Hepatitis B vaccination is the most effective way to prevent hepatitis B virus infection. Hepatitis B vaccination usually requires 3 injections, according to 0, 1, 6 months program, i.e., after the first vaccination, the second and third injections are given at intervals of 1 month and 6 months, and the earlier the vaccination time, the better. children under 15 years old are the main target of Hepatitis B vaccine because their immune system is not yet fully developed, and they will easily become chronic hepatitis B patients after infection with the Hepatitis B virus. Although hepatitis B vaccine can effectively prevent hepatitis B, not all children are suitable for immediate vaccination. First of all, as far as preterm infants are concerned, preterm infants of HBsAg-negative pregnant women with unstable vital signs should first be treated for related diseases before receiving hepatitis B vaccine. If the preterm baby is <2000 g, the vaccination should be started after the baby reaches 2000 g. Preterm babies born to HBsAg-positive pregnant women need to wait until their vital signs are stabilized before receiving the first dose of the vaccine, even though weight is not a concern. In addition to this, there are also contraindications to the Hepatitis B vaccine. Although the World Health Organization has basically not stipulated other contraindications for hepatitis B vaccination except for allergy, China has still made some stipulations from the safety point of view. Generally speaking, vaccination is not recommended when there are acute diseases or serious chronic diseases, acute attacks of chronic diseases and fever. There are some neurological diseases, such as epilepsy and degenerative diseases, China also as a contraindication to hepatitis B vaccination. Parents should carefully and objectively explain the situation to the doctor when their children are vaccinated, and should suspend the vaccination when encountering the above problems. Meanwhile, side effects and substandard antibody titers may also be encountered during hepatitis B vaccination. The most common side effect of Hepatitis B vaccination is localized pain at the injection site, which can usually be relieved in a few hours. Some may have hard nodules, which will also subside after a while. Under normal circumstances, after the 1st injection of hepatitis B vaccine, most anti-HBs are still negative or below the lower limit of detection, parents do not need to worry too much; about 1 week after the 2nd injection, the anti-HBs will turn positive, that is, 35~40d after the start of the vaccination has immunity against hepatitis B virus; the 3rd injection can make the level of anti-HBs rise significantly and prolong the years of protection. If the antibody level is lower than 10 mg international unit/ml (mIU/ml) after 3 injections, the original hepatitis B vaccine and the dose can be used to receive 3 injections according to the 0, 1, 6-month program again; or receive 1 injection of 60 micrograms recombinant yeast hepatitis B vaccine. 1~2 months after the 2nd vaccination of 3 shots or 1 shot of 60 micrograms of hepatitis B vaccine, test the surface antibody of hepatitis B virus in the blood, if it is still negative, you can get another shot of 60 micrograms of hepatitis B vaccine. How long can a child be protected by hepatitis B vaccination? Children who receive the hepatitis B vaccine and develop surface antibodies to the hepatitis B virus are generally protected for at least 12 years. As time goes by, the level of hepatitis B virus surface antibody in vaccinated persons will gradually decrease, and some of them may not even be able to detect hepatitis B virus surface antibody. If you are not a high-risk group for hepatitis B, you do not need to be revaccinated. However, if they are high-risk groups, as they are at higher risk of contracting hepatitis B, it is best for them to receive a booster shot of hepatitis B vaccine when their blood surface antibody against hepatitis B virus becomes negative.