Hepatitis A virus(HAV)is one of the most common infectious etiologies of acute hepatitis worldwide.The virus is known to be transmitted fecal-orally,resulting in symptoms ranging from asymptomatic infection to fulminant hepatitis.HAV can also be transmitted through oral-anal sex.Residents from regions of low endemicity for HAV infection often remain susceptible in their adulthood.Therefore,clustered HAV infections or outbreaks of acute hepatitis A among men who have sex with men and injecting drug users have been reported in countries of low endemicity for HAV infection.The duration of HAV viremia and stool shedding of HAV may be longer in human immunodeficiency virus(HIV)-positive individuals compared to HIV-negative individuals with acute hepatitis A.Current guidelines recommend HAV vaccination for individuals with increased risks of exposure to HAV(such as from injecting drug use,oral-anal sex,travel to or residence in endemic areas,frequent clotting factor or blood transfusions)or with increased risks of fulminant disease(such as those with chronic hepatitis).The seroconversion rates following the recommended standard adult dosing schedule(2doses of HAVRIX 1440 U or VAQTA 50 U administered6-12 mo apart)are lower among HIV-positive individuals compared to HIV-negative individuals.While the response rates may be augmented by adding a booster dose at week 4 sandwiched between the first dose and the 6-mo dose,the need of booster vaccination remain less clear among HIV-positive individuals who have lost anti-HAV antibodies. Hepatitis A virus (one HAV) is one of the most common infectious etiologies of acute hepatitis virus. The virus is known to be transmitted fecal-orally, resulting in symptoms ranging from asymptomatic infection to fulminant hepatitis. HAV can also be transmitted through oral-anal sex.Residents from regions of low endemicity for HAV infection often remain susceptible in their adulthood.Therefore, clustered HAV infections or outbreaks of acute hepatitis A among men who have sex with men and injecting drug users have been reported in countries of low endemicity for HAV infection. duration of HAV viremia and stool shedding of HAV may be longer in human immunodeficiency virus (HIV) -positive individuals compared to HIV-negative individuals with acute hepatitis A. Current guidelines recommend HAV vaccination for individuals with increased risks of exposure to HAV such as from injecting drug use, oral-anal sex, travel to or residence in endemic areas, frequent clotting factor or blood transfusions) or with increased risks of fulminant disease (such as those with chronic hepatitis). The seroconversion rates following the recommended standard adult dosing schedule (2doses of HAVRIX 1440 U or VAQTA 50 U administered 6-12 mo apart) are lower among HIV-positive individuals compared to HIV-negative individuals.While the response rates may be augmented by adding a booster dose at week 4 sandwiched between the first dose and the 6-mo dose, the need of booster vaccination remain less clear among HIV-positive individuals who have lost anti- HAV antibodies.