In 2015, 3.5% of the world's population were living with hepatitis B virus (HBV) chronic infection. If untreated, HBV infection can lead to cirrhosis, hepatocellular carcinoma, and death; currently, 66% of all viral hepatitis deaths are attributable to HBV [1]. HBV transmission can be perinatal or horizontal, through needlesticks, exposure to infected blood, or sexual transmission. The main preventive measure for HBV is vaccination [2].

In Mexico, HBV vaccination for all children and adolescents was implemented in 1999 [3]. By 2012, the prevalence of immunity attributable to vaccination was 44.7% in the Mexican population aged 10 to 25 years [4]. By 2000, the estimated HBV prevalence was 0.21% and since then there have been no updates [5]. In 2015, the Global Health Sector Strategy on viral hepatitis 2016-2021, proposed to reduce the incidence of viral hepatitis by 90% and mortality by 65% [6]. To achieve this goal, we need information on HBV infection in the population. Therefore, we aimed to estimate the prevalence of hepatitis B surface antigen (HBsAg) in the Mexican population.

Data were obtained from the 2018-19 National Health and Nutrition Survey (ENSANUT, for its Spanish acronym). The survey is nationally representative of the population living in community households using a probabilistic, stratified, two-stage, and cluster sampling. Research procedures were approved by the Ethical, Research, and Biosecurity Committee of the National Institute of Public Health. More details about the design of ENSANUT are available elsewhere [7]. A probabilistic sample of 2,280 adults aged 20 years and older (18.4% of the total sample) was obtained for HBsAg determination, representing a total population of 81,445,000 individuals.

Using the ENSANUT 2018 questionnaire, we collected sociodemographic characteristics and risk factors for viral hepatitis, such as age, sex, marital status, country region, urban area, and socioeconomic status. The population aged 20 to 49 years had additional questions about sexual behaviors: age of sexual initiation, condom use in the first and last sexual intercourse.

The prevalence of HBsAg was 0.51% (95% CI 0.19, 2.33), which represents 411,000 cases. The prevalence was 0.54% (95% CI 0.14-2.51%) in women and 0.46% (95% CI 0.12-1.67%) in men. There were no cases of hepatitis B in the population aged 20 to 39 years nor aged 50-59 years. All cases in the 40-49 age group were women (0.24%, 95% CI 0.03-1.68%), conversely, all cases in the ≥70 age group were men (0.77, 95% IC 0.24-2.45%).

Table 1 presents the national prevalence of HBsAg in the Mexican population according to sociodemographic characteristics. Participants aged 60-69 years, people with middle or high-school education, and widows had the highest prevalence of HBsAg. Mexico City reported a higher prevalence of HBsAg compared to other regions. Sociodemographic characteristics such as sex, socioeconomic status, condom use in the first and last sexual intercourse, and age of sexual initiation were not associated with HBV infection (Table 1).

In 2018, the prevalence of HBsAg was 0.51% (95% CI 0.19, 2.33) among the Mexican adult population. This represents an increase with respect to the 2000 National Health Survey when the estimated HBsAg prevalence was 0.21% (95% CI: 0.11–0.37) in the same age group [5]. Unfortunately, surveys are not comparable because in year 2000 HBsAg determination was restricted to those with positive antibodies against HBV core antigen (anti-HBVc), while in 2018, HBsAg was determined in all samples.

Sociodemographic characteristics and sexual behaviors were not associated with HBV prevalence [2]; however, this finding should be interpreted carefully, given the sample size limitations. Our results suggest that HBsAg is more prevalent in people over 60 years. While it is difficult to provide a clear explanation for this finding, the low prevalence for the group aged 20 to 39 years could be related to access to HBV vaccination, which was estimated to cover 44.7% of those aged 10 to 25 years in 2012, six years prior to this new survey [4].

ENSANUT 2018 did not collect detailed information on sexual practices or needle exposure. In addition, only HBsAg was determined, thus, we cannot assess if infection was resolved [2]. We could be underestimating the real prevalence because HBV DNA can be detectable in serum or hepatic tissue in the absence of HBsAg. However, hidden hepatitis infection is rare and has been reported in specific immunocompromised subgroups [9]. Finally, ENSANUT is a household community survey and may fail to capture groups at higher risk of HBV infection, leading to an underestimation of the prevalence.

The 2018 estimated prevalence of HBsAg in the Mexican adult population might be higher than previously reported. Vaccination and screening strategies towards the elimination of viral hepatitis should be reinforced to further reduce the prevalence over the next years. Studies that monitor the prevalence of HBV in key groups are needed.