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Hepatitis B virus (HBV) infection remains an important global health problem despite the availability of a safe and effective vaccine; >350 million people worldwide are chronically infected [1]. Infection with HBV in adults most often results in self-limited, acute hepatitis that confers protective immunity and causes no further disease. In contrast, most children fail to clear the virus, resulting in chronic infection in 90% of children who are infected perinatally. Chronically infected children are generally asymptomatic but are at risk of developing liver cirrhosis and hepatocellular carcinoma [2, 3]. Since 1992, the World Health Organization has recommended global vaccination against HBV, and by the end of 2009, 177 countries had implemented a universal HBV immunization program for newborns, infants, and/or adolescents. Countries with a low level of HBV endemicity, such as Japan and many northern European countries, have adopted a strategy in which vaccination is offered to individuals at high risk of infection [4]. Spread of HBV occurs through contact with blood or other bodily fluids of an infected person. In countries with low endemicity, the spread of HBV is caused predominantly by sharing contaminated equipment during drug injections and through sexual contact. In this issue of the Journal, Komatsu et al report their findings of high levels of HBV DNA in tear specimens from children with chronic HBV infection and show how inoculation with these specimens led to HBV infection in chimeric mice. Furthermore, they report high levels of HBV DNA in saliva, sweat, and urine samples from children with chronic HBV infection, results that are supported by several studies [5–8]. In countries where only high-risk groups are vaccinated, these findings are of clinical importance when assessing the real and the perceived risk of horizontal transmission of HBV. Children with chronic HBV are more prone than adults to be HBV e antigen positive and to have a high viral load. At the same time, children are more likely to have contact with each other’s body fluids, such as saliva and tears, and therefore have a high risk of horizontal transmission. Horizontal transmission is especially important in children who are at a high risk of acquiring chronic, asymptomatic infection when exposed to HBV. That children often are asymptomatic after infection with HBV lets them enter the large pool of chronic carriers unnoticed. The report by Komatsu et al provides further evidence of the risk of horizontal transmission, although the precisemechanisms of transmission are unknown. It may possibly be due to contact of nonintact skin or mucous membranes with tears, saliva, or blood-containing secretions. Transmission from sharing personal care items such as toothbrushes may also occur [9]. As Komatsu and colleagues state, physicians from countries with an HBV vaccination strategy targeting at-risk individuals are keen to know whether various body fluids are sources of HBV transmission; in addition, they are concerned whether these strategies are effective in preventing HBV infection. It is believed that the ideal HBV immunization strategy is to implement universal vaccination for children or adolescents [4]. By this strategy, lower carrier rates of HBV surface antigen in children are seen within relatively few years [10], but it is important to state that several immunization strategies are needed in areas of low endemicity until universal vaccination has been in place for 20–40 years. The issue of cost-effectiveness is important in countries where HBV has a low level of endemicity, but although the strategy focused on at-risk individuals might initially appear to be a costReceived and accepted 12 October 2011; electronically published 16 April 2012. Correspondence: Ida Louise Heiberg, MD, Department of Pediatrics 460, Hvidovre Hospital, University of Copenhagen, Kettegård Allé 30, 2650 Hvidovre, Denmark (ida.heiberg@ gmail.com). The Journal of Infectious Diseases 2012;206:464–5 © The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. permissions@oup.com. DOI: 10.1093/infdis/jis294

Horizontal Transmission of Hepatitis B Virus--Why Discuss When We Can Vaccinate?