Occult B hepatitis – still a risk for transfusions?
Claudio Velati, Transfusion Medicine and Haematology Department, Sondrio Hospital, Italy
NAT screening for HBV has been introduced when multiplex technology was available only in countries where the endemic of HBV was on medium/high level and where the screening with other serological tests, like anti-HBc, was impossible without impair self-sufficiency in blood procurement. The first goal of NAT testing introduction in blood screening was to increase the sensibility of screening tests in order to impair transfusion of blood units from subjects in initial acute phase of infection with no other serological markers (window period shortage). But after HBV NAT implementation the great majority of NAT HBV positive/HBsAg negative subjects are represented by repeat donors with other serological markers of previous contact with HBV (anti-HBc and anti-HBs), with low viral load and totally silent from the clinical point of view. In the 2001–2008 period 7,436,996 blood units were tested, in Italy, for HBV DNA and 383 resulted positive: 20 of them (1:372.000) were subjects in acute phase of the infection and seroconverted, 363 (1:20.000) were OBI. Similar pictures are observed in other Mediterranean and East Europe Countries. These data could induce all mathematical models, finalized to calculate the residual risk, to consider HBV as the main infectious risk in transfusion field. But we can observe, where HBV DNA blood screening is in progress since some years, a downward trend of OBI blood donors: in Italy 61.9×106 subjects have been observed in 2005 and 42.2×106 in 2008 (p < 0.05). On the contrary, acute cases are stable. In addition, there are some doubts about the actual capacity of OBI subjects to infect the recipient: some experiences showed that HBV DNA of OBI subjects is less efficient in transmitting the infection than DNA of acute phase subjects.Nevertheless, HBV transfusion transmitted infections from blood donors with low viral load are still reported. In conclusion blood screening with both HBsAg and HBV DNA test is, at the moment, the best way for HBV safe transfusion; in the meantime it limits donors deferral only to those who are really at risk of transmitting a HBV infection. In the future the occurrence of blood donors with OBI will decrease due to progressive deferral of these subjects and to the vaccination campaigns against HBV in progress in countries at medium/high level of HBV endemic.
The impact of hepatitis B virus (HBV) infection on the whole world population remains still high: more than two billion people are positive for one or more serological HBV markers and, among these, in a third the HBV is actively replicating.
It is possible to identify three different geographical areas with respect to the prevalence of HBsAg carriers:
- • areas at high prevalence (>2% of carriers): Africa, China, South-East of Asia;
- • areas at intermediate prevalence (0·5–2%): South Europe;
- • low prevalence (<0·5%): USA, North Europe, Australia.
In the three areas, epidemic and ways of transmission are different [1,2]. Before the availability of diagnostic tests for HBV infection, approximately 6% of multi-transfused patients were thought to be infected by HBV, but, even after the implementation of specific serological markers in blood screening, the residual risk of transmitting a HBV infection by transfusion remains the highest with respect to the other transmissible viruses [3,4].
Conditions of HBV transmission by transfusion
Clinical and serological backgrounds of possible HBV transmission by transfusion are the following:
- • acute and asymptomatic HBV infection in the blood donor with circulating virions;
- • previous HBV infection in the blood donor, without evidence of chronic hepatitis, but with circulating virions. This condition has been recently defined as occult hepatitis B virus infection (OBI): presence of HBV DNA in the liver (with or without detectable HBV DNA in the plasma) of subjects testing HBsAg negative by currently available assays. When detectable, the amount of HBV DNA in the plasma is usually very low (<200 IU/ml). On the basis of the HBV antibody profile, OBI may be distinguished as sero-positive-OBI (anti-HBc and/or anti-HBs positive) and sero-negative-OBI (anti-HBc and anti-HBs negative) .
We have to emphasize that the transmission of HBV to the recipient can happen if the screening tests are not able to evidence the presence of the virus. Another important factor to consider is the virus capability to infect the recipient that is influenced by the viral load, the phase of the infection in the blood donor and the immunological condition of the patient.
Tests and screening strategies
Tests used in biological validation of blood units, with respect to HBV, are divided into three categories:
- • tests that directly identify a virus structural component (as HBsAg, NAT tests);
- • tests that identify an antibody, produced by the immune system of the subject, that is a sign of a previous contact with the virus (as anti-HBc, anti-HBe);
- • ‘surrogate’ tests that are an indirect sign of the liver damage because of the contact with the virus (as ALT).
All over the world, the HBsAg is the most used in blood screening and it is nowadays considered mandatory.
In some countries at low endemic for HBV (North Europe, USA), in addition to HBsAg, anti-HBc test has been introduced. The rationale of the use of anti-HBc in blood screening is as follows:
- • it is a serological marker of recent and previous infection and is persistent in the time,
- • it may be the only marker present in the acute phase of the infection after HBsAg disappearance,
- • although the low specificity, the number of blood donors deferred from donation is limited if endemic is low,
- • may be a further safety measure in addition to HBsAg and may be an alternative to NAT implementation for HBV.
In some countries at intermediate or high HBV prevalence (South and East Europe), NAT testing for HBV DNA has been introduced in addition to HBsAg.
The advantage to introduce HBV DNA testing is as follows:
- • it is a direct marker of the virus,
- • it is able to reduce the window period in acute infection,
- • it allows deferral of only blood units that could actually be responsible for HBV transmission.
Moreover, the presence of HBV DNA in HBsAg−/anti-HBc+ subjects could give information about:
- • just recovered infection where HBsAg is no more detectable, but where low levels of DNA are still present,
- • chronic HBV carrier with HBsAg under detectable levels,
- • infection from HBV mutants with low viral load and not detectable by current screening tests.
NAT testing has been firstly introduced in blood screening to identify HCV infections because of the wide capacity to reduce the window period in comparison with EIA tests.
Public concern about safety in blood transfusion, particularly for the risk of infectious diseases transmission, has strongly influenced the decision to implement NAT blood screening for HCV and HIV and, in fact, the residual risk has been dramatically decreased. NAT screening for HBV has been introduced only when multiplex technology was available, only in countries where the endemic of HBV was on medium/high level and where the screening with other serological tests, like anti-HBc, was impossible without impair self-sufficiency in blood procurement.
The first goal of NAT testing introduction in blood screening was to increase the sensibility of tests to impair transfusion of blood units from subjects in initial acute phase of infection with no other serological markers (window period shortage). But after HBV NAT implementation, the great majority of NAT HBV-positive/HBsAg-negative subjects are represented by repeat donors, 50–55 years old, with other serological markers of previous contact with HBV (anti-HBc and anti-HBs), with low viral load and totally silent from the clinical point of view (OBI).
Epidemiology of HBV in blood donors
In the 2001–2008 period, 7 436 996 blood units were tested, in Italy, for HBV DNA and 383 resulted positive: 20 of them (1:372·000) were subjects in acute phase of the infection and seroconverted, 363 (1:20·000) were OBI.
The serological picture in OBI subjects was described as follows: 16·6 % of donors had anti-HBc only; 15·7% were anti-HBc- and anti-HBe-positive; 49·6% were anti-HBc- and anti-HBs-positive and 18·1% were anti-HBc-, anti-HBe- and anti-HBs-positive. Considered as a whole, 67·7% of the donors with occult HBV had anti-HBs, 74·5% of whom at titres considered protective (10 mIU/ml) .
Similar pictures are observed in other Mediterranean and East Europe Countries. In Spain, the overall yield of HBV DNA-positive/HBsAg-negative blood units was 1:21 282, but 1:95 817 was collected during the window period (WP) and 1:29 482 was OBI . In Poland, the frequency was 1:200 000 and 1:47 619, respectively, for WP and OBI units . Similar studies are reported in Greece  and in South Africa . These data induce all mathematical models, finalized to calculate the residual risk, to consider HBV as the main infectious risk in transfusion field .
But we can observe, where HBV DNA blood screening is in progress since some years, a downward trend of OBI blood donors: in Italy, 61.9 × 106 subjects have been observed in 2005 and 42.2 × 106 in 2008 (P < 0·05). On the contrary, acute cases are stable, as reported in Table 1.
Table 1. Trend of WP and OBI HBV DNA–positive/HBsAg–negative subject (×106) in 5 years of observation in Italy (SIMTI Study, 2009)
|HBV WP|| ||3·5||1·9||3·6||2·3|
In the past, some cases of post-transfusion HBV hepatitis because of HBsAg-negative/anti-HBc-positive blood donors have been reported. In fact, in these cases, HBV DNA testing was not performed or performed with methods at low sensitivity and not comparable with current ones. In the most recent cases HBV transmission is always documented by HBV DNA in donor and recipient [4,11–13].
In addition, there are some doubts about the actual capacity of OBI subjects to infect the recipient: some experiences showed that HBV DNA of OBI subjects is less efficient in transmitting the infection than DNA of acute phase subjects. A systematic review carried out by the Japanese Red Cross showed that blood components collected from donors with occult HBV infection with low levels of anti-HBc were more than 10 times less infectious than donations collected from donors in the window phase .
In addition, there is evidence that infectious risk caused by transfusion of blood units or components derived from donors with occult HBV with or without anti-HBs may be different. According to Allain and colleagues , blood units from donors with occult infection containing anti-HBs are unlikely to be infectious, while those without anti-HBs (anti-HBc–positive alone or in copresence with anti-HBe) may be more infectious, but the matter is still controversial [15,16].
Nevertheless, HBV transfusion transmitted infections from blood donors with low viral load are still reported and recently a HBV transmission from a donor with OBI with anti-HBs has been documented .
Gerlich and colleagues  considered transmission of occult HBV as a rare but potentially dangerous event. They described three cases of fulminant hepatitis B with fatal outcome in immunosuppressed patients who were given anti-HBs–negative blood containing low concentrations of mutated forms of occult HBV, only detectable with sensitive ID-NAT. De novo post-transplant HBV infection in liver recipients from anti-HBc alone or anti-HBc- and anti-HBs–positive donors suggests caution when anti-HBc– and anti-HBs–positive blood donations are given to immunodeficient patients .
At present, the blood screening with both HBsAg and HBV DNA tests seems to be the best option to further minimize the residual risk of HBV transmission through transfusion because it detects HBsAg-nonreactive units both in a preacute phase and in a late occult stage of infection.
In the meantime, it limits donors deferral only to those who are really at risk of transmitting a HBV infection. In the future, the occurrence of blood donors with OBI will probably decrease because of progressive deferral of these subjects and to the vaccination campaigns against HBV in progress in countries at medium/high level of HBV endemic.
The Italian information quoted in this manuscript are obtained from the “SIMTI Study” and the Author thanks all the Italian Transfusion Services for procuring data and the board of the Study (L. Fomiatti, L. Baruffi, L. Romanò and A. Zanetti) for data processing.
No conflicts declared.