Avidity as a marker of protection against hepatitis E during a genotype 1 outbreak in South Sudan

Background: In settings where hepatitis E virus (HEV) is highly endemic, anti-HEV IgG seroprevalence approaches saturation, yet how often disease occurs in previously infected individuals, and how protective natural genotype 1 immunity is, remains unmeasured. IgG concentration cannot answer this as levels rise within days of symptom onset masking any pre-existing levels; IgG avidity, maturing over months, may serve as a useful tool to understand reinfections. Methods: We enrolled 1007 suspected HEV patients and measured anti-HEV IgG avidity in IgG-positive confirmed cases and an age-stratified sample of IgG-positive test-negatives (n = 366) in enhanced surveillance around a 2022 vaccination campaign in Bentiu, South Sudan. Using high avidity (avidity index greater than or equal to 50%) as a marker of past infection, we estimated (i) the reinfection fraction, (ii) protection from prior infection using a test negative design and (iii) associations with viral load and infection severity. Results: Median avidity index was 11% in cases versus 77% in test-negatives. Among unvaccinated cases, 11.6% (28/242) carried high-avidity IgG (indicative of mature immunity and therefore reinfection), rising with age (odds ratio 1.6 per decade, p=0.006). Mature immunity was associated with 95% lower odds of disease (adjusted OR 0.05, 95% CI 0.03-0.08), robust to threshold and case-definition. High-avidity cases were less often viremic (29% vs 78%) at matched time since onset, though we found no differences in liver function biomarkers between patients with high-avidity and low-avidity. Conclusions: Most HEV disease occurred in previously uninfected individuals, and prior immunity was strongly protective against detected disease. Avidity provides a population-level tool to read the immune landscape where seroprevalence may be uninformative.