The second dose of Hepatitis B vaccine should be given when the infant reaches one month of age. If the vaccine is not given in time, it should be replenished as early as possible, with no exact maximum delay time. 1. The vaccination program of hepatitis B vaccine is 0, 1 and 6, respectively, one dose should be given within 24 hours of birth, at the age of 1 month and at the age of 6 months. The interval between the second dose and the first dose should be not less than 28 days, the interval between the third dose and the second dose should be not less than 60 days, and the interval between the third dose and the first dose should be not less than 4 months. 2. If the second dose of hepatitis B vaccination is delayed for various reasons, the vaccination should be made up as soon as possible. Because the longer the interval, the immunization effect will be reduced. Meanwhile, infants and young children need to be vaccinated with many kinds of vaccines within 6 months of age, and postponing the vaccination will also affect the immunization of other vaccines.