In 1992, the World Health Organization (WHO) recommended the integration of the hepatitis B vaccine into the national immunization programs of all highly endemic countries by 1995 . Following the WHO's recommendation, the HBsAg prevalence in China fell among children born after 1992  and more so for children born after 2002, when the hepatitis B vaccine was fully integrated into routine infant immunization . Our study confirms those findings. In Anhui province, HBsAg prevalence among children aged 0-4 years is merely 0.3%, which is lower than children aged 5 and up.
The carrier rate for HBsAg is 7.44% among all participants. To compare this figure with the national serosurvey, the carrier rate in our paper was standardized based on the 2002 national population constituent ratio, which was 6.56%. The standardized rate for HBsAg prevalence among the general population aged 1-59 years is 6.71%, which is lower than the national serosurvey rate in 2006 and lower than the rates from the Anhui serosurveys in 1979 and 1992. Again, we attribute the falling rates to the universal immunization program for infants. The prevalence of HBsAg is low among children age < 5 years but this increases with age, which indicates an inverse relationship between age and likelihood of vaccination. This suggests the need for an immunization program to encourage both older children and adult populations to be vaccinated as well. The Chinese government is currently implementing policies aimed at these populations to control hepatitis B, such as HBsAg screening of blood for transfusion, control of blood exposure in medical settings, and the management and treatment of HBV-infected persons .
The multivariate logistic regression analysis revealed that among children, the main risk factor for HBsAg positivity is age. Children aged 0-4 have more access to the hepatitis B vaccine and a better hepatitis B vaccine service than children aged 5 onwards. Thus is because the routine Expanded Programme on Immunization (EPI) has both improved and been more emphasized since 2002. In addition, the financial barriers to vaccination have become less of an issue because of the recent rising economic development in Anhui province. More pregnant women are being screened for HBsAg, and better medical records are being kept. This allows a hospital to determine if the expectant mother requires testing if HBsAg screening was not previously performed. The infants of those women found positive will then be given the HBsAg vaccine and Hepatitis B Immunoglobulin (HBIG) within 24 hours after birth.
Among adults aged 16-59 years, the risk factors include being male, a history of operations, a family member who is HBsAg positive, and not being immunized. These factors are in agreement with recent seroprevalence studies [15, 16]. Among adults aged 60+ years old, the greatest risk factor was a history of blood transfusions. This finding suggests that horizontal HBV transmission and health-care-related factors are mainly responsible for the prevalence among all adults. Horizontal transmission and mother-to-infant transmission of HBV are demonstrated by strong family clustering occurring through frequent exposure to blood (e.g., through contact with skin lesions), saliva (e.g., through sharing of toothbrushes and candy), or breast milk [17, 18]. These factors are not specific to resource-poor settings, as it has also been described within families living in the United States and Europe [19–21].
Health-care-related transmission has long been recognized as an important source of new HBV infections worldwide. While most HBV transmissions have occurred during invasive surgical or obstetric procedures , our findings indicate that people with any operational history are also at risk for HBV infection. There are three possible routes for infection during an operation: the first from a surgeon to a patient, the second from contaminated surgical instruments to a patient, and the third from an HBV-positive patient to another patient staying in the same hospital room [22–24]. Throughout China, nosocomial infection control is conducted stringently in provincial and municipal level hospitals but insufficiently in certain county and town level hospitals as well as several private hospitals where invasive surgical or obstetric procedures are frequently performed [25, 26]. Surgeon-to-patient transmissions of hepatitis B were essentially eliminated when the vaccination of health care workers became routine . However, preventing all health-care-related transmissions of HBV requires a comprehensive approach that includes administering nosocomial infections, consistently providing hepatitis B vaccination to healthcare personnel, enforcing stricter measures to reduce blood exposure between healthcare workers and patients, and having all surgical teams committed to promoting and maintaining a safe work environment constantly [28, 29].
An interesting finding is that a history of blood transfusions is the greatest risk factor for HBV infection among adults who are more than 59 years old. Although recent investigations have suggested that blood transfusion remains a major risk factor, at least partly due to the presence of occult HBV infection (OBI) among blood donors , blood transfusions have become safer in recent decades in China due to the issuing of laws and regulations . The Ministry of Health initially introduced routine strict serum HBsAg screening in all blood centres in the early 1980s. By 1998, the Law for Donating Blood was issued, enacting regulations for the Management of Blood in Clinical Facility and further enhancing the safety of blood transfusions and safety in the use of serum related products. Transfusion-related HBV infection caused by unsafe blood has been satisfactorily controlled thereafter . However, serological screening of blood donors was first performed in a few blood centres as early as in the late 1970s, and many people could have been infected then before regulations were established. As such, the higher prevalence rates among the elderly could in part be attributed to previous contaminated blood transfusions.
The carrier rate for HBsAg was 2.0% among vaccinated participants aged 0-14 years, and 6.2% over 15 years. This suggests that 141 vaccinated participants failed to respond to the hepatitis B vaccine, and became sero-positive for HBsAg. One possibility is that hepatitis B vaccine did not have an effect on certain people. Another is that several hepatitis B vaccines were not preserved well in cool chains, causing it to be ineffective. A third may be that a number of HBV infected persons did not know their infected status. Although the number is small, failed responses to the vaccine may warrant further investigation.
One limitation of our study is that HBsAg prevalence may be overestimated among children and underestimated among adults because younger participants with HBV infection were more likely to be brought to the survey site by their parents or other guardians, while vaccinated adult participants were less likely to take part in our survey. Another is recall bias. Participants were first asked, "Were you vaccinated with the hepatitis B vaccine?", followed by, "Did you receive 3 injections?", and finally, "When were you vaccinated?" If the answers for the first two questions are both "yes," the participant will be included in the vaccinated variable. Thus, some unvaccinated participants may end up becoming vaccinated participants.