Teamwork makes the dream work : systemic and mucosal immunity in shaping protection against SARS-CoV-2

COVID-19 triggered an unprecedented scientific response, and although knowledge gaps persist, large parts of the world had access to efficacious vaccines only a year after the virus first appeared. A detailed understanding of mechanisms of protection against infection, and not only against severe disease, is essential for comprehending the transition from the uncontrolled viral transmission to a low-grade, endemic circulation of this and future emerging viruses. The studies in this thesis focus on the role of serum and mucosal antibody responses in shaping protection against SARS-CoV-2 infection.

Study I is a four-week screening study of 368 triple vaccinated health care workers, with a cumulative SARS-CoV-2 infection incidence of 22%. In this study, we assessed protection against Omicron infection in relation to ancestral spike- specific IgG levels in serum and explored their potential role in reducing viral load. We found that high levels of vaccine-induced spike-specific serum antibodies were associated with both protection against infection and lower viral load in those who became infected. Additionally, we suggested that the added protection against Omicron infection observed in individuals with hybrid immunity is largely mediated by mucosal spike-specific IgA.

In Study II, we assessed protection against SARS-CoV-2 Omicron infection in relation to ancestral spike-specific IgA levels in nasal secretions, as well as the impact of prior infection on mucosal immune responses, in 337 healthcare workers who also participated in Study I. We observed a 65% reduction in the risk of infection among individuals with nasal spike IgA levels above the 75th percentile at study inclusion compared to the rest. Additionally, we identified a temporal relationship between rising nasal IgA levels and declining viral load during infection. A trend toward more rapid nasal spike IgA responses in reinfected individuals was also noted.

In Study III, we investigated the duration of mucosal immune responses elicited during infections in Studies I and II, and the protection associated with those responses. We found that the significantly reduced risk of infection among participants with nasal spike IgA levels above the 75th percentile at baseline (Study I and II inclusion) persisted for eight months, with a Hazard Ratio of 0.55 (p<0.05). Furthermore, we assessed the durability of nasal spike IgA responses and reported that, seven months after infection, 58% of participants experiencing their first infection and 94% of reinfected participants still had nasal spike IgA levels above the level corresponding to the 75th percentile at baseline.

In Study IV, we assessed protection against the newly emerged Omicron BQ strains in relation to nasal spike IgA and found robust protection, incidence rate ratio of 0.42 [95% CI 0.26-0.70] for individuals with detectable nasal spike IgA, with a linear association between nasal spike IgA levels and protection against infection. We also found that nasal spike IgA demonstrated a stronger ability to bind viral variants compared to serum spike IgG. Interestingly, the protection against infection associated with serum spike IgG in this study was no longer significant when adjusted for nasal spike IgA levels.

Study V was a retrospective cohort study investigating factors influencing nasal spike IgA levels. Prior mucosal viral encounter, i.e. infection, was paramount to nasal spike IgA development. We observed an increased likelihood of nasal spike IgA detection for as long as >22 months following the most recent infection and identified a boosting effect by repeated infections. We also found that parenteral vaccination was associated with lower nasal spike IgA levels, with an adjusted OR for detectable nasal spike IgA of 0.24 [95% CI 0.08-0.68] among individuals with 1-3 vaccine doses compared to those who were unvaccinated. Additionally, the temporal infection-vaccination sequence influenced nasal spike IgA levels, with higher levels in participants infected prior to vaccination as compared to those with break-through infection as first viral encounter. This finding is of importance to mucosal vaccine development, particularly to replicating vaccine platforms where systemic immunity may reduce viral replication and the resulting inflammation that shapes the subsequent mucosal immune response.

The observations in this thesis illustrate the transition from early pandemic control-where serum-derived protection from vaccination was paramount-to the current phase of endemic transmission, where mucosal immune responses play a central role in population immunity. We argue that with evolution of immune evasive strains and concurrent development of mucosal defences, mucosal spike IgA replaces serum spike IgG as the most relevant correlate to protection against infection. Although parenteral vaccination and systemic responses are crucial in protection against severe COVID-19, a vaccine platform that evokes a robust mucosal response may be more potent in inducing population immunity, with a reduction of viral transmission and variant evolution.

List of scientific papers

I. Marking U, Havervall S, Greilert-Norin N, Bladh O, Christ W, Gordon M, Ng H, Blom K, Phillipson M, Mangsbo S, Alm JJ, Smed-Sörensen A, Nilsson P, Hober S, Åberg M, Klingström J, Thålin C. Correlates of protection and viral load trajectories in Omicron breakthrough infections in triple vaccinated healthcare workers. Nat Commun. 2023 Mar 22;14(1):1577. PMID: 36949041. https://doi.org/10.1038/s41467-023-36984-1

II. Havervall S*, Marking U*, Svensson J, Greilert-Norin N, Bacchus P, Nilsson P, Hober S, Gordon M, Blom K, Klingstrom J, Åberg M, Smed- Sörensen A, Thålin C. Anti-Spike Mucosal IgA Protection against SARS- CoV-2 Omicron Infection. N Engl J Med. 2022 Oct 6;387(14):1333-1336. Epub 2022 Sep 14. PMID: 36103621. https://doi.org/10.1056/NEJMc2209651

III. Marking U, Bladh O, Havervall S, Svensson J, Greilert-Norin N, Aguilera K, Kihlgren M, Salomonsson AC, Månsson M, Gallini R, Kriegholm C, Bacchus P, Hober S, Gordon M, Blom K, Smed-Sörensen A, Åberg M, Klingström J, Thålin C. 7-month duration of SARS-CoV-2 mucosal immunoglobulin-A responses and protection. Lancet Infect Dis. 2023 Feb;23(2):150-152. Epub 2023 Jan 11. PMID: 36640796. https://doi.org/10.1016/S1473-3099(22)00834-9

IV. Marking U, Bladh O, Havervall S, Greilert-Norin N, Gordon M, Alm JJ, Blom K, Åberg M, Klingström J, Thålin C. Mucosal IgA protects against BQ.1 and BQ.1.1 infection. Lancet Infect Dis. 2023 Aug;23(8):e272-e273. Epub 2023 Jul 12. PMID: 37453441. https://doi.org/10.1016/S1473-3099(23)00421-8

V. Marking U, Bladh O, Aguilera K, Pongracz T, Havervall S, Greilert-Norin N, Blom K, Klingström J, Wang Y, Åberg M, Thålin C. Systemic SARS-CoV-2 vaccination shapes subsequent generation of mucosal IgA responses - a retrospective cohort study. [Manuscript]

* Shared first authorship