Mother-to-child transmission is the main route of transmission of HBV in China, and about 30% - 50% of chronic HBV carriers are infected by this route. Although the current hepatitis B vaccine combined with hepatitis B immunoglobulin scheme has achieved excellent results, about 5% - 10% of transmission prevention failure still occur. More than 90% of newborns will develop chronic infection after HBV infection, and about one quarter will eventually develop cirrhosis and / or hepatocellular carcinoma, which is extremely serious. Therefore, improving and optimizing the current transmission prevention technology and program to further increase the success rate of HBV mother-to-child prevention is to reduce the transmission and prevalence of HBV in China, reduce the burden of hepatitis B disease, and achieve the "three-year plan" and "two rates" goals.

Currently, pregnant women's blood hepatitis B virus load ≥ 2 × 10\^5 IU/mL before delivery is the main risk factor that affects transmission prevention and whether infant infection occurs. For pregnant women with high HBV viral load, antiviral therapy during pregnancy can further reduce mother-to-child transmission of HBV. Previous studies have confirmed that tenofovir disoproxil fumarate (TDF) 300mg as a single therapy or combination therapy has good safety and antiviral activity, and can be used as a high viral load for chronic HBV infection as a first-line antiviral treatment for pregnant women. Tenofovir alafenamide (TAF) is a first-line treatment for chronic hepatitis B. Its specification is 25mg. It is an RNA-dependent DNA polymerase inhibitor. The agent is an oral prodrug of tenofovir (TFV), similar to TDF. Because the dosage is much lower than TDF, the incidence of adverse reactions is significantly lower than TDF, but the clinical efficacy is equivalent to TDF. Studies have reported that TAF treatment of HBeAg-positive chronic hepatitis B patients, at 96 weeks HBsAg and HBeAg negative rate, serological conversion rate, ALT normalization rate, etc. were not statistically different from the TDF treatment group. The incidence of ≥ 5% reduction in hip bone and spine bone density, as well as ≥ 5% decrease in estimated glomerular filtration rate (eGFR) from baseline were significantly lower than those in the TDF treatment group. TAF is recommended as the drug of choice for the treatment of chronic hepatitis B.

Extensive past data on pregnant women (over 3,000 exposure outcomes) have shown that there are no malformations or fetal / newborn toxicity associated with TDF. However, no such data exists for TAF. Presently, TAF has been approved by the State Food and Drug Administration (SFDA) and marketed in China in December 2018. SFDA Label has suggested that TAF may be considered during pregnancy if necessary. It has not been reported whether the application of TAF in pregnant women can achieve better effects and safety of mother-to-child transmission prevention, and is the focus of this current study.

This is a multicenter, prospective, triple arm cohort study from the gestational age of 28 weeks in pregnancy to post-partum week 28. The enrollment from 4 centers (Beijing You'an Hospital, Capital Medical University, the Fourth Affiliated Hospital of Harbin Medical University, the third hospital of Qinhuangdao city, Inner Mongolia Tongliao infectious disease hospital) will be caped for sample balance. Consecutive 120 HBeAg-positive and HBV DNA levels ≥ 2 × 10\^6 IU/mL pregnant women will be enrolled to receive TAF (25mg oral daily) treatment from the aforementioned 4 centers, consecutive 120 HBeAg-positive and HBV DNA levels ≥ 2 × 10\^6 IU/mL pregnant women will be enrolled to receive TDF (300mg oral daily) treatment, and a historical cohort of consecutive highly viremic mothers with HBV DNA levels ≥ 2 × 10\^6 IU/mL will be retrospective enrolled as the control group. Patients in the TAF-treated group will receive TAF starting at week 28 of gestation until delivery (if the liver function is abnormal at delivery, ie ALT≥5×ULN, the treatment can be continued according to the wishes of the pregnant woman and the laboratory tests). All infants will receive passive-active immunoprophylaxis. According to the mother's wishes, the mother's milk is collected every day for 5-7 days for TAF concentration determination. In this trial, when the creatinine clearance is less than 15 mL/min, the subjects will discontinue the drug permanently. Subjects with permanent discontinuation (either before or after discontinuation according to the protocol) were followed up every 4 weeks until 16 weeks after discontinuation, or until they received new antiviral treatment. The primary endpoint was the rate of mother-to-child transmission in TAF-treated group compared with control group. The tolerance and safety in TAF-treated group, including congenital malformation rate of infants. The secondary efficacy endpoint was the decrease of HBV DNA level at delivery, the clearance and seroconversion rate of HBsAg or / and HBeAg, ALT normalization and other adverse events of mothers and infants.

For comparison, there will be two comparative arms of mothers whose pregnancies were followed at a single center, Capital Medical University, Beijing Youan Hospital. Arm B will consist of 120 consecutive HBeAg-positive and HBV DNA levels ≥ 2 × 10\^6 IU/mL pregnant women enrolled to receive TDF (300mg oral daily) treatment, with all treatment variables constant with the TAF treatment group to act as a control. The TDF treatment for arm B will start at week 28 of gestation until delivery. Arm C will consist of 360 HBeAg-positive and HBV DNA levels ≥ 2 × 10\^6 IU/mL pregnant women that did not receive any antiviral treatment during pregnancy. This third group is a historical cohort that will be retrospectively enrolled as an additional comparative arm for further comparison and analysis.