Immunity to HEV is not life-long as serum anti-HEV IgG does decline several years after initial infection [31]. The duration of IgG immunity can last as long as 12 years after acute infection, but reinfection has been documented and therefore immunity to HEV may be limited [11]. The waning of HEV IgG over an individual’s lifetime is consistent with our observed decreased in seroprevalence among those >50 years old, especially in the Lop Buri cohort.
Previous efforts to determine the burden of HEV infection in Thailand using HEV seroprevalence survey found 23.3% in occupational high-risk group and 6.5% in ethnic Hmong population in the northern Thailand [16, 32]. The overall seroprevalence of 37% found in this study was higher than the 9–22% [33] from a previous Thai study, which may be due to better detection sensitivity. It was also higher than the 14% from another study, which only surveyed Thai men between the ages 18–30 [34]. Globally, prevalence of anti-HEV IgG varies and previous studies conducted in the general population included mostly adults. For example, the overall HEV seroprevalence was 5.9% in Korea [24], 4.6%–6.7% in Japan [35], and 17% in Germany [36]. Among blood donors, the rate of HEV antibodies found were 4.9% in Switzerland [37], and 3.7% in Japan [38].
Detection of HAV antibodies generally indicate past infection and its presence is generally recognized as lifelong [8]. Prior to 1980, >97% of Thais ≥16 years old tested positive for anti-HAV [39]. Improved sanitation and hygiene over the past three decades, however, have helped to reduce exposure to HAV infection and thus HAV immunity in the general population. As a result, sporadic outbreaks in recent years have rendered young Thais vulnerable to HAV infection [40]. Our previous assessment of the overall anti-HAV seroprevalence in the Thai population was 27%, but it was as high as 71% among those residing in regions bordering Myanmar [41]. Similarly, higher HAV seroprevalence in Narathiwat may result from reduced access to better sanitation or exposure to unsafe drinking water or contaminated produce. Consistent with this observation is the many documented outbreaks of HAV in southern Thailand including Narathiwat [42–44].