10] .
This study aimed to determine influenza epidemiological, clinical, and virological profiles of patients who attended sentinel sites in DRCongo between 2009 and 2018.
2. Material and Method
1) Framework of the study
Our study focused on the eleven sentinel surveillance sites and the national influenza laboratory in the DRCongo, between January 2009 and December 2018. These sites were located both in the capital Kinshasa and in five DRCongo provinces and were also geographically representative, financially accessible, staffed with qualified health care providers, equipped with a cold chain and regularly supplied for sample collection. Two sites were medical clinics intended for ILI outpatients and nine were hospitals for both inpatients and outpatients. In Kinshasa, the following health facilities where selected: Kinshasa provincial general hospital, Kalembelembe pediatric hospital, Kingasani hospital Centre, Boyambi and RVA medical clinics. In provinces, they were: Kinkanda and Matadi Referral General Hospital in Kongo-Central, Kenya and Kisanga Referral General Hospitals in Haut-_Katanga, Charité Maternelle hospital in Nord-Kivu, and the Dipumba Referral General Hospitals in Kasai-Oriental, Boyambi and RVA medical Clinics were ILI sites, while other were ILI and SARI sites.
2) Type of study
We conducted a cross-sectional with secondary data of notified suspected influenza cases, tested with RT-PCR, from January 2009 through December 2018.
3) Study population
The study population was constituted of patients who had attended the eleven sentinel sites between January 1, 2009, and December 31, 2018 suffering for respiratory infection. As eligible case in this study, we included any inpatient or outpatient who had visited a sentinel site for respiratory issue, fulfilled ILI or SARI case definition, undergone nasopharyngeal and/or oropharyngeal sampling and tested with RT-PCR. On the other hand, we excluded from this study any other patients who has not met these requirements.
4) Sampling technique
We conducted an exhaustive sampling to obtain a large sample size for increasing the representativeness of the study population, and consequently reducing the selection bias. Of 380,867 patients who had visited the 11 sentinel sites for respiratory issues, 32,708 were ILI and SARI notified cases, among them 19,622 sampled cases. A total of 18,461 were tested with RT-PCR amid sampled cases. Those patients who performed the RT-PCR constituted our size sample, as shown in Figure 1. As statistical unit, we considered any patient who ultimately performed RT-PCR testing, from January 1, 2009, through December 31.
5) Data collection
We extracted data from the database of notification forms sent by the sentinel sites to the national influenza laboratory. We had also recourse to the surveillance system’s database of the ministry of health. All data were collected between December 2021 and March 2022 using EPIINFO.7.
6) Data processing and analysis
We processed and analysed variables like sex, age, provenance site, patient category (ILI or SARI), identified viral type and subtype and the period of illness. For that, we used Excel, Stata 16.0 for calculating frequencies, percentages as well as measures of association by using bivariate analysis, with p-value and brut odd ratio measures. For quantitative variable like age, we calculated the median as a measure of central tendency.
7) Ethical considerations
We required and obtained the IRB approval.
3. Results
Sex Analysis
Of 18461 samples tested, 1795 (9.7%) were positive, among them 1273 (79%) were ILI and 522 (21%) SARI. Positive men were 927 (51.6%), while positive women were 868 (48.4%). By Comparing positive ILI cases to positive SARI cases according to sex, men represented 51% and 54%, respectively, while 49% of positive women were positive for ILI and 46% for SARI. These differences were not statistically significant. The p-value was greater than 0.05 for both situations, as shown in Table 1.
Analysis by age
The median age of patients was four years old (range 4 - 98). By considering the age groups according to the positive cases (n = 1795), the age group < 2 years ranked first with a total of 560 cases (31.1%) of positive cases, while the age group age ≥ 65 years ranked last with 27 cases (1.5%). There were 14 cases with missing age (0.8%).
Regarding positive cases, 5 - 14 years old age group ranks first with 24% of ILI cases, while for SARI the age group < 2 years old came first with 52%. Furthermore children under five years old pilling up <2 years and 0 - 4 years old totalise 969 (54%) cases. An association between age and positive influenza cases was found in 2 - 4 and 5 - 14 years old age groups with a p-value < 0.05.
Analysis according to disease symptoms.
In this study, we considered frequently encountered symptoms in ILI and SARI patients, like fever, cough, sore throat, cold, headache, and dyspnoea. Regarding ILI patients, Table 1 shows that cough came first (98%), followed by colds (96%), fever (84%), dyspnoea (58%), sore throat (36%), and finally headaches (24%). For SARI patients, cough was still kept the first rank (98%), followed by fever (93%), dyspnoea (90%), and colds (89%). Sore throat and headaches were less frequent symptoms, respectively, with 64 and 17%. For both ILI
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Table 1. Socio-demographic and clinical characteristics of enrolled patients.
and SARI patients an association between fever ≥ 38˚C, sore throat, absence of headache, and positive Influenza was found with a p-value < 0.05. On the other hand the same association was established for SARI patients, with also cold in addition.
Analysis according to age and the viral type and subtypes.
Regarding the repartition of positive cases (n = 1795) according to age groups and to viral type and subtype, the results were illustrated in Table 2. This shows that there were 1221 positives cases (68%) of type A virus, 566 cases (31.5%) of type B virus, and 8 co-infected cases A&B types. Among the 1221 positive cases of type A virus, the A(H3N2) subtype accounted for 595 cases (46.6%), followed by the A(HIN1)pdm09 (pandemic) subtype with 548 cases (44.6%), and the A(H1N1)s (seasonal) subtype with 11 cases (0.9%). In addition, 8 co-infected A(H3N2) and A(H1N1)pdm09 cases (0.3%) and 92 no subtyped cases (7.5%) were reported. This table also shows that by calculating the percentages of each type by age group, the type B had respectively 33.2% and 23.3% of cases in <2 years and 2 - 4 years age groups vs. 30% and 22.6% for the type A in the same age groups.
Analysis of positive case by provenance and category of patient
Of 1795 positive cases, 1273 were ILI (71%), and 522 SARI (29%). Considering the ILI-positive cases, Boyambi medical clinic occupied the first place with 22.5%, followed by RVA medical clinic, which totalled 21.8%. For SARI positive cases, the first place went to Kalembelembe paediatric hospital with 43.8%, followed by Kingasani hospital with 22.0%, as shown in Figure 2
Analysis by quarters and years
As regard to positive cases, the first quarter occupied the ranking top with 35.4% of cases, followed by the second quarter, with 30.4%, the fourth quarter with 29.8%, and finally, the third quarter came last with 4.5% of cases, as shown in Table 3.
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Table 2. Distribution of positive cases according to age groups and viral types and subtypes.
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Figure 2. Percentage of positive ILI and SARI case of influenza viruses in sentinel sites.
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Table 3. Distribution of positive cases per quarter and per year according to the types and subtypes of the influenza virus.
* Predominant viral strain during the year.
By comparing the frequency of viral strains in circulation, the A(H3N2) subtype was more frequent in the year’s first quarter, whereas the type B was predominant in the second and third quarters, while the A(H1N1)pdm09 subtype was more important in the fourth quarter.
Concerning, the distribution by year, the size of the annual confirmed influenza cases varied between 32 and 271 with a median of 232 as illustrated in Table 3. This table shows that 2009 ranked first with 15.1%, followed by 2013 with 14%, then 2014 with 13.6%, and in the last position, 2018 with 1.8%. In addition, the subtype A(H1N1)pdm09 was the most frequent strain in 2009, 2012, 2015, 2016, and 2017. Nevertheless, the A(H3N2) strain was predominant in 2011, 2014, and 2018, while the type B was dominant in 2010, 2013, and 2018. From 2010, the seasonal A(H1N1)s strain disappeared.
About the circulation of the predominant viral strains, we have observed over the course of ten years the following pattern: A(H1N1)pdm09-B-A(H3N2)-A(H1N1)pdm09 cycle.
4. Discussion
1) Sense of study
Our study aimed to address the epidemiological, clinical and virological characteristics of people affected by seasonal influenza who visited sentinel surveillance sites from 2009 to 2018. For that, we should answer the following questions: Whom were these people affected by the flu? What were the clinical symptoms that they presented? Where did they live? What types and subtypes of the virus were they infected?
2) Statement of main results
The results of our study showed that: influenza accounted for 9.8% of other severe acute respiratory infections; children under five year old were the most affected by the flu in the DRCongo; symptoms such as fever ≥ 38˚C, sore throat, absence of headache, were associated with influenza infection; viral strains A(H3N2), A(H1N1)pdm09 and B circulated each year concomitantly, but in different proportions; the circulation pattern of viral strains was A(H1N1)-B-A(H3N2)-A(H1N1)pdm09; influenza outbreaks were observed more in the first, second and fourth quarters of the year; influenza season has two waves(the first occurs in the fourth quarter, and the second in the second quarter).
3) Discussion of material differences in results
Our study showed that positive cases of the influenza virus accounted for 51.6% for men and 48.4% for women, respectively. This difference is not significant, p > 0.05. This result differs from some studies where female sex was a risk factor [11] [12] [13] but is similar to those found in many African countries [4] [5] .
Concerning age, the results of our study showed that the age groups of <2 years and 2 - 4 years represented approximately 66% of SARI positive cases and 46% of ILI positive cases, hence the interest in direct priority interventions in the fight against influenza to this target group like vaccination and case management. KATZ et al. in 2009 also found in Kenya similar results, in a study on the findings of the first six years of influenza sentinel surveillance. According to this study, the age group under five years and specifically that under two years was the most affected, with 33% of cases [14] . Several African studies had also shown the heavy burden children under five carry on morbidity and mortality from influenza [2] - [9] [15] [16] . In addition, the absence of data on the follow-up of complications and mortality by age group on the notification forms did not allow us to determine the age group at risk of complications and death.
Regarding the distribution of confirmed positive cases according to age groups and the viral type and subtype, the results of our studies showed that there were more positive cases of the A (H3N2) viral subtype in <2 years, 2 - 4 years age groups, and beyond 40 years than positive cases for the viral subtype A(H1N1)pdm09. On the other hand, the A(H1N1)pdm09 subtype was predominant in of 5 - 14 and 15 - 39 years age groups.
In Zambia, Theo et al. in 2009 had obtained similar results that showed children under five years old were more affected by the viral subtype A (H3N2) and type B. Nevertheless, the difference was found in the slices affected by A(H1N1)pdm09 which in Zambia [7] concerned people aged 5 to 24 years old, whereas in our study, the infected people were between 5 and 39 years old. Studies conducted by Dalhatu in 2010 in Nigeria [5] and Homaira in 2009 in Bangladesh [17] showed that the A(H1N1)pdm09 subtype was not significant below five years of age. However, the results obtained by these two researchers differ in the age group of people affected by this subtype. The first found that the A(H1N1)pdm09 subtype was important between 5 and 17 years and greater than 65 years. The second instead found the age range between 5 and 55 years. The difference in results between these two researchers is explained, among other things, by the difference in the age groups used.
Compared to the symptoms manifested by patients, the cough was not associated with the Influenza virus infection, which is considered one of the main symptoms of this infection. This result is different from those found in many studies [18] [19] [20] . In addition, headaches were inversely associated with influenza infection. This finding differs to that reported by Barakat et al. in 2011, and Hirves et al. in 2012 [16] [18] . The reason would be that cough should be accompanied by fever to be a predictor of influenza infection.
Regarding positive cases according to their category and provenance, our study found that ILI cases represented 79% and SARI cases, 21%. Three assumptions could explain this difference. The first is the presence of two sentinel sites which only collected ILI cases (RVA and Boyambi medical clinic). The second would be the low sensitivity of the SARI case definition compared to the ILI case [21] , and the third stems from the second would be the often late arrival of SARI cases at the hospital beyond the time required for the sample.
As for the provenance of the patients, Kalembelembe paediatric hospital recorded 43.8% of SARI cases. We believe this result is justified insofar as this site is a paediatric reference hospital at the provincial level and therefore receives severe cases, including SARI cases.
Our study found that the third quarter (July, August and September) had fewer positive cases of the Influenza virus than other quarters. This period could therefore conducive for the vaccination campaign. As a possible explanation, this quarter coincides with the dry season, while the first quarter (January, February and March) and the fourth quarter (October, November and December) experience outbreaks during the rainy season. Other countries with tropical climates in Africa and Asia, like Kenya, Rwanda, Thailand, Singapore and Bangladesh [22] - [28] had experienced influenza epidemics during the rainy season.
Compared to the circulation of viral types and subtypes, we found that subtype A(H1N1)pdm09 was the most frequent strain in 2009, 2012, 2015, 2016 and 2017. On the other hand, the H3N2 strain was predominant in 2011, 2014 and 2018, while type B was more frequent in 2010, 2013 and 2018. Unlike the A(H1N1)pdm9 virus was predominant in DRCONGO in 2009, West Africa was under the grip of the A(H3N2) virus during the same period. When the type B virus was predominant in the DRCongo and southern Africa in 2010, West Africa was mainly affected by the A(H1N1)p subtype [29] . This situation shows that influenza viruses circulate differently from one region to another. Some studies have incriminated temperature and humidity as factors of variability [30] [31] [32] [33] [34] . These factors differ according to geographical areas.
Furthermore, our study showed a pattern of viral circulation in the form of a loop in the following order A(H1N1)pdm09-BA-(H3N2)-A(H1N1)pdm09. In Cameroon, a study conducted by Njoum in 2014 found the same pattern between 2009 and 2013. Therefore, this circulation pattern would constitute a research hypothesis that could target several countries.
In addition, we have noted a decrease of reported cases in 2016, 2017 and 2018. This statement was due to funding reduction for the surveillance system. Our study also showed that 2009 had more cases than other years due to the A(H1N1)pdm09 pandemic.
4) Limit of the study
Regarding the clinical characteristics, we could not obtain data on the evolution of hospitalised patients. For example, we would have liked to know how many hospitalised patients were healed and how many died.
5. Conclusions
The human infection with influenza virus represented 9.8% of acute respiratory infections in the DRCongo. This illness rages during the rainy session and most often affects children, particularly those under five years old, with a predominance of SARI cases compared to ILI cases. This study also showed that the A(H1N1)pdm09, H3N2 and type B subtypes were circulating concomitantly. According to the annual dominant strains, the human influenza virus circulates in a sequence of A(H1N1)pdm09-B-A(H3N2)-A(H1N1)pdm09 in loop form. The A(H1N1)pdm09 strain has wiped out the A(H1N1)s (seasonal) strain. This pandemic virus, assorted from three strains (human, poultry, and pig), is sufficient proof that influenza remains a threat to public health against which the sentinel surveillance system should be strengthened within one health framework.
What is known about this subject?
· Children under 5 are the most affected by influenza infection;
· Influenza infection is associated with rainfall season;
· The strains circulating in the DRCONGO are A(H1N1)p, H3N2 and type B.
What does your study bring new?
· In the viral circulation, there would be an alternation according to the dominant strains of the order…A(H1N1)p-BA(H3N2)-A(H1N1)pdm09;
· The DRCongo influenza season has two waves: The first occurs in the fourth quarter, and the second in the second quarter.
Acknowledgements
We want to thank the Head of the Community Health Department of the School of Public Health, the staff of the Influenza sentinel sites for collecting samples and epidemiological data and the national influenza laboratory staff for testing samples.