(a) What is hepatitis B? An infectious disease caused by hepatitis B virus (HBV) infection, mainly inflammation and necrotic lesions of the liver, is called viral hepatitis B, or “hepatitis B” for short. Hepatitis B is a major disease that threatens human health, and HBV infection is prevalent worldwide, but the intensity of HBV infection varies greatly from region to region. According to the World Health Organization, about 2 billion people worldwide have been infected with HBV, 350 million of whom are chronically infected with HBV, and about 1 million people die each year from liver failure, cirrhosis and liver cancer (HCC) caused by HBV infection. The results of the 2006 national epidemiological survey of hepatitis B show that the HBsAg carriage rate of the general population aged 1-59 in China is 7.1 8%, and the HBsAg carriage rate of children under 5 years old is only 0.96%. According to this projection, the existing chronic HBV infection in China is about 93 million people. Among them, there are about 20 million cases of chronic hepatitis B. (The cause of hepatitis B is HBV, which belongs to the family of hepatophilic DNA viruses (hepadnaviridae), with a genome length of about 3.2 kb and a part of double-stranded circular DNA. However, HBV can be inactivated by 65°C for 10 h, boiling for 10 min or high pressure steam. ethylene oxide, glutaraldehyde, peroxyacetic acid and iodoform also have a good inactivation effect on HBV. HBV is a blood-borne disease, mainly transmitted by blood (such as unsafe injection, etc.), mother-to-child and sexual contact. In China, pediatric hepatitis B infection is still mainly caused by mother-to-child transmission. Due to strict HBsAg screening of blood donors, HBV infections caused by blood transfusions or blood products have become less frequent. Transmission through broken skin and mucous membranes is mainly due to the use of medical devices that are not strictly sterilized, invasive diagnostic and surgical procedures, unsafe injections, especially drug injections, etc. Other infections such as pedicures, tattoos, earring piercings, accidental exposure of medical personnel at work, and sharing of razors and toothbrushes can also be transmitted. Mother-to-child transmission occurs mainly in the perinatal period, mostly from contact with the blood and body fluids of H BV-positive mothers during delivery. The risk of HBV infection is increased by unprotected sexual contact with HBV-positive people, especially those who have multiple sexual partners. HBV is not transmitted through the respiratory and digestive tracts, so daily study, work or living contacts, such as working in the same office (including sharing office supplies such as computers), shaking hands, hugging, living in the same room, eating in the same restaurant and sharing toilets without blood exposure, generally will not transmit HBV. epidemiological and experimental studies have not found that HBV can be transmitted by blood-sucking insects (mosquitoes, bedbugs, etc.). (C) Natural history of pediatric hepatitis B The natural history of HBV infection in infancy can generally be artificially divided into four phases: ① Immune tolerance phase: characterized by positive serum HBsAg and HBeAg, the highest HBV DNA load (often >2×106IU/ml, equivalent to 107 copies/m1), but normal serum glutamate transaminase (ALT) levels, no significant abnormalities in liver histology and can be maintained for several years or even decades, or mild inflammatory necrosis, no or only slow progression of liver fibrosis. (②Immune clearance stage: manifested by serum HBV DNA titer >2000 IU/ml (equivalent to l 04 copies/m1), accompanied by continuous or intermittent elevation of ALT, moderate or severe inflammatory necrosis of liver histology, liver fibrosis can progress rapidly, and some patients can develop cirrhosis and liver failure. ③ inactive or low (non) replication stage: manifested by HBeAg negative, anti-HBe positive, HBV DNA consistently below the minimum detection limit, normal ALT levels, liver histology without inflammation or only mild inflammation; this is the result of HBV infection gaining immune control, most patients in this stage have a greatly reduced risk of cirrhosis and HCC, in some patients who have sustained HBV DNA transitions for several years The spontaneous HBsAg serological conversion rate is 1% to 3%/year. ④ Reactive phase: Some patients in the inactive phase may have 1 or several episodes of hepatitis, mostly presenting as HBeAg negative and anti-HBe positive [partly due to low or no HBeAg expression levels due to promoter (BCP) variants in the pre-C region and/or the basic core region of the C gene], with active HBV DNA replication and persistent or recurrent abnormal ALT, becoming HBeAg-negative chronic hepatitis B. These patients may progress to liver fibrosis, cirrhosis, decompensated cirrhosis, and HCC. Some patients may also develop spontaneous HBsAg disappearance (with or without anti-HBs) and reduced or undetectable HBV DNA, and thus the prognosis is often good. A small number of patients in this stage may return to HBeAg-positive status (especially in immunosuppressed states such as with chemotherapy). Not all people infected with HBV go through these 4 stages. Only a minority (about 5%) of neonatal HBV infections can spontaneously clear HBV, while most have a long immune tolerance period and then enter the immune clearance phase, but most adolescents and adults infected with HBV have no immune tolerance period and enter the immune clearance phase directly. positive chronic hepatitis B.