Christina Sølund[1,2*] & Alexander P. Underwood[1,2*], Carlota Fernandez-Antunez[1,2], Signe Lysemose Villadsen[1,2], Ulrik Fahnøe[1,2], Signe Bollerup[1], Anni Assing Winckelmann[1,2], Santseharay Ramirez[1,2], Nina Weis[1,3#] & Jens Bukh[1,2#].

Affiliates

Department of Infectious Diseases, Copenhagen University Hospital, Hvidovre, Denmark[1]. Copenhagen Hepatitis C Program (CO-HEP), Department of Infectious Diseases, Copenhagen University Hospital, Hvidovre and Department of Immunology and Microbiology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark[2]. Department of Clinical Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark[3]. *Shared first authorship, #shared last authorship.

Abstract

Background:
Given the importance of vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the prevention of severe coronavirus disease 2019 (COVID-19), detailed long-term analyses of neutralizing antibody responses are required to inform immunization strategies. In this study, neutralizing antibody titers to an ancestral SARS-CoV-2 isolate were comprehensively analyzed longitudinally in those previously infected with SARS-CoV-2, vaccinated against COVID-19, and a complex mix thereof with up to two years follow up. Furthermore, variant cross-neutralizing potential of individuals that displayed high neutralizing titers was analyzed.

Methods:
The Clinical, Virological and Immunological COVID-19 (CVIC) study is a prospective cohort study of individuals infected by SARS-CoV-2 or vaccinated against COVID-19, with either a homologous prime-boost (Comirnaty®) or heterologous prime (Vaxzevria®) – boost (Comirnaty® or Spikevax®) vaccine regimen. A total of 103 individuals with non-hospitalized COVID-19, 55 individuals with hospitalized COVID-19, 109 individuals that were vaccinated with a homologous prime-boost regimen and 22 individuals that were vaccinated with a heterologous prime-boost regimen against COVID-19 were recruited at Copenhagen University Hospital, Hvidovre, Denmark during the time period April 2020 – March 2021. Figure 1 shows the time scale and blood collection time points. Plasma was screened for anti-SARS-CoV-2 receptor-binding domain antibodies via the WANTAI enzyme-linked immunosorbent assay (ELISA) in those infected to confirm seroconversion and in those vaccinated to confirm no previous SARS-COV-2 infection before – and seroconversion after vaccination. All subjects were screened for neutralization in Vero E6 cell-culture experiments using a SARS-CoV-2 isolate and selected individuals with high neutralizing titers were further screened against a delta and an omicron SARS-CoV-2 isolate.

Results:
Both infection- and vaccine-induced neutralizing responses appeared to follow similar decay patterns, whereby there was a decline of neutralizing responses until 6 months post symptom onset/vaccination, which were then sustained thereafter. Data also showed that the level of the plateau was reflected by the initial level of neutralizing responses detected. Following vaccination, individuals previously infected displayed more durable neutralizing responses compared than prior to vaccination. Further, data shows that vaccination after infection and booster vaccination in those previously vaccinated increases cross-neutralizing potential to both delta and omicron SARS-CoV-2 isolates.

Conclusion:
This study found that, regardless of SARS-CoV-2 infection or COVID-19 vaccination, the durability of neutralizing responses was the same. However, boosting of neutralizing responses through further antigenic exposure showed that the durability of these responses could be enhanced, likely through increasing expansion of long-lived antigen-specific plasmablasts. In regard to cross-neutralization, vaccination in those previously infected and booster vaccination or infection in those previously vaccinated greatly increased the level of cross-neutralization to the delta and omicron variants, suggesting that boosting using current vaccine formulas can provide enhanced protection to divergent SARS-CoV-2 variants and thus potentially prevent severe COVID-19 disease.