Immune boosting by B.1.1.529 (Omicron) depends on previous SARS-CoV-2 exposure

Importance of infection history
A long-term study of healthcare workers in the United Kingdom has allowed their history of infection and vaccination to be traced precisely. Reynolds et al. found some unexpected immune-damping effects caused by infection with a heterologous variant to the latest wave of infection by the Omicron/Pango lineage B.1.1.529. The authors found that Omicron infection boosted immune responses to all other variants, but responses to Omicron itself were muted. Infection with the Alpha variant provided weaker boosting for Omicron-specific responses. Furthermore, Omicron infection after previous Wuhan Hu-1 infection failed to boost neutralizing antibody and T cell responses against Omicron, revealing a profound imprinting effect and explaining why frequent reinfections occur. —CA

B.1.1.529 (Omicron) and its subvariants pose new challenges for control of the COVID-19 pandemic. Although vaccinated populations are relatively protected from severe disease and death, countries with high vaccine uptake are experiencing substantial caseloads with breakthrough infection and frequent reinfection.
We analyzed cross-protective immunity against B.1.1.529 (Omicron) in triple-vaccinated health care workers (HCWs) with different immune-imprinted histories of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection during the ancestral Wuhan Hu-1, B.1.1.7 (Alpha), and B.1.617.2 (Delta) waves and after infection during the B.1.1.529 (Omicron) wave in previously infection-naïve individuals and those with hybrid immunity, to investigate whether B.1.1.529 (Omicron) infection could further boost adaptive immunity. Spike subunit 1 (S1) receptor binding domain (RBD) and whole spike binding, live virus neutralizing antibody (nAb) potency, memory B cell (MBC) frequency, and T cell responses against peptide pools and naturally processed antigen were assessed.
B and T cell recognition and nAb potency were boosted against previous variants of concern (VOCs) in triple-vaccinated HCWs, but this enhanced immunity was attenuated against B.1.1.529 (Omicron) itself.

Furthermore, immune imprinting after B.1.1.7 (Alpha) infection resulted in reduced durability of antibody binding against B.1.1.529 (Omicron), and S1 RBD and whole spike VOC binding correlated poorly with live virus nAb potency. Half of triple-vaccinated HCWs showed no T cell response to B.1.1.529 (Omicron) S1 processed antigen, and all showed reduced responses to the B.1.1.529 (Omicron) peptide pool, irrespective of SARS-CoV-2 infection history. Mapping T cell immunity in class II human leukocyte antigen transgenics showed that individual spike mutations could result in loss or gain of T cell epitope recognition, with changes to T cell effector and regulatory programs. Triple-vaccinated, previously infection-naïve individuals infected during the B.1.1.529 (Omicron) wave showed boosted cross-reactive S1 RBD and whole spike binding, live virus nAb potency, and T cell immunity against previous VOCs but less so against B.1.1.529 (Omicron) itself. Immune imprinting from prior Wuhan Hu-1 infection abrogated any enhanced cross-reactive antibody binding, T cell recognition, MBC frequency, or nAb potency after B.1.1.529 (Omicron) infection.

Vaccine boosting results in distinct, imprinted patterns of hybrid immunity with different combinations of SARS-CoV-2 infection and vaccination. Immune protection is boosted by B.1.1.529 (Omicron) infection in the triple-vaccinated, previously infection-naïve individuals, but this boosting is lost with prior Wuhan Hu-1 imprinting. This “hybrid immune damping” indicates substantial subversion of immune recognition and differential modulation through immune imprinting and may be the reason why the B.1.1.529 (Omicron) wave has been characterized by breakthrough infection and frequent reinfection with relatively preserved protection against severe disease in triple-vaccinated individuals