The study, published in BMC Infectious Diseases, analyzed 4,668 hepatitis E cases drawn from China’s National Notifiable Disease Reporting System, a nationwide passive surveillance network to which clinicians and laboratories must report diagnoses of legally designated infectious diseases. Over the six-year window, Shanghai recorded an average annual notification rate of 3.14 cases per 100,000 population. That figure places hepatitis E firmly on the map of locally relevant infectious diseases in the city, even though it rarely attracts the public attention devoted to respiratory pathogens or foodborne outbreaks. Hepatitis E virus, or HEV, is transmitted primarily through the fecal-oral route, most often via contaminated water, and through zoonotic pathways, particularly the consumption of undercooked pork, game meat, and shellfish from infected animals.

To understand the disease’s rhythm in time, the researchers applied temporal scan statistics, a method that slides a window of variable length across the surveillance timeline and asks whether case counts within any given interval exceed what would be expected by chance. The analysis identified a statistically significant temporal cluster spanning January 1, 2017 to May 31, 2019, with a relative risk of 1.35 and a log likelihood ratio of 52.18, well beyond the threshold of statistical significance. In practical terms, cases accumulated during this early window at a rate roughly a third higher than the six-year baseline. The team also detected a recurring seasonal signature: notifications clustered in the months from December through May, a winter-to-spring peak that echoes patterns reported in other regions of China and is often linked to seasonal dietary customs, including festival-period consumption of raw or undercooked animal products.

The spatial dimension of the analysis proved more surprising. When the researchers ran a spatial-temporal scan across the entire six-year period, they found one statistically significant cluster with a relative risk of 1.73 and a log likelihood ratio of 135.74. Communities with the highest raw notification rates tended to sit in the southeastern parts of the city, but the most likely cluster identified by the scan statistic was not there. Instead, it was located in the urban core. When the team repeated the scan year by year, the same pattern held: the most likely cluster consistently fell in central urban districts, with secondary clusters appearing in suburban towns on the metropolitan periphery. The discrepancy between where cases are most numerous and where the statistical signal of clustering is strongest is itself informative, because scan statistics adjust for underlying population size and expected case counts rather than simply flagging the tallest bars on a map.

That adjustment matters. Dense urban neighborhoods generate large numbers of cases in absolute terms simply because so many people live there, but a cluster statistic asks a subtler question: are there more cases than the local population structure would predict? The fact that the urban core repeatedly emerged as the most likely cluster suggests that something about these districts, beyond sheer headcount, elevates hepatitis E transmission or detection. Possibilities include greater reliance on food purchased from restaurants and markets, more frequent consumption of seafood and pork dishes, higher turnover of food handlers, or simply better access to healthcare and laboratory testing, which would raise the probability that infections are recognized and reported. The study’s design cannot distinguish among these mechanisms, a limitation the authors acknowledge explicitly.

To probe what distinguishes clustered communities from the rest, the researchers turned to binary logistic regression, a statistical technique that estimates the odds of membership in a high-risk cluster as a function of community-level characteristics. Three variables emerged as significant. Population density showed the strongest association: communities with higher density had markedly higher odds of falling into a high-risk cluster, with an odds ratio of 7.367. The count of shopping malls, used as a proxy for commercial activity and food-service intensity, was also positively associated, with an odds ratio of 1.531. Intriguingly, distance to the nearest river showed a negative association, with an odds ratio of 0.742, meaning that communities closer to rivers had higher odds of clustering. This last finding is consistent with the hypothesis that waterways and the aquatic food chains they support, including shellfish and fish harvested or sold in riverside markets, may play a role in HEV transmission, although the ecological nature of the analysis means the link remains speculative.

The authors are careful, and rightly so, about how far these associations can be pushed. Because the analysis operates at the level of communities rather than individuals, it is vulnerable to the ecological fallacy: a community-level correlation does not establish that the people within a cluster acquired their infections through the hypothesized route. A mall-dense neighborhood may have many cases not because its residents eat at malls but because mall density tracks with other unmeasured features of urban life. Similarly, proximity to a river may correlate with historical settlement patterns, sanitation infrastructure, or dietary traditions rather than with any direct waterborne exposure. The researchers describe their findings as ecological and hypothesis-generating, and they emphasize that confirming the actual transmission routes of hepatitis E in Shanghai will require individual-level epidemiological studies, including case-control designs that compare exposures of confirmed cases with those of matched controls.

Even with those caveats, the study carries practical weight for public health planning. Shanghai sits within the Yangtze River Delta, one of the most economically dynamic and densely interconnected regions in the world, and its surveillance system feeds into national and global assessments of viral hepatitis burden. The World Health Assembly has set targets for eliminating viral hepatitis as a public health threat, and hepatitis E, though often self-limiting in healthy adults, can be devastating for pregnant women, who face elevated risks of fulminant liver failure, and for people with chronic liver disease or compromised immune systems. Knowing that risk concentrates in dense, commercially active, riverside urban communities gives health authorities a concrete template for targeting interventions, whether that means food-safety inspections in high-risk districts, health education campaigns timed to the December-to-May seasonal peak, or enhanced testing of at-risk populations such as pregnant women in clustered neighborhoods.

The methodological approach also deserves attention from the wider infectious-disease community. Spatial-temporal scan statistics, originally developed for cancer cluster detection and later adapted for communicable disease surveillance, have become a standard tool for turning routine notification data into actionable geographic intelligence. By combining them with community-level socioeconomic and environmental covariates in a regression framework, the Shanghai team demonstrated a pipeline that other cities with robust notifiable-disease systems could replicate. The work also underscores the value of long surveillance windows: a six-year dataset made it possible to separate a one-off elevated period in 2017 through 2019 from a stable seasonal rhythm and a persistent geographic core, distinctions that shorter studies would blur.

What remains to be resolved is the biology behind the map. Genotyping of viral sequences from urban and suburban cases could reveal whether a single transmission chain or multiple introductions drive the clusters, and molecular epidemiology could test whether zoonotic strains from the pork supply chain dominate in commercial districts while water-associated genotypes predominate near rivers. Seroprevalence surveys could measure how much asymptomatic infection the notification data miss, since hepatitis E is frequently mild or silent in young, healthy adults. Until such studies are done, the Shanghai analysis stands as a carefully constructed hypothesis: that in a modern megacity, hepatitis E risk is written into the urban fabric itself, concentrated where people, commerce, and waterways converge. It is a hypothesis that public health officials in Shanghai, and in riverine megacities across Asia and beyond, now have both the reason and the roadmap to test.

Subject of Research: Spatial-temporal epidemiology and community-level risk factors of hepatitis E virus infection in Shanghai, China

Article Title: Identification of the spatial-temporal cluster and risk factors of hepatitis E from 2017 to 2022 in Shanghai, China

Article References: Zhi-Tao, M., Ling-Xiao, Q., Kai-Yun, C., Xin, S., Di, X., Zhao-He, L., Yi-Han, L., Kang, C., Jing, L., & Hong, R. (2026). Identification of the spatial-temporal cluster and risk factors of hepatitis E from 2017 to 2022 in Shanghai, China. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14463-4

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14463-4

Keywords: hepatitis E, hepatitis E virus, spatial-temporal analysis, disease cluster, Shanghai, epidemiology, public health surveillance, risk factors, population density, zoonotic transmission, viral hepatitis, logistic regression

The study rests on a simple but powerful epidemiological premise: antibodies of the immunoglobulin G class persist for years after infection or vaccination, so their prevalence in a population acts as a cumulative record of exposure. Because hepatitis A and hepatitis E are both transmitted primarily through the faecal-oral route, their seroprevalence patterns are tightly coupled to sanitation, water quality, hygiene practices and socio-economic conditions. When a community improves its water and sanitation infrastructure, the age at which children first encounter these viruses tends to rise, shifting the burden of susceptibility toward older individuals in whom infection is more likely to cause symptomatic disease. This phenomenon, known as epidemiological transition, is one of the central concerns for vaccination policy in low- and middle-income countries undergoing rapid urban development.

To test whether such a transition was underway in Vellore, the team drew on biobanked serum samples from 600 participants in the 2013 survey and 558 participants in the 2022 survey, all aged between one and forty years. Each sample was tested for IgG antibodies specific to hepatitis A virus and hepatitis E virus. The investigators then stratified seroprevalence estimates by age, gender and residential cluster, allowing them to detect not only overall changes between the two survey years but also geographic heterogeneity within the city. Statistical comparisons of seropositivity between years were carried out using proportion tests or Fisher’s exact tests, depending on the structure of the data.

The hepatitis A results were striking in their consistency. Seroprevalence exceeded 80 percent among children aged one to five years in both surveys and climbed to 100 percent among individuals aged sixteen and above, in 2013 as well as 2022. No statistically significant differences in hepatitis A seroprevalence were observed between the two survey years in any age stratum. In practical terms, nearly every resident of urban Vellore had been infected with hepatitis A virus by early adulthood in both eras, and infection continued to occur early in childhood. The virus, in this setting, remains firmly endemic, and the population’s collective immunity remains high.

Hepatitis E told a different story. IgG seroprevalence against this virus was low among children and adolescents in both surveys and increased progressively with age, a pattern consistent with sporadic rather than sustained childhood transmission. Among adults aged twenty-six to forty years, seroprevalence rose from 18 percent in 2013, with a 95 percent confidence interval of 12 to 27 percent, to 24 percent in 2022, with a 95 percent confidence interval of 16 to 32 percent. Although this increase suggests a possible gradual accumulation of exposure in adulthood, the change did not reach statistical significance, and the authors are careful not to overinterpret it. Across both viruses, no significant gender-based differences in seropositivity were detected.

Cluster-wise analysis added a spatial dimension to these findings. Hepatitis A seroprevalence was uniformly high across all residential clusters sampled in Vellore, reflecting the pervasive nature of early-life exposure to the virus throughout the urban environment. Hepatitis E seroprevalence, by contrast, was both low and heterogeneous across clusters, indicating that exposure to this virus is patchy and likely driven by localised factors such as intermittent contamination of water supplies, sanitation gaps in specific neighbourhoods, or differences in food handling practices. This heterogeneity matters for surveillance design, because a citywide average can easily mask pockets of elevated risk where outbreaks of hepatitis E, which can be particularly dangerous for pregnant women, may originate.

Beyond simple prevalence counts, the study employed more sophisticated quantitative tools to probe hepatitis A transmission dynamics. The researchers applied contact matrix and mixture modelling to the serological data to estimate the effective reproduction number, Re, for hepatitis A in each survey year. Mixture modelling, which treats the antibody distribution in a population as a blend of distributions from susceptible and immune individuals, allows researchers to infer the force of infection even from cross-sectional snapshots. The analysis yielded an Re below 1 in both serosurveys, indicating that secondary transmission of hepatitis A was not sustaining epidemic growth in either era, despite the accumulation of susceptible individuals in older age groups over time. This finding suggests that while the age profile of susceptibility may be shifting slowly, the underlying transmission intensity has not yet crossed the threshold that would fuel outbreaks among older, more vulnerable populations.

The implications for vaccination policy are significant. In many middle-income countries that have improved sanitation, hepatitis A has transitioned from a disease of early childhood, where infection is usually asymptomatic, to one affecting older children and adults, in whom clinical illness and occasional severe outcomes are more common. This shift is the classic argument for universal childhood hepatitis A vaccination: immunising children early both protects them and reduces circulation, indirectly shielding older susceptible individuals. The Vellore data show that this transition, while anticipated, has not yet materialised in measurable form; immunity remains high and acquired early, and the effective reproduction number remains below one. The authors note that their findings should inform age-specific considerations for hepatitis A vaccination policy in urban India as it undergoes transition, implying that the window for deciding on vaccination strategy remains open but should be monitored with continued serosurveillance.

For hepatitis E, the picture is one of limited but persistent and age-dependent exposure. The low seroprevalence among the young means that a large fraction of the population reaches adulthood without prior immunity, and the gradual rise in seropositivity with age reflects cumulative adult exposure. The authors highlight the need for targeted hepatitis E surveillance, particularly because the virus causes substantial morbidity in pregnant women and can trigger large outbreaks when water supplies are compromised. The modest, non-significant rise in adult seroprevalence between 2013 and 2022 is consistent with continued low-level transmission that could accelerate under adverse conditions, making sustained monitoring essential.

Perhaps the clearest message of the study is a reaffirmation of the value of water, sanitation and hygiene interventions. The stability of hepatitis A immunity over the decade, and the continued containment of hepatitis E transmission below epidemic thresholds, support continued investment in WASH infrastructure as the backbone of enteric virus control in urban India. At the same time, the study demonstrates the power of repeated, geographically stratified serosurveys using biobanked samples to detect epidemiological change before it becomes clinically visible. As Indian cities continue to grow and modernise, the balance between endemic early-childhood infection and emerging adult susceptibility will determine whether hepatitis A vaccination becomes a public health priority, and whether hepatitis E remains a smouldering, localised threat or flares into wider outbreaks. The Vellore data provide a rigorous baseline against which that future can be measured.

Subject of Research: Decadal changes in IgG seroprevalence of hepatitis A and E virus in urban Vellore, India

Article Title: Decadal changes in IgG seroprevalence of hepatitis A and E virus in Urban Vellore, India: Persistent Endemicity or Epidemiological Shift?

Article References: Decadal changes in IgG seroprevalence of hepatitis A and E virus in Urban Vellore, India: Persistent Endemicity or Epidemiological Shift?. (n.d.). https://doi.org/10.1186/s12879-026-14413-0

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14413-0

Keywords: Hepatitis A, Hepatitis E, IgG seroprevalence, serosurvey, enteric infections, WASH, epidemiological transition, urban India, effective reproduction number, vaccination policy, Decadal, changes