AIM To investigate the protective efficacy ofH2 strain attenuated live hepatitis A vaccines(H2-strain vaccines)in hepatitis A(HA)outbreaks.METHODS With the permission of theirparents,5551 pre-school and grade 1-3 primaryschool children were inoculated with 1 dose(10~(6.5)TCID_(50))of H2-strain vaccines in anonrandomized,controlled trial conducted inFucheng County,Hebei Province in May 1997.Another 6485 children in the same grades andcompatible in gender and age were enrolled ascontrols.Epidemiological and serological surveywas conducted to evaluate the protectiveefficacy of the vaccines.ELISA was used todetect serum IgM anti-HAV.RESULTS HA outbreak started in early May1998,peaked in the middle of the same month,and lasted about 80 days.Overall 302 HA caseswere found,192(53.58%)were 5-9 years old.One vaccinee and 25 control cases were found tohave hepatitis A,which account for 0.28%(1/356)and 5.92%(25/422)of all vaccinees andcontrols in the 14 villages,respectively.The protective efficacy of vaccines was 95.27%(95% Cl:85.83%-104.72%).In subjects testedfor anti-HAV IgM from 13 villages,1(0.40%)overt and 11(4.06%)asymptomatic HAV caseswere found in 271 vaccinees,but 21(6.69%)ofovert and asymptomatic ones were found in 314controls.CONCLUSION H_2 strain vaccines were excellent in preventing overt hepatitis A, but not so effective in preventing asymptomatic hepatitis A virus infection. A booster dose might be needed to get permanent reliable immunity. AIM To investigate the protective efficacy ofH2 strain attenuated live hepatitis A vaccines (H2-strain vaccines) in hepatitis A (HA) outbreaks.METHODS With the permission of theirparents, 5551 pre-school and grade 1-3 primaryschool children were inoculated with 1 dose (10 ~ (6.5) TCID_ (50)) of H2-strain vaccines in anonrandomized, controlled trial conducted in Fucheng County, Hebei Province in May 1997. Another 6485 children in the same grades andcompatible in gender and age were enrolled ascontrols.Epidemiological and serological surveywas conducted to evaluate the protective efficacy of the vaccines. ELISA was used to detect serum IgM anti-HAV.RESULTS HA outbreak started in early May 1998, peaked in the middle of the same month, and lasted about 80 days. Overall 302 HA caseswere found, 192 (53.58%) were 5-9 years old. One vaccinee and 25 control cases were found to have hepatitis A, which account for 0.28% (1/356) and 5.92% (25/422) of all vaccinees and controls in the 14 villages, respectively cy of vaccines was 95.27% (95% Cl: 85.83% -104.72%) In subjects tested as anti-HAV IgM from 13 villages, 1 (0.40%) overt and 11 (4.06%) asymptomatic HAV cases were found in 271 vaccinees, but 21 (6.69%) of of atrtrt and asymptomatic ones were found in 314controls.CONCLUSION H_2 strain vaccines were excellent in preventing overt hepatitis A, but not so effective in preventing asymptomatic hepatitis A virus infection. A booster dose might be needed to get permanent reliable immunity.