Hepatitis B virus (HBV) infection amongst children in Senegal: current prevalence and seroprotection level

Introduction Hepatitis B virus (HBV) infection is highly endemic in Senegal. HBV vaccine of all children has been introduced in 1999 and included in the Expanded Programme on Immunization in 2004. The aim of this study was to assess the HBV prevalence and immunity status against HBV amongst children in Senegal. Methods Between March and August 2016, consecutive children aged from 6 months to 16 years old were recruited in outpatient department of three main children hospitals in Senegal. Serum samples were analyzed for HBV serology (HBsAg, HBcAb, HBsAb) using ARCHITECT analyzer. Children with HBsAb levels ≥ 10 IU/l) were considered as seroprotected against HBV. Results During the study period, 295 children fulfilled the criteria for the study and were further analyzed. Three children were HBsAg positive giving a seroprevalence at 1.1% (95% CI: 0.2-3.3), 12/267 (4.5%, 95% CI=2.3-7.7) had positive HBcAb and 226/295 (76.6%, 71.4-81.3) had positive HBsAb including 191 (77.3%, 71.6-82.4) with isolated HBsAb related to previous active immunization. However only 165 children (56%, CI 50-62) had seroprotective HBsAb levels (HBsAb ≥ 10 UI/L) and 63 (21.4, 16.8-26) had a strong seroprotectiondefined by HBsAb ≥ 100 IU/L. Conclusion Our results suggest that although HBV prevalence has significantly decreased in children in Senegal following a better HBV vaccine coverage, the number of children correctly seroprotected is insufficient (56%). Assessing the levels of HBsAb and providing HBV vaccine boosters should be considered in children in Senegal.


Introduction
Hepatitis B virus (HBV) infection is a major public health problem and a leading cause of morbidity and mortality globally, affecting approximately 250 million persons worldwide [1] and accounting for 650,000 deaths annually [2]. Most of these deaths occur in resource-limited countries in Asia and Africa. Without effective preventive and therapeutic interventions chronic hepatitis B (CHB) infection will lead to an estimated 11.8 million deaths by 2030, primarily as a result of cirrhosis and hepatocellular carcinoma (HCC) [3]. The World Health Organization (WHO) has recently incorporated HBV elimination in its global health agenda and plans a 90% reduction of anew HBV cases and a 65% reduction of HBVrelated mortality by 2030. In order to achieve these ambitious targets prevention of HBV transmission in particularly in endemic countries should be urgently improved. Effective HBV vaccines have been developed in the early 80s and in 2009, the WHO has recommended the introduction of the vaccine into the national immunization programs of all endemic countries. However, the current coverage of three doses of HBV vaccines remains imperfect with an estimate below 80% in 2015 in Africa [4]. In sub-Saharan Africa, the seroprevalence of Hepatitis B s antigen (HBsAg) in the adult population is high, estimated over 8% [1,5]. In Senegal, 85% of the population has at least one marker of previous or current HBV infection and the prevalence of HBsAg varies between 11 to 17% [6].
Following three doses of vaccine given at 4 weeks interval, it has been demonstrated that 90-99% of healthy neonates, children, adolescents and adults will develop protective levels of HBs antibody (HBsAb) [7] defined by serum levels ≥ 10 IU/L although a level below 10 IU/L does not necessarily indicates loss of immunity [8].
In Senegal, one of the most endemic countries for HBV in the world, the HBV vaccine has been introduced in 1999 and included in the Expanded Programme on Immunization (EPI) only in 2004 with recommended injections in neonates at the age of 1, 2 and 3 months using a recombinant vaccine. Since January 2016 the Senegalese ministry of health has been providing the HBV birth dose vaccine within the 24 hours of life to all infants born in Senegal in order to comply with the WHO guidelines and eventually control the burden of HBV-related liver diseases in Senegal. Following the 2016 call from the WHO and world health assembly to control and eliminate HBV worldwide, the aim of the following study was to evaluate the current immunization profiles against HBV in a large number of Senegalese children aged from 6 months to 16 years.

Characteristics of the study population: Between March and
August 2016, out of 416 consecutive children who visited the study centers, 295 were aged between 6 months and 16 years old and their parents accepted to participate to the study. General characteristics of the study children are summarized in Table 1. The vast majority of the children (34 (46%)) were aged below 5 years and 61 (21%) were more than 10 years.  (Table 1). In children aged under 5 years, HBsAg prevalence was slightly higher at 1.6% (95% CI: 0. 2-5.8). In children aged 5-10 years, 2 had a positive HBsAg giving a seroprevalence at 1.6% (95% CI: 0. 2-5.8).

Markers of HBV infection
No positive cases of HBsAg were noted in children over 10 years, but the sample size in that age group was small (n = 61). There was no significant difference in the prevalence of HBsAg by age and sex (1.4% versus 0.8%, p = 0.55). Of the 295 children, 69 (23% (95% CI = 19-29)) had HBsAb levels under 2 IU/L and were therefore considered as non-immunized.

Seroprevalence of HBcAb
There was no statistical significant difference between both sexes (56.6% and 55.1% in boys and girls respectively, p = 0.82).

Discussion
This study aimed to assess the seroprevalence of HBsAg amongst children (6 months-16 years old) and the level of seroprotection Page number not for citation purposes 4 against HBV in Senegal in 2016. Our findings provide two main messages 1) more than 10 years after the introduction of the vaccine within the EPI, the HBsAg seroprevalence amongst children in Senegal has significantly decreased (1.1%) as compared to the high prevalence reported in the 90s (> 15.%) [9]. This indicates a great impact of the integration of the HBV vaccine within the EPI as it has been observed in Asian and Western countries [10]. 2) Almost 80% of the children were vaccinated against HBV but only a small proportion (56%) were correctly sero-protected suggesting that Children hospitalized in two hospitals in Dakar tested for HBsAg between 2009-2010 had a low prevalence of HBsAg (0.2%) [12].
However, no significant difference was found between these 2 studies (p = 0.139). Compared with national studies, our overall HBsAg carrier rate was lower than those in 930 children born from HIV-seropositive mothers (3.0%) [13] and 462 babies including 88 born to 80 HBsAg (+) mothers (HBsAg (+) (17%) than to HBsAg (-) (4%) women [14]. Moreover compared with international studies, the HBsAg prevalence observed in our study (1.1%) was lower than those found in Nigerian children aged 2 months to 15 years (13.9%) [15], healthy Indonesian children (3.1%) [16] and HIV-infected children in Swaziland (1.4%) [17]. The discrepancy observed with other African countries is likely associated with several factors: a modest decrease in the HBsAg prevalence among African women, from 13.8% in 1992 to 11.57% in 2012 [14,18] and different vaccine coverage over the African continent. According to the 2015 WHO-Unicef immunization report the HBV 3 doses vaccine coverage has improved in Senegal and was estimated at 89% in Senegal [6].
In our study we found that 77.3%, (95%CI 71.6-82.4) had isolated positive HBsAb suggesting a HBV vaccine coverage close to 80% in our study population. HBV vaccination or infection induces the production of HBsAb, an immunoglobulin able to neutralize HBsAg in case. Along with other effector immune functions, the protective HBsAb can block the spread of HBV in the body. HBsAb levels persist in the blood for a long time but gradually decrease with age.
Our study confirms this effect (Figure 2). Our results are close to those reported by other authors in Senegal showing that the rate of positive HBsAb was 61 % in hospitalized children aged between 3 month to 6 years (p=0.302) [12]. Our results are however lower than those found in other HBV endemic countries where HBV vaccine coverage rates among children range from 90 to 98%. [19,20]. We also found that the seroprotective rate of HBsAb levels was higher in males (56.6%) as compared to females (55.1%) (p > 0.05) suggesting gendre-guided immunity responses. In contrast previous studies reported better protective levels of HBsAb in vaccinated children (70%) [21] and 84.2% [22] with higher levels in girls as compared to boys. Such discrepancy could be attributed to differences in the primary immune response to vaccination, the type of vaccines used, the quality of vaccine storage (broken cold chain for instance) age groups, nutritional status and socioeconomic factors or race factors [23].
In our study protective level of HBsAb decreased with age from 65% in children below 5 years to 15.3% in those above 10 years.
Similar findings were reported in Egypt and China where the protection rate was inversely correlated with age [24,25]. A study conducted in Yemen assessed the hepatitis B vaccine coverage and immune response in children below ten years and reported a 45.2% children with low levels of HBs (< 10 UI/L) despite a full course of 3 doses vaccine [26]. These results suggested a poor response to HBV immunization in African children [27]. Interestingly we did not find any occult hepatitis B in the children enrolled in our study. In a recent study conducted in China, a low rate of occult hepatitis B (1.26%) was observed in 1.192 community children [28]. Our study has some limitations: first the parents were not surveyed on the history of hepatitis B in the family. Second we were not able to assess the number of HBV vaccine doses that were administrated to the children. It has been previously demonstrated that 3 doses are the best method to get effective protection against HBV and is better than one single dose. Third, we did not collect the HIV and What is known about this topic  The seroprotection rate of vaccinated subjects was less than 80%; hepatitis B is a real public health problem with prevalence above 8% in our countries; immunization coverage remains inadequate with seroprotection rates below 80%;  Eighty percent of those vaccinated with HBV did not have seroprotective antibody levels;  Only 90% of healthy people will develop protective levels of anti-HBs antibodies (≥ 10 IU/L) after taking 3 doses of HBV vaccines at 4 week intervals.

What this study adds
 This study shows a very low prevalence of this infection in children;  This study shows the need, on the one hand, to evaluate the seroprotection of subjects after taking the three doses in order to propose boosting immunity and on the other hand to make an evaluation of the quality of vaccines, the vaccine storage system and the their usage.