Genotype A is characterized by an insertion of six nucleotides at the carboxyl end of the core gene. Genotype A is dominant in Northwest Europe and North America. Additionally, some strains of genotype A have been found in the Philippines, Hong Kong, and in some parts of Africa and Asia. Subgenotype A2 is dominant in Europe, A1 is prevalent in Asia and most of Africa, A3 is found in the Cameroon and Gambia, A6 (currently named A4) and quasi-subgenotype A3 (which includes the strains previously named A4 from Mali, A5 from Nigeria, and A7 from Cameroon) have been isolated from other regions [32].

Genotype B is distributed throughout Asia and has been classified into nine subgenotypes to date. B1(Bj), the subgenotype without recombination with genotype C in the precore/core region, is distributed in Japan [33, 34]. Genotype B is mainly prevalent in Southeast Asia, but can also be found in the Pacific islands. Subgenotype B5, obtained from a Canadian Inuit population [35], represents genotype B without recombination with genotype C in the precore/core region, as opposed to the other subgenotypes of B, which do show this recombination [33]. Subgenotype B1 is the most likely ancestor of B5, which was possibly carried by the indigenous peoples during migration from Siberia and Alaska to North America and Greenland [36, 37].

Genotype C is mainly prevalent in Southeast Asia, but can also be found in the Pacific islands [38]. According to Paraskevis et al. [39], genotype C is the oldest HBV genotype. It has the highest number of subgenotypes, C1–C16 [40, 41], reflecting the long duration of being endemic in humans. A large number of these subgenotypes circulate in Indonesia [40]. Subgenotype C4 is exclusively found in the indigenous people of northern Australia [42], who are descended from a founder group that emigrated from Africa at least 50,000 years ago [43].

Genotype D is the genotype most widely distributed globally. It is found in northeastern Europe, the eastern and central Mediterranean, northern Africa, and the Middle East. Furthermore, it is highly prevalent in the Indian subcontinent and in a group of islands in the Indian Ocean with high endemic levels of HBV (Nicobar and Andaman) [44], and has also been identified in Oceania [43]. Nine HBV/D subgenotypes (D1–D9) have been described to date [45]. D1 is the most prevalent subgenotype in Greece, Turkey, and North Africa [46, 47]; D2 in northeastern Europe (Russia, Belarus, and Estonia) and Albania [48, 49]; and D3 in Italy and Serbia [50, 51]. D4 is the dominant subgenotype in Oceania [40], D5 in primitive tribes living in India, where a number of different D subgenotypes are also found [52], D6 in Papua New Guinea and Indonesia [53], and D7 in Tunisia and Morocco [54, 55]. Finally, the recently described D8 and D9 subgenotypes are found in Nigeria and India.

Genotype E is characterized by a three-nucleotide deletion in the preS1 region. Genotype E is mainly dominant in West Africa [56]. Genotype E is rarely found outside of Africa, except in individuals of African descent. Although it is found over a large geographical area, it is interesting to note that it has a very low degree of genetic diversity: the isolates studied by means of phylogenetic analysis do not segregate into distinct subgenotypes, but are included in a single monophyletic group [57]. This observation suggests that it has a relatively recent evolutionary history among humans and, despite the forced immigration of West African slaves [57], the absence of any significant spread among Afro-Americans indicates that it was probably rare in West Africa at the time of the slave trade and before the nineteenth century. The only documented finding of its presence in America is a report by Alvarado et al., who identified nine HBV-infected individuals carrying genotype E in 2010 in the relatively isolated Afro-American community of Quibdò, Colombia [58]. All of these strains were identified by means of their two-nucleotide synapomorphy in the S region, thus forming a highly significant monophyletic group.

Genotype F is indigenous to America, and is the most prevalent HBV genotype in Central and South America, and among the Amerindians of the Amazon basin [59, 60]. Genotype F is classified into four subgenotypes (F1–F4), which are further subdivided into different clades [43]. F1 is highly prevalent in Central America, Alaska, and southeast America [61, 62]; F2 is highly prevalent in Venezuela, and is also present in Brazil [61]; F3 is present in central (Panama) and northern Latin America (Colombia and Venezuela); and F4 is present in Bolivia and Argentina [63]. The presence of HBV-F among the Amerindian population suggests the long evolution of this s train. In the study by Alvarado et al. on the molecular epidemiology and evolutionary dynamics of HBV/F in Colombia [64], it was found that HBV/F3 was the most prevalent subgenotype in Colombia, and its origin was suggested to be in Venezuela. This is probably the oldest F subgenotype, as it is closely related to genotype H [61, 64].

In 2000, genotype G was defined as the seventh HBV genotype from a strain isolated from a French patient [11]. Genotype G harbors a 36-nucleotide insertion in the core region and a genome length of 3248 base pairs. The HBe antigen was detected in the sera of individuals infected with genotype G, despite the presence of two stop codons in the precore region, which should not allow production of HBe antigen [65]. Stuyver et al. proposed that genotype G might have a unique mechanism that allowed production of the HBe antigen. However, we revealed that the sera of all the individuals infected with genotype G showed coinfection with genotype A [66]. Thus, the HBe antigen in the sera of individuals infected with genotype G was produced due to the coinfection with genotype A HBV, which does not have a stop codon in its precore region. Much evidence has accumulated showing that genotype G was not exceptionally associated with co-infection with HBV of other genotypes [67, 68]. Genotype G was identified frequently in homosexual men, and demonstrated very low genome diversity.

Phylogenetic analysis has demonstrated that genotype H is closely related to genotype F. Genotype H is prevalent in Mexico in both the indigenous populations and the mestizos (individuals of mixed descent), suggesting that this genotype has a long history among the descendants of the Aztecs, preceding the arrival of Europeans [12, 69]. Considering that genotype F demonstrates a wide range of diversity, it has been proposed that genotype H should be classified as a subgenotype of genotype F [70].

In 2008, sequence analysis of the complete genome of a single isolate from a Vietnamese male showed that it was closely related to three previously described “aberrant” Vietnamese strains [15, 71] and a ninth genotype, I, was proposed [13]. This proposal was not accepted, because the mean genetic divergence of these four strains from genotype C was 7 % and the recombination analysis was not robust [72]. Subsequently, sequences derived from isolates obtained from Laos [73], the Idu Mishmi tribe in northeast India [74], a Canadian of Vietnamese descent [75], and China [76] have expanded the number of these sequences. The nucleotide divergence of most of these sequences relative to genotype C was at least 7.5 %, with good bootstrap support for the group, thus meeting the criteria for genotype assignment [77]. Two subgenotypes, I1 and I2, with serological subtypes adw 2 and ayw 2, respectively, were described [73]. Genotype I is a recombinant of genotypes A/C/G and an indeterminate genotype [73–76], which clusters close to genotype C when the complete genome is analyzed, and with genotype A in the polymerase gene region [76]. The genotype A and C regions are closely related to subgenotypes A3 and C3, respectively [73–76].