This study describes the epidemiology of acute hepatitis B and chronic HBV infection in Norway between 1992-2009. In accordance with the results from other European countries, acute hepatitis B most commonly occurs amongst injecting drug users [5, 11]. Chronic infection is mostly affecting immigrants from high endemic parts of the world [6, 12].
Nearly all the diagnosed cases of HBV infection are reported to the national surveillance system as laboratories mandatorily report all tests that have positive HBV markers. Clinicians also report a large proportion (80%) of the laboratory reported cases of acute hepatitis B. Therefore, there is a good overview on the disease in Norway. Nevertheless additionally to existing under-reporting only 30% of acute HBV cases would develop jaundice, which is the main reason to seek medical care among HBV patients .
There is no data in the surveillance system as to whether the person has been exposed to more than one risk factor (i.e. intravenous drug use and sex); although clinicians can report both. Nevertheless it is difficult to recognize which exposure caused the infection first. There is no data on HIV co-infection or other sexually transmitted infections.
Data for chronic HBV infection is not as accurate as for acute hepatitis B: exposure is unknown in 83% of notifications. It is difficult for clinicians to obtain information from immigrant populations from highly endemic countries, reflecting low reporting coverage.
The hepatitis B incidence in Norway is in the same range as other Nordic countries (Figure 1). Although, it is difficult to compare data from these countries as some surveillance systems do not distinguish between acute and chronic infection .
The increase in the late 1990s was due to a nationwide outbreak of hepatitis A and hepatitis B among IDUs [10, 14]. Similar outbreaks were observed in Finland, Sweden and the Baltic countries, mainly among groups of non-immune IDUs [14, 15]. These outbreaks coincided with increased number of IDUs and reduced drug prices [10, 14, 16].
While the estimated number of IDUs was stable between 2004 and 2008 , the number of IDUs with acute hepatitis B has decreased. According to the Norwegian Institute for Alcohol and Drug Research it might be a result of decrease in heroin users in the country which is probably related to a substantial increase in the availability of substitution treatment during the period . Stabilization in the number of IDUs has also been observed in other European countries . The decreasing incidence might also be due to vaccination of IDUs and their close contacts. Screening of a group of IDUs in Oslo (2009) indicates 35% vaccination coverage (unpublished data). The number of carriers (anti-HBc positive) among IDUs in Oslo has decreased from 50% in 2002 to 45% in 2007 . In addition, following the large outbreak in late 1990s, the number of susceptible individuals has probably been considerably reduced.
Even though Norway is one of the countries with well established preventive measures for marginalised populations like IDUs, sexual activity with this risk group contributes to further spread of the infection. The proportion of cases infected through sexual activity has slightly increased in early 2000s following the outbreak among IDUs and has increased in 2009 (Figure 3). A larger proportion of males among those with acute hepatitis B is explained by the preponderance of males amongst IDUs. Surveillance data suggests sexual activity to be the second most important exposure. This is well recognized in other low prevalence countries [20, 21]. For more than half of the cases of acute hepatitis B sexual orientation is not specified and this group is mostly represented by males (62%), partly covering all types of sexual orientation. The lack of information about sexual orientation is due to the ethical considerations of personal identification of the patient, therefore this data is not included in the notification form, preventing clinicians of asking for this kind of information.
Homosexual orientation was mentioned by 11% of those with known sexual orientation, all males. MSM are a well recognized risk group for hepatitis B and other STIs [11, 22]. Vaccination is in place for this group in Norway as in other countries . Although, vaccination coverage is not monitored, a web-based survey among MSM in Norway in 2007  revealed a coverage of 47% among respondents (unpublished data). This is in line with a study from USA . The injecting drug use among Norwegian MSM is unknown, however it is interesting to note that injecting drug use was uncommon among MSM recruited in the Amsterdam Cohort Studies .
Age distribution of the cases of acute hepatitis B with high incidence starting in age group 16-19 years reflects the onset of injecting drug use and sexual activity [17, 25].
Although the mode of transmission was unknown in 83% of the chronic hepatitis B cases, we assume that almost all have been infected at birth or early in childhood .
The increase of newly diagnosed chronic hepatitis B in the last years of the study period is closely related to the increasing number of immigrants from countries with a higher prevalence of hepatitis B. The immigrant population represents 10% of the Norwegian population, with a twofold increase in residents of Asian, African, South and Central American origins since the early 90's, and a six fold increase in residents of Eastern European origin (SSB).
Immigrant populations from highly endemic countries have an impact on overall prevalence . However, further infection from the immigrant population is quite limited . Additionally, a molecular study by Fisker et al (2004) suggests that immigrants pose a smaller risk to general population than IDUs .
Hepatitis B screening of immigrants is in place in Norway. However, there is little data regarding the uptake of testing and vaccination amongst these populations. Certain immigrant populations are both hard to reach and to convince as to the merits of testing, this may be due to cultural differences.
Countries with selective vaccination strategy have low prevalence of the disease and behavioural risk groups are the main source of the infection. Low vaccination coverage among MSM and IDUs are in line with some studies, therefore suggesting a reconsideration of a universal vaccination strategy [21, 27, 28].