1 Introduction

Hepatitis B virus (HBV) is a partially double-stranded DNA virus belonging to the genus Orthohepadnavirus and the virus family Hepadnaviridae [1]. Chronic HBV infection affects between 257 and 400 million people worldwide [2,3,4]. Globally, approximately 29% of cirrhosis-related deaths are attributed to HBV [5]. Hepatitis B now ranks as the 15th leading cause of global mortality worldwide [6].

According to the European Association for the Study of the Liver (EASL), HBV infection is classified into five phases: (I) HBeAg-positive chronic infection, (II) HBeAg-positive chronic hepatitis, (III) HBeAg-negative chronic infection, (IV) HBeAg-negative chronic hepatitis, and (V) HBsAg-negative phase or occult HBV infection [7]. Occult HBV infection was defined by a panel of experts as the presence of HBV DNA in the liver (with detectable or undetectable HBV DNA in the blood) in those who tested negative for HBsAg using currently available diagnostics [8]. For HBV testing, the current WHO clinical guidelines recommend an initial HBsAg test. This approach is also applicable to high-risk populations, such as people infected with the hepatitis C virus, those on hemodialysis, and those with advanced chronic liver disease of unknown etiology [9]. Unfortunately, this strategy poses the risk of overlooking OBI.

OBI can be categorized as seropositive or seronegative, defined by serum markers of HBV infection. The majority of cases are seropositive [10]. Seropositive OBI is characterized by the detection of anti-HBc antibodies with or without anti-HBs, while seronegative OBI is characterized by undetectable antibodies, both anti-HBc and anti-HBs [11]. The clinical impact of OBI includes the following: First, it plays a significant role in the progression of liver diseases, including hepatocellular carcinoma and liver cirrhosis; second, it can spread to healthy individuals even at extremely low viral load levels. Third, immunosuppressive therapies in patients with OBI may trigger HBV reactivation [12].

Globally, the overall prevalence of OBI was 0.2% (95% CI: 0.1–0.4) in HBsAg-negative blood donors [13]. The prevalence of OBI was generally higher in countries with low economic status; for instance, in Africa, OBI prevalence in HBsAg-negative blood donors was 5% (95% CI: 0.7–12.6) [13]. Regardless of the endemicity, OBI prevalence was high in high-risk groups: 5.5% (95% CI 2.9–8.7) in low-endemicity countries, 5.2% (2.5–8.6) in intermediate-endemicity countries, and 12% (3.4–24.7) in high-endemicity countries [14].

Despite several studies addressing the prevalence of OBI, the pooled prevalence of OBI in Egypt remains unknown, especially in specific subpopulations such as blood donors, those with liver-related conditions, multi-transfused patients, patients with malignancies, and healthcare workers. So, we conducted this review to completely synthesize the available data and fill this knowledge gap.

2 Material and methods

2.1 Search strategy

A comprehensive literature search of the following databases was conducted from inception to October 2022 using the following keywords: occult hepatitis B virus infection or occult HBV infection or OBI and Egypt in MEDLINE [PubMed], Scopus, Google Scholar, and Web of Science. The review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement and was registered in PROSPERO with registration number CRD42022368147.

2.2 Inclusion and exclusion of studies

The following are the inclusion criteria:

  1. 1.

    Only primary studies (cross-sectional, case–control, or cohort studies) of participants residing in Egypt

  2. 2.

    Studies reporting the prevalence of occult HBV infection (HBsAg-negative and anti-HBc positive, or HBsAg-negative and anti-HBc-negative, or HBsAg-negative with unknown anti-HBc) were considered eligible regardless of the molecular detection technique of OBI

  3. 3.

    Studies published in English without a time limit

The following are the exclusion criteria:

  1. 1.

    Studies that were not conducted in Egypt or on Egyptian immigrants

  2. 2.

    Non-human studies

  3. 3.

    Full text is not available

  4. 4.

    Case reports, review articles, and conference abstracts

Studies were selected based on the aforementioned “inclusion and exclusion criteria” by three independent authors (H.K., S.O., and E.S.E.). All disagreements were settled by consensus.

2.3 Data extraction

The data extraction was conducted by three investigators (A.Az., H.K., and F.A.G.) and cross-checked by (E.S.E. and S.O.). From each included study, the following was extracted: the last name of the first author, publication time, study period, age, region, population, number of participants recruited to the study, number of participants tested for HBV DNA, OBI cases, HBV-DNA detection technique, and serological criteria used to test HBV DNA. For reports that address OBI genotyping, the number of occult HBV cases undergoing genotype analysis and their distribution among different genotypes were extracted.

2.4 Quality assessment

The quality of the included studies was checked using the “Joanna Brigg Critical Appraisal Checklist for Prevalence Studies” by two independent reviewers (S.O. and E.S.) and cross-checked by F.A.G. and A.Az.

2.5 Data synthesis

I-squared and Cochran’s Q were used to measure the heterogeneity between the studies, and based on the random effects model, results were reported as proportions with a 95% confidence interval (CI). Analyses of the subgroups were conducted based on the target population. Sensitivity analyses were conducted using the leave-one-out approach to test the robustness of the results.

All statistical analyses were performed using Open Meta Analyst (CEBM, University of Oxford, Oxford, UK).

Publication bias testing by funnel plot and associated tests were not conducted as they do not produce reliable results for meta-analysis of proportions [15].

3 Results

3.1 Study selection

A total of 677 records were identified through searching databases. There were 131 duplicates removed. The remaining 546 publications were then evaluated by title and abstract, and 446 articles were found to be irrelevant and excluded. The remaining 100 articles were evaluated for eligibility by full text, of which 50 were excluded, and a total of 50 studies reporting 62 estimates of occult HBV (Fig. 1) fulfilled our inclusion and exclusion criteria and were included in our review.

Fig. 1
figure 1

PRISMA flow chart outlining the process of article selection

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3.2 The characteristics of the included studies

The characteristics of the studies included are shown in Table 1. A total of 62 estimations of OBI were reported by 50 studies: 19 in patients who were HBsAg-negative and anti-HBc-positive and 43 in patients who were HBsAg-negative. Among the 43 studies conducted on HBsAg-negative patients, 8 were conducted on patients undergoing hemodialysis, 4 were on multi-transfused patients, 16 were on patients with chronic hepatitis C infection, 5 were on patients with hepatocellular carcinoma (HCC), 3 were on patients with liver cirrhosis, 2 were on blood donors, 2 were on children with cancer, 2 were on healthy adults and children, and one was on high-risk children born to HBsAg-positive mothers.

Table 1 Characteristics of the included studies
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Among 19 studies on HBsAg-negative and anti-HBc-positive patients, there were 6 on blood donors, 4 on patients with chronic hepatitis C infection, 3 on patients with hepatocellular carcinoma (HCC), 2 on patients on hemodialysis, one on healthcare workers (HCWs), and one on each of the following: patients with hematological malignancies, HIV patients, and patients with liver cirrhosis.

The included studies were all published between 2010 and 2022, with only three publications dated between 2008 and 2009 [26, 32, 53].

The quality of the included studies is presented in Additional file 1: Table S1.

3.3 Occult hepatitis B virus prevalence in HBsAg-negative patients

The overall prevalence of OBI among multi-transfused patients was the highest at 41% [95% CI: 23–59] (Fig. 2).This rate was estimated from 4 studies that included 285 HBsAg-negative patients tested for HBV DNA with high heterogeneity (I2% = 90.58). The pooled prevalence of OBI among patients on hemodialysis, patients with chronic hepatitis C infection, patients with HCC, and patients with liver cirrhosis was 17% [95% CI: 10–25], 10% [95% CI: 7–13], 24% [95% CI: 15–32], and 13% [95% CI: 1–25] (Figs. 3, 4, 5, and 6), respectively. The summary of meta-analysis results for the prevalence of occult hepatitis B is presented in Table 2.

Fig. 2
figure 2

The pooled prevalence of OBI in HBsAg-negative, multi-transfused patients

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Fig. 3
figure 3

The pooled prevalence of OBI in HBsAg-negative patients on hemodialysis

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Fig. 4
figure 4

The pooled prevalence of OBI in HBsAg-negative chronic HCV-infected patients

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Fig. 5
figure 5

The pooled prevalence of OBI among HBsAg-negative patients with HCC

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Fig. 6
figure 6

The pooled prevalence of OBI in HBsAg-negative patients with liver cirrhosis

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Table 2 Summary of meta-analysis results for the prevalence of occult hepatitis B in Egypt
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3.4 Occult hepatitis B virus prevalence in HBsAg-negative and anti-HBc-positive patients

The pooled prevalence of OBI among blood donors, patients with chronic hepatitis C infection, and patients with HCC was 12% [95% CI: 7–17], 15% [95% CI: 8–22], and 31% [95% CI: 1–60] (Figs. 7, 8, and 9), respectively. The summary of meta-analysis results for the prevalence of occult hepatitis B is presented in Table 2.

Fig. 7
figure 7

The pooled prevalence of OBI in HBsAg-negative and anti-HBc-positive blood donors

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Fig. 8
figure 8

The pooled prevalence of occult hepatitis B infection in HBsAg-negative and anti-HBc-positive chronic HCV-infected patients

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Fig. 9
figure 9

The pooled prevalence of occult hepatitis B infection in HBsAg-negative and anti-HBc-positive HCC patients

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3.5 OBI genotypes distribution

Ten reports, as shown in Table 3, addressed the genetic background of OBI, with a total number of 150 HBV genome analyses of which 68 (45.3%) were of genotype D (Fig. 10). The majority of reports (7/10) reported that genotype D predominated. However, Esmail et al., Zaki et al., and Elmaghloub et al., reported that genotypes B, E, and C were the most prevalent, [20, 21, 64] (Table 3).

Table 3 OBI genotype analysis among the included studies
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Fig. 10
figure 10

OBI genotypes distribution in Egypt

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3.6 Sensitivity analysis

Sensitivity analysis using the leave-one-out approach indicated that the combined estimates are reliable and do not depend on one study (see Additional file 1: Figs. S2-S10).

4 Discussion

Blood-borne hepatitis viruses are a major issue in underdeveloped nations. Egypt has one of the highest HCV prevalences in the world and is regarded as an intermediate area for HBV infection. However, the national burden of occult HBV in Egypt is still unclear. To the best of our knowledge, this is the first systematic review and meta-analysis that highlight the rise in occult HBV infection in Egypt, particularly in blood donors and high-risk populations, at which interventions should be focused.

In Egypt, HBV screening in blood banks is based only on the detection of HBsAg. However, nucleic acid amplification testing (NAT) is implemented in some large blood banks [48]. Furthermore, occult HBV infection and infection during the pre-seroconversion window period are undetectable using HBsAg assays [66]. Our study revealed that about one-tenth of blood donors who are HBsAg-negative and anti-HBc-positive have OBI in Egypt, which is consistent with a global meta-analysis among blood donors that revealed a rate of 10.0% (95% CI: 5.0–16.4) in the Eastern Mediterranean Region [13]. Oluyinka et al. suggested pre-testing Nigerian blood donors for occult HBV infections using NAT and/or anti-HBc, even if they tested negative for HBsAg, to eliminate or at least lessen the risk of HBV infection transmission [67]. However, implementing such a strategy in Egypt may not be feasible due to the high cost of nucleic acid testing as well as the need for specially trained personnel and equipment, which may not be available in many of the country’s blood banks. Furthermore, the overall sensitivity and specificity of anti-HBc as a screening tool to identify OBI were unsatisfactory (77% and 76%, respectively) [14]. So, using the anti-HBc biomarker alone to screen donated blood for occult HBV infection is not recommended, even in resource-limited countries [14].

OBI is highly prevalent among patients with end-stage renal disease (ESRD) on maintenance hemodialysis due to the frequent need for blood transfusions [68,69,70]. A meta-analysis of OBI in Sudan revealed a high incidence of OBI among patients on hemodialysis at 13.36%, which is lower than our findings of 17% [70], despite Sudan having a higher HBV prevalence than Egypt [4]. Based on our findings, we recommend screening for OBI in patients on hemodialysis, adherence to standard hygiene precautions, and patient isolation if OBI is detected.

Transfusion-transmitted infection (TTI) represents a real concern for transfusion services. Multi-transfused patients, such as those with thalassemia major, hemophilia, or sickle cell disease, are especially vulnerable to TTI [71]. According to the current review, multi-transfused patients have the highest OBI prevalence among the HBsAg-negative population (41%). In Iran, OBI among patients with thalassemia major was 1.16% [72]. In Palestine, no OBI cases were reported among patients with thalassemia and sickle cell anemia [73].

Occult HBV infection has frequently been identified in patients with chronic HCV infection, which has been linked to hepatocellular carcinoma and even severe liver damage [74,75,76,77,78]. The mechanism underlying the increased frequency of OBI in those with chronic HCV infection may include HCV molecules interfering with HBV replication, leading to OBI with reduced HBV replication [12, 79]. According to our findings, OBI was found in a significant number of HBsAg-negative patients with hepatitis C in Egypt, with a pooled prevalence of 10%, which was higher than the rate of 7.76% estimated by a similar review among Iranian HCV-positive patients. The prevalence was higher (compared with HBsAg-negative patients) at 15% among HBsAg-negative and anti-HBc-positive patients with hepatitis C infection. Several studies have demonstrated that the prevalence of OBI is not associated with the presence of anti-HCV antibodies in patients on hemodialysis [80,81,82].

According to Abu El Makarem et al. and Ismail et al., patients on hemodialysis with and without chronic HCV infection did not show any significant differences in the prevalence of OBI [16, 17]. Omar et al., El-Sherif et al., Mahmoud et al., and Selim et al. showed that liver aminotransferases were statistically higher in patients with dual OBI/HCV infection than those with mono-HCV infection [31, 33,34,35], which is consistent with other studies [83,84,85]. In contrast, Naga et al., Thabit et al., Taha et al., and Sheneef et al. reported that liver aminotransferases were not statistically higher in patients with dual OBI/HCV infection compared to those with mono-HCV infection [36, 37, 39, 40], which was consistent with other studies [86,87,88,89]. Therefore, the relationship between OBI and liver enzyme flare in patients with chronic HCV infection remains inconclusive. According to our findings, OBI is also frequent in patients with HCC and cirrhosis, indicating that OBI may play a role in the progression of cirrhosis and the development of HCC.

There is a paucity of studies that assessed OBI prevalence among vaccinated children. For instance, a pilot study from Taiwan revealed a 10.9% prevalence among HBV-vaccinated children [90]. Another study found that in HBsAg-negative and anti-HBc-positive subjects, OBI frequency was lower in the unvaccinated (1.7%) than in the vaccinated (4.8%) [91]. Only one study, by Elrashidy et al., evaluated the prevalence of OBI among 170 HBsAg-negative vaccinated children and adolescents with no OBI cases identified among them [49]. However, further studies with large sample sizes and long-term follow-ups are needed. There was only one study that assessed OBI in high-risk infants born to HBsAg-positive mothers by Fouad et al., demonstrating that among 64 children delivered to HBsAg-positive mothers who received HBV immunoprophylaxis (HBV vaccine and HBIG) at birth, only one case developed OBI with anti-HBc negativity [50]. Therefore, OBI may occur in infants born to HBsAg-positive mothers despite immunoprophylaxis, and being anti-HBc-negative does not rule out OBI.

4.1 Limitation

First, the studies involved different methods of screening and kits, a resulting in variation in the sensitivity and specificity that could account for the difference in prevalence rates between the different study publication periods. Second, some studies had a small sample size. Third, there is no data about OBI prevalence in some regions. Fourth, the paucity of publications that assesse OBI in the post-vaccination era and healthy individuals. Fifth, although we stratified the included studies into subgroups to minimize the heterogeneity, there may be other sources of heterogeneity that cannot be identified. Nevertheless, our review provides crucial data on the prevalence of OBI in Egypt and highlights the high-risk populations at which intervention should be primarily targeted. Our results suggest that HBV eradication efforts should take occult HBV infection into account as a global health concern and enhance affordable access to nucleic acid testing.

5 Conclusion

OBI is a major public health issue. Its clinical importance derives from the fact that OBI can spread to healthy individuals even at extremely low viral load levels. Additionally, immunosuppression has the potential to restart viral replication, which can result in life-threatening liver decompensation. The current study highlights the high prevalence of OBI among blood donors and high-risk groups, specifically patients on hemodialysis, multi-transfused patients, chronically HCV-infected patients, patients with HCC, and patients suffering from liver cirrhosis. The implementation of HBV nucleic acid amplification testing (NAT) may increase the safety of blood transfusions by excluding all HBV DNA-positive donations. However, the cost-effectiveness of these tests should be investigated. In addition, more research is required to strengthen the current evidence and describe the prevalence of OBI among vaccinated individuals.