Significant Reduction in Seroprevalence of Antibodies Against Hepatitis A across Thailand, 2024.

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Learn more: PMC Disclaimer | PMC Copyright Notice Am J Trop Med Hyg . 2025 Mar 18;112(6):1329–1334. doi: 10.4269/ajtmh.24-0702 Significant Reduction in Seroprevalence of Antibodies Against Hepatitis A across Thailand, 2024 Phattharaporn Inma 1 , Pornjarim Nilyanimit 1 , Nasamon Wanlapakorn 1 , Ratchadawan Aeemjinda 1 , Sumeth Korkong 1 , Prangnapitch Wihanthong 2 , Narong Thawinwisan 3 , Pichet Puedkuntod 4 , Watcharanan Tinnaitorn 5 , Montana Foonoi 6 , Pornsawan Meechin 7 , Yong Poovorawan 1, 8, * Author information Article notes Copyright and License information 1 Centers of Excellence in Clinical Virology, Chulalongkorn University, Bangkok, Thailand; 2 Division of Epidemiology, Department of Disease Control, Ministry of Public Health, Nonthaburi, Thailand; 3 Ayutthaya Provincial Health Office, Office of the Permanent Secretary, Ministry of Public Health, Ayutthaya, Thailand; 4 Buriram Provincial Health Office, Office of the Permanent Secretary, Ministry of Public Health, Buriram, Thailand; 5 Trang Provincial Public Health Office, Office of the Permanent Secretary, Ministry of Public Health, Trang, Thailand; 6 Uttaradit Hospital, Uttaradit, Thailand; 7 Faktha Hospital, Uttaradit, Thailand; 8 The Royal Society of Thailand (FRS(T)), Sanam Sueapa, Dusit, Bangkok, Thailand Financial support: This research was funded by the Health Systems Research Institute, the Center of Excellence in Clinical Virology at Chulalongkorn University, King Chulalongkorn Memorial Hospital, the MK Restaurant Group, the Aunt Thongkham Foundation, and the Education and Public Welfare Foundation. Additionally, P. Nilyanimit received support from the Second Century Fund fellowship at Chulalongkorn University. Disclosures: This research received ethical approval from the Ethics Committee of the Faculty of Medicine, Chulalongkorn University (IRB Number 787/66). Before participation, all subjects and their legal guardians were provided with a comprehensive explanation of the study’s objectives and procedures, and written informed consent was obtained. The study was conducted under Good Clinical Practice guidelines and the ethical principles outlined in the Declaration of Helsinki. Authors’ contributions: All authors were involved in the conception and design of the study. P. Wihanthong, N. Thawinwisan, P. Puedkuntod, W. Tinnaitorn, M. Foonoi, and P. Meechin collected specimens and data. R. Aeemchinda and S. Korkong performed the laboratory tests. P. Inma and P. Nilyanimit analyzed the data. P. Inma, N. Wanlapakorn, and Y. Poovorawan drafted the manuscript. All authors critically revised the manuscript for intellectual content and approved the version to be published. All authors agree to be accountable for all aspects of the work. Current contact information: Phatharaporn Inma, Pornjarim Nilyanimit, Nasamon Wanlapakorn, Ratchadawan Aeemchinda, and Sumeth Korkong, Centers of Excellence in Clinical Virology, Chulalongkorn University, Bangkok, Thailand, E-mails: bam21142@hotmail.com , mim_bhni@hotmail.com , nasamon.w@chula.ac.th , aeemjinda.r@gmail.com , and sumeth.kor@gmail.com . Prangnapitch Wihanthong, Division of Epidemiology, Department of Disease Control, Ministry of Public Health, Nonthaburi, Thailand, E-mail: wnop_049@hotmail.com . Narong Thawinwisan, Ayutthaya Provincial Health Office, Office of the Permanent Secretary Ministry of Public Health, Ayutthaya, Thailand, E-mail: korea02@gmail.com . Pichet Puedkuntod, Buriram Provincial Health Office, Office of the Permanent Secretary Ministry of Public Health, Buriram, Thailand, E-mail: drpichet@gmail.com . Watcharanan Tinnaitorn, Trang Provincial Public Health Office, Office of the Permanent Secretary Ministry of Public Health, Trang, Thailand, E-mail: tenten.gucci@gmail.com . Montana Foonoi, Uttaradit Hospital, Uttaradit, Thailand, E-mail: ple.md0205@gmail.com . Pornsawan Meechin, Faktha Hospital, Uttaradit, Thailand, E-mail: nefertalentiti@hotmail.com . Yong Poovorawan, Centers of Excellence in Clinical Virology, Chulalongkorn University, Bangkok, Thailand, E-mail: yong.p@chula.ac.th . * Address correspondence to Yong Poovorawan, Center of Excellence in Clinical Virology, Faculty of Medicine, Chulalongkorn University, Rama IV Rd., Pathumwan, Bangkok 10330, Thailand. E-mail: yong.p@chula.ac.th Received 2024 Oct 18; Accepted 2024 Dec 18; Issue date 2025 Jun. © 2025, American Society of Tropical Medicine and Hygiene PMC Copyright notice PMCID: PMC12139558  PMID: 40101293 ABSTRACT. Hepatitis A virus (HAV) is an RNA virus that causes acute hepatitis and is transmitted via the fecal–oral route. It has historically been highly endemic in Thailand, where most children develop lifelong immunity after infection. Economic development and improved sanitation have reduced HAV transmission, but immunity levels have declined, raising concerns about potential future outbreaks. This study aims to assess the seroprevalence of HAV antibodies in Thailand in 2024, 10 years after the last national survey in 2014, and to evaluate current immunity levels to inform public health strategies. A cross-sectional study was conducted in a population aged 6 months to 80 years across Thailand’s geographic regions. A total of 4,312 serum samples were tested for anti-HAV antibodies using the chemiluminescent microparticle immunoassay. The seroprevalence data were compared with findings from previous surveys in 2004 and 2014. The study showed a significant decline in population immunity to HAV, with the age at which 50% of individuals had antibodies increasing from 36 in 2004 to 42 in 2014, and to 47 years in 2024. A majority of the population remained susceptible to HAV, particularly among younger age groups. Thailand has transitioned to low HAV endemicity, with a large proportion of the population lacking immunity. Despite the absence of significant outbreaks in recent decades, the risk of future outbreaks remains, particularly from imported cases. Enhanced surveillance and vaccination strategies are necessary to prevent future HAV transmission and manage public health risks. INTRODUCTION Hepatitis A virus (HAV) is a member of the Heptovirus genus within the Picornaviridae family. 1 It is an RNA virus that primarily causes acute hepatitis. In severe cases, HAV infection can lead to hepatic failure and mortality. 2 Although infections in children tend to be less severe, the severity of HAV infection increases with age, particularly among individuals with underlying chronic conditions such as cirrhosis or those with compromised immune systems. 3 Hepatitis A virus is a nonenveloped virus known for its environmental resilience, exhibiting resistance to chemicals such as acid, ether, and alcohol. 3 The transmission of HAV occurs via the fecal–oral route, making the virus highly prevalent in regions characterized by inadequate sanitation. 4 Historically, Thailand has exhibited a state of high HAV endemicity for more than four decades. During this period, the majority of infections occurred in childhood, leading to the acquisition of lifelong immunity within the adult population. 5 Although HAV comprises a single serotype, it includes multiple genotypes. 6 Importantly, immunity acquired through natural infection provides lifelong protection against HAV. 4 With the nation’s economic, social, and sanitation advancements, an increasing proportion of children and young adults have been deprived of natural immunity, thereby heightening their susceptibility to outbreaks. Between 2001 and 2005, Thailand experienced frequent HAV outbreaks, particularly in the southern and northern regions. 7 Further improvements in sanitation led to a decline in outbreak frequency, with the most recent significant outbreak occurring in Bueng Kan Province in 2013. This outbreak was effectively contained within 4 to 6 months through the implementation of rigorous sanitation control measures. 8 Thailand has not experienced any significant outbreaks of HAV in the past decade, but this is not due to HAV vaccine use. Although the HAV vaccine has been available for more than two decades, including inactivated vaccines requiring two doses 6 months apart and, more recently, a live attenuated H2 strain vaccine requiring a single dose at 2 years of age, the high cost of the vaccine has restricted its accessibility to individuals who can afford it. It remains a voluntary immunization that has not yet been incorporated into the National Immunization Program (NPI). As a result, the epidemiology of HAV in Thailand is primarily shaped by economic factors, advancements in public health infrastructure, and improvements in sanitation, rather than by widespread vaccination efforts. 9 Despite ongoing improvements in living conditions and sanitation, there has been a marked decline in the incidence of HAV infection, resulting in a significant portion of the population lacking immunity to the virus. Recent studies have consistently documented a progressive reduction in HAV immunity within Thailand. A national survey conducted in 2014 indicated that a majority of the population remained nonimmune, with the median age at which 50% of individuals exhibited seropositivity for anti-HAV antibodies reaching 42 years. 9 This widespread lack of immunity in a predominantly unvaccinated population presents a considerable risk for future outbreaks. Epidemiological studies on the seroprevalence of HAV antibodies in Southeast Asia are scarce, particularly in the context of the region’s transition to low endemicity. However, Thailand has consistently conducted such studies. 10 The present study represents a comprehensive national survey designed to assess the seroprevalence of anti-HAV antibodies in the Thai population. Conducted a decade after the previous survey in 2014, this study aims to provide insights into the current epidemiological status of HAV immunity, with particular emphasis on the pediatric population. In the absence of significant outbreaks in recent years, immunity in children is predominantly acquired through vaccination. The findings from this study will guide the formulation of effective strategies for disease control and prevention, particularly in the event of a resurgence of HAV outbreaks. MATERIALS AND METHODS This research was part of a broader national study entitled “Evaluation of the Impact of the Hepatitis B Vaccine Immunization Program as Part of the Expanded Program on Immunization (EPI) Following Three Decades of Implementation and Seroprevalence of Hepatitis A, B, and C in Thailand.” The primary objective of this study was to determine the incidence and seroprevalence of hepatitis A, B, and C within the Thai population, with assessments conducted at 10-year intervals. The initial evaluation was conducted in 2004, followed by a subsequent assessment in 2014. Study population and study sites. A cross-sectional study was conducted among individuals aged 6 months to 80 years, encompassing representative provinces from Thailand’s geographic regions, including Uttaradit (North), Buriram (Northeast), Ayutthaya (Central), and Trang (South). A random sample of 4,312 participants was selected from a total population of 6,069 collected from May to August 2024. The age distribution of the study cohorts is detailed in Table 1 . All participants were confirmed to be in good health, free from chronic diseases, and exhibited no symptoms indicative of HIV infection. Moreover, none of the participants were receiving immunosuppressive medications or had undergone screening for hepatitis B and C, following the protocols of the National Health Security Office (NHSO). Blood samples, ranging from 3 to 7 milliliters of clotted blood, were collected, centrifuged to extract the serum, anonymized, and preserved at a temperature of −20°C until laboratory analysis. Table 1. Number of participants in different age groups in each province studied Age Ayutthaya Trang Uttaradit Buriram Total M F M F M F M F 6 months–2 years 38 41 43 40 47 35 35 46 325 3–4 years 41 40 30 45 36 37 36 39 304 5–6 years 34 39 49 34 28 51 37 38 310 7–8 years 39 43 36 33 32 44 42 40 309 9–10 years 38 41 42 36 29 45 39 40 310 11–12 years 29 36 26 40 29 37 39 23 259 13–14 years 20 41 31 35 20 39 22 37 245 15–16 years 28 35 27 32 19 42 31 36 250 17–18 years 23 38 27 36 41 25 36 27 253 19–20 years 27 39 25 34 40 23 26 33 247 21–30 years 84 107 65 121 90 91 72 108 738 31–40 years 20 56 14 58 24 51 21 51 295 41–50 years 13 28 5 35 16 25 14 27 163 51–60 years 7 39 7 33 14 29 11 33 173 >60 years 9 28 11 23 12 18 15 15 131 Total 450 651 438 635 477 592 476 593 4,312 Open in a new tab F = female; M = male. Laboratory testing. All 4,312 serum samples were tested for anti-HAV antibodies using the chemiluminescent microparticle immunoassay method on the automated Architect system, manufactured by Abbott Laboratory (Wiesbaden, Germany). The testing procedures were conducted at the Center of Excellence in Clinical Virology, Faculty of Medicine, Chulalongkorn University. The results of the analysis were classified as positive (reactive) or negative (nonreactive) according to the manufacturer’s guidelines. STATISTICAL ANALYSES The data are presented as percentages of individuals with immunity to hepatitis A across various age groups and geographic regions in Thailand, along with an overall national percentage. The findings are compared with seroprevalence data from 2004 and 2014. The age at which 50% of the population demonstrated immunity to hepatitis A was recorded, and the estimated proportion of the Thai population remaining susceptible to hepatitis A in 2024 has been assessed. RESULTS A comprehensive serosurvey for hepatitis A, B, and C was conducted on a national scale, involving the random selection of 4,312 serum samples for the hepatitis A antibody study from a total of 6,069. The study encompassed a diverse population across various age groups and included representative provinces from Thailand’s geographic regions: Uttaradit (North; 1,069 cases), Ayutthaya (Central; 1,101 cases), Buriram (Northeast; 1,069 cases), and Trang (South; 1,073 cases). Serum samples were collected from both urban and rural areas, with the demographic distribution of participants, including population numbers and sex breakdowns for each province, presented in Figure 1 . Additionally, the age distribution within each province demonstrated a higher proportion of children compared with adults and the elderly, as detailed in Table 1 . Figure 1. Open in a new tab Map of Thailand showing the locations of the provinces that represent each region in this study. The corresponding number of participants from each province is detailed in the accompanying table. The results indicated that 206 individuals, comprising 4.8% of the study population, had received the hepatitis A vaccine. In contrast, a significant majority, ∼95%, either had no history of hepatitis A vaccination, were uncertain of their vaccination status, or lacked information about their vaccination history. Among those vaccinated, the majority had received the hepatitis A vaccine in the childhood groups. The assessment of immunity against hepatitis A, determined by the presence of anti-HAV IgG, revealed that the majority of individuals across the four provinces lacked immunity. Immunity levels increased with age, particularly among adults over 40 years old, and further rose, nearing 95%, in individuals aged 60 and above, as illustrated in Figure 2 . Figure 2. Open in a new tab The graph illustrates the detection of antibodies against hepatitis A (anti-hepatitis A virus [HAV] IgG) in each province, as well as the overall prevalence of anti-HAV IgG in Thailand across different age groups. A comparison of the seroprevalence data for anti-HAV IgG with previous studies conducted in 2004 and 2014 indicates a continuous decline in immunity against hepatitis A over time. The age at which 50% of participants tested positive for anti-HAV IgG increased from 36 years in 2004 to 42 years in 2014, and further to 47 years in 2024, as illustrated in Figure 3 . Figure 3. Open in a new tab A comparison of the detection of anti-hepatitis A virus IgG across different age groups in studies conducted in 2004, 2014, and 2024. The majority of the Thai population still lacked immunity against hepatitis A or was susceptible to infection. By assessing the susceptibility rates among different age groups, an overall estimate was made of the total number of people in Thailand 11 who were still at risk or susceptible to HAV infection: on average, 88.5%, which corresponds to ∼39.7 million individuals ( Table 2 ). Table 2. Estimation of the susceptible population to hepatitis A infection in Thailand, 2024 Age Groups (years) Populations Anti-HAV IgG Estimated Populations Positive (%) Negative (%) HAV, Immune HAV, Susceptible 0–5 3,437,387 8.31 91.69 285,647 3,151,740 6–10 3,603,170 4.74 95.26 170,790 3,432,380 11–15 3,898,932 2.40 97.60 93,574 3,805,358 16–20 3,988,754 2.13 97.87 84,960 3,903,794 21–30 9,078,836 3.83 96.17 347,719 8,731,117 31–40 9,313,464 8.47 91.53 788,850 8,524,614 41–50 10,148,426 46.63 53.37 4,732,211 5,416,215 51–60 9,794,386 78.61 21.39 7,699,367 2,095,019 >60 11,823,811 94.66 5.34 11,192,419 631,392 Total 65,087,166 11.50 88.50 25,395,539 39,691,627 Open in a new tab HAV = hepatitis A virus. This estimate quantifies the number of individuals in the Thai population who remain susceptible to hepatitis A infection. The current population of Thailand, obtained from official statistics registration systems, the Department of Provincial Administration, Ministry of Interior, Kingdom of Thailand ( https://stat.bora.dopa.go.th/stat/statnew/statMONTH/statmonth/#/mainpage ). DISCUSSION The HAV remains a significant global health concern, particularly in developing countries with inadequate public health infrastructure, where the incidence of infection is notably high. 12 In children, HAV infections are typically asymptomatic or result in only mild symptoms. Among children under 6 years of age, symptomatic hepatitis A occurs in ∼7.7% of cases. 13 However, the proportion of symptomatic cases increases with age, reaching ∼50% in children aged 6–12 years. 14 In adults, particularly those over 40, the fatality rate is ∼1%, 15 and the risk of severe outcomes is heightened in individuals with chronic conditions such as cirrhosis. 16 In hyperendemic regions, such as parts of sub-Saharan Africa and South Asia, under-resourced public health systems contribute to widespread early exposure to HAV. In these areas, most infections occur asymptomatically during childhood, leading to lifelong immunity and minimizing the burden of hepatitis A as a public health issue. However, as public health systems improve and access to clean water and sanitation increases, the age of infection shifts to older populations. This transition has been observed in regions like some parts of Southeast Asia and the Middle East, where improving public health infrastructure has led to intermediate endemicity, resulting in more symptomatic cases and heightened public health concerns. In low-endemic countries, such as those in North America and most of Western Europe, high public health standards have nearly eliminated HAV transmission domestically. However, sporadic outbreaks occur, often linked to imported cases from areas with higher endemicity. These dynamics highlight the importance of vaccination policies and public health surveillance tailored to endemicity levels and regional socioeconomic conditions. 12 This study identifies Thailand as a country with very low endemicity for HAV infection (<50% of individuals are immune by the age of 30 years), consistent with reports by Jacobsen KH and the WHO. 17 , 18 Globally, hepatitis A vaccination strategies primarily target high-risk groups that are more likely to experience severe disease or contribute to outbreaks. These groups include patients with chronic liver disease, people traveling from low-endemic countries to areas with intermediate or high endemicity, men who have sex with men, people who inject drugs, individuals experiencing homelessness, migrants and refugees, and incarcerated persons. 18 Historically, more than 50 years ago, Thailand’s public health infrastructure was underdeveloped, and hepatitis A was a prevalent infection. Natural immunity was acquired early in life, and by adulthood, most individuals had developed immunity. Consequently, hepatitis A was not regarded as a significant public health issue because most infections occurred during childhood, often presenting with asymptomatic manifestations. However, with improvements in public health systems over the past 30 to 50 years, the incidence of symptomatic HAV infections has increased, particularly among children. 13 , 14 In the last two decades, rapid national development has propelled Thailand into a state of low endemicity, with minimal HAV transmission. Today, Thailand is classified as having very low endemicity, similar to developed countries, with only imported cases posing a potential risk. Previous studies have demonstrated a correlation between HAV seroprevalence and socioeconomic development, as reflected by Thailand’s rising gross domestic product per capita and improvements in public health indicators, such as reductions in infant mortality rates. National surveys comparing data from 2004 and 2014, 9 , 19 as well as the current study in 2024 reveal a continuous decline in population immunity to HAV, with seroprevalence rates now ∼50% among specific age groups corresponding to the ages of 36, 42, and 47, respectively. This trend underscores Thailand’s sustained progress in social, economic, and public health domains, categorizing the country as one of very low endemicity. Despite the decrease in population immunity, there has been no significant outbreak of HAV in the past two decades. Nonetheless, ongoing surveillance remains crucial to preventing future outbreaks. In the present study, a portion of children exhibited detectable antibodies to HAV, likely attributable to vaccination, which has been available for over 30 years, although it remains nonmandatory. The high cost of the vaccine has limited its accessibility, resulting in a relatively small number of vaccinated children. The majority of the Thai population lacks immunity to hepatitis A, making them susceptible to infection and increasing the risk of a large outbreak. In neighboring countries like Laos, the age at which 50% of the population has developed antibodies to hepatitis A is ∼20 years younger than in Thailand. 20 Additionally, higher rates of hepatitis A infection, as indicated by anti-HAV IgG testing, have been observed among migrant workers entering Thailand. 21 This ongoing transmission of hepatitis A in Southeast Asia poses a risk of the virus spreading into Thailand. Given the basic reproductive number of 2.1–2.8, 22 achieving herd immunity requires 52–64% of the population to be immune. Thus, widespread vaccination, or initiating vaccination programs in early childhood as part of the national vaccination policy, is essential to prevent outbreaks. This approach would help mitigate potential health and economic impacts. 23 Additionally, the continuous monitoring of hepatitis A is crucial to prevent future outbreaks, and conducting a cost–benefit analysis of vaccination efforts is important for guiding national policies. In our study, Uttaradit, representing the northern region of Thailand, demonstrated a lower anti-HAV seroprevalence compared with other regions. This finding is consistent with previous studies conducted 10 years ago, 9 which also reported lower seroprevalence rates in the northern region relative to other parts of Thailand. Several factors likely contribute to this geographic variation, including differences in socioeconomic conditions, vaccine coverage, health education, and other region-specific characteristics. Further studies are needed to better understand the impact on anti-HAV seroprevalence among the different regions of Thailand. Our research is part of a nationwide survey, encompassing a large and geographically diverse sample from different regions of Thailand. A significant limitation, however, is this study’s dependence on self-reported vaccination histories. Many participants, particularly older children and adults, were unsure of their vaccination status, which may impact the accuracy of the seroprevalence data. Additionally, the high cost of the hepatitis A vaccine means that its availability is currently restricted to the private sector and is not part of the NPI or covered by the NHSO, resulting in most of the population remaining unvaccinated. Future studies that incorporate more detailed vaccination records could enhance the reliability of the findings. In conclusion, Thailand is currently in a phase of very low endemicity for hepatitis A. A significant proportion of the population has not been exposed to the virus and therefore lacks immunity. Although the HAV vaccine is available, its high cost and voluntary administration have resulted in low vaccination rates among children. To mitigate the risk of future outbreaks, it is imperative to implement robust surveillance systems, educational programs, and prevention strategies. The findings from this study are critical for informing national health planning, particularly in the development of future HAV vaccination policies. ACKNOWLEDGMENTS We would like to thank the Thailand Ministry of Public Health study coordinators, Uttaradit, Buriram, Ayutthaya, and Trang provincial health officers, and the staff of the health promotion center for their dedication, cooperation, and support. REFERENCES 1. Miller MJ, 1993. Summary of current nomenclature, taxonomy, and classification of various microbial agents. Clin Infect Dis 16: 612–613. [ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] 2. Cooksley WG, 2000. What did we learn from the Shanghai hepatitis A epidemic? J Viral Hepat 7 Suppl 1: 1–3. [ DOI ] [ PubMed ] [ Google Scholar ] 3. Pramoolsinsap C, Poovorawan Y, Hirsch P, Busagorn N, Attamasirikul K, 1999. Acute, hepatitis-A super-infection in HBV carriers, or chronic liver disease related to HBV or HCV. Ann Trop Med Parasitol 93: 745–751. [ DOI ] [ PubMed ] [ Google Scholar ] 4. World Health Organization, 2023. Hepatitis A . Available at: https://www.who.int/news-room/fact-sheets/detail/hepatitis-a/ . 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