title: Immunity to hepatitis A when outbreaks of infection in men who have sex with men (MSM) are rare.
abstract: BACKGROUND
  Epidemics of hepatitis A among men who have sex with men (MSM) have decreased significantly in recent years although the level of immunity that is required to prevent epidemics has not been studied. Our aim was to determine the level of immunity to hepatitis A among MSM.
  
  
  METHODS
  This was a retrospective study conducted using notifications of Hepatitis A in Victoria from 1991 to 2010, serological testing for hepatitis A among MSM attending Melbourne Sexual Health Centre (MSHC), and vaccination records among MSM attending MSHC.
  
  
  RESULTS
  Hepatitis A notifications declined from 370 to 47 and the male to female ratio declined from 4.2 to 0.9 in Victoria between 1991 to 2010. Between 2002 and 2011, there were 12,064 individuals MSM seen at MSHC of whom 3055 (25%) were tested for hepatitis A antibodies and 1180 (39%) had antibodies. The proportion of MSM who were tested for hepatitis A rose significantly over time (P<0.01), but the proportion of these with hepatitis A antibodies did not (P=0.28). Hepatitis A antibodies were more common in MSM over 30 (54%) compared to those 30 or less (32%), (Crude Odds Ratio 2.5 (95% confidence interval 2.1-2.9)) and were uncommon in MSM under 20 (19%). Vaccination against hepatitis A was recorded in 49% of 660 clinical files of MSM who attended the centre between 2003 and 2011 and did not change over time (P=0.42) but was significantly more common in those over 30 years of age (P<0.005).
  
  
  CONCLUSIONS
  Hepatitis A is rare in MSM in Victoria where levels of immunity are about 40-50%. As outbreaks have occurred when levels of immunity were around 30%, maintaining vaccination levels over 40-50% is important if outbreaks are to be prevented. The lower levels of immunity in younger MSM create the potential for outbreaks in this sub-group.
authors.affiliation: Melbourne Sexual Health Centre, Alfred Hospital, Melbourne, Australia. ajith weera1964@yahoo.com
authors.forename: Ajith P
authors.initials: AP
authors.lastname: Weerakoon
authors.forename: Marcus Y
authors.initials: MY
authors.lastname: Chen
authors.forename: Tim R H
authors.initials: TR
authors.lastname: Read
authors.forename: Catriona
authors.initials: C
authors.lastname: Bradshaw
authors.forename: Christopher K
authors.initials: CK
authors.lastname: Fairley
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018563:Disease Notification / Q000706:statistics & numerical data; D004196:Disease Outbreaks; D005260:Female; D006506:Hepatitis A / Q000453:epidemiology* / Q000276:immunology*; D035922:Hepatitis A Antibodies / Q000097:blood; D022362:Hepatitis A Vaccines / Q000008:administration & dosage; D018451:Homosexuality, Male; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D014611:Vaccination / Q000706:statistics & numerical data; D014739:Victoria / Q000453:epidemiology; D055815:Young Adult
chemical_list: D035922:Hepatitis A Antibodies; D022362:Hepatitis A Vaccines