The rebound of endemic respiratory viruses following the COVID-19 pandemic was marked by atypical transmission dynamics, with children experiencing increased disease burden and out-of-season epidemics as restrictions relaxed. Here we used serology from a newly developed quantitative multiplex assay to assess the post-pandemic immunity debt, a drop in immunity due to a lack of endemic virus circulation during COVID-19. We assessed age-specific antibody dynamics in Seattle, Washington, US, across a broad range of respiratory viruses, including influenza, respiratory syncytial virus, seasonal coronaviruses, and SARS-CoV-2. We found that respiratory virus immunodynamics differed between individuals <5 years of age and older individuals, with young children experiencing both larger boosts and quicker waning of antibodies across pathogens. We confirmed that these patterns are upheld in a non-pandemic setting by analyzing influenza serological data from South Africa. We incorporated our serological insights into an influenza transmission model calibrated to epidemiological data from Seattle and show that consideration of age-specific immunodynamics may be important to anticipate the effects of pandemic perturbations.

The authors have declared no competing interest.

E.T.M. is supported by 75N93021C00015 from NIAID. Funding for the Seattle Flu Study was provided by Gates Ventures and the Howard Hughes Medical Institute. JAH is supported by a Wellcome Trust Early Career Award (grant 225001/Z/22/Z).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This study was approved by the Institutional Review Board of Seattle Childrens Hospital. This study was granted a waiver of consent since it used residual clinical samples and existing clinical data.

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I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

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Non-identifiable individual-level serology data and aggregated surveillance data will be shared upon publication.