Wastewater antibody signatures reflect SARS-CoV-2 infection and immunization patterns in the population

Wastewater-based epidemiology (WBE) has been widely used to track SARS-CoV-2 transmission using viral RNA, but its capacity to capture population immunity remains poorly defined. Although antibodies can be recovered from wastewater, the relationship between wastewater antibody signals, individual-level shedding dynamics, and community-wide infection and immunity patterns has not been systematically established. We conducted a three-year longitudinal study (2020–2022) across urban and rural communities in Thailand, covering approximately 18 million people. SARS-CoV-2 viral RNA and anti-SARS-CoV-2 IgG concentrations were quantified in wastewater from diverse facility types. To calibrate wastewater signals, faecal viral RNA and IgG shedding kinetics were characterised in 412 individuals from the same communities. Deconvolution models were applied to infer infection incidence from wastewater measurements, and lagged regression analyses assessed associations with confirmed cases, mortality, and vaccination coverage. Faecal viral RNA shedding peaked early after symptom onset and declined rapidly, whereas anti-SARS-CoV-2 IgG exhibited delayed onset, peaked around 50 days, and persisted for several months. These distinct kinetics produced temporally offset wastewater signals, with RNA-based incidence consistently preceding antibody-based estimates. Prior to vaccine rollout, wastewater incidence was dominated by viral RNA signals, while antibody levels remained low. Following widespread vaccination and successive infection waves, antibody-based incidence increased and gradually converged with RNA-based estimates, particularly in highly vaccinated urban areas. Wastewater antibody levels showed strong temporal autocorrelation and positive lagged associations with confirmed cases and vaccination, with weaker associations with mortality. Integrating viral RNA and antibody measurements in wastewater, informed by individual-level shedding dynamics, enables concurrent assessment of infection burden and population immunity.