Use of SARS-CoV-2 serology to assess risk of SARS-CoV-2 breakthrough infection (BTI) after COVID-19 vaccination is officially discouraged.1  Yet some experts suggest that low SARS-CoV-2 anti–spike protein receptor-binding domain (anti-RBD) antibody measurements could identify individuals with a suboptimal response to vaccination who may be at increased risk of BTI.2 

We conducted a prospective cohort study among health care workers (HCWs) in a large integrated health care system spanning 15 medical centers across Southern California (see Supplemental Table for locations in the supplemental digital content at https://meridian.allenpress.com/aplm in the January 2024 table of contents) to determine whether lower anti-RBD concentrations or undetectable anti-nucleocapsid antibodies (anti-N) were associated with higher risk of BTI (institutional review board approval 12792).

Beginning in September 2021, we recruited 512 fully vaccinated HCWs (Table) through company newsletters and email to submit a blood sample for serology and weekly saliva for polymerase chain reaction testing. Individuals were followed from date of blood collection until March 26, 2022, roughly spanning the Omicron BA.1 and BA.2 surge in California (December 12, 2021, to March 26, 2022).3 

Serology was measured with the Roche Elecsys Anti-SARS-CoV-2 (anti-N) and Anti-SARS-CoV-2 S (anti-RBD) immunoassays and the Siemens ADVIA Centaur SARS-CoV-2 Total (anti-RBD) immunoassay. As there are no recommended serologic cutoffs associated with BTI risk, we used cutoffs consistent with other studies for Roche anti-RBD (<144/≥144) and Siemens anti-RBD (<1.44/≥1.44).4 

BTI was identified from weekly saliva self-collection for SARS-CoV-2 polymerase chain reaction (Roche and ThermoFisher), positive clinical results noted in the electronic medical record, or participant report of outside positive results. Cox proportional hazards regression was used to compare BTI rates by serologic cutoffs, adjusting for participant demographics, comorbidities, and prior SARS-CoV-2 infection and vaccination.

Of the 512 participants, 315 were negative and 197 were positive by Roche anti-N serology; 21 had Roche anti-RBD concentrations less than 144 and 491 had concentrations 144 or more; and 21 had Siemens anti-RBD concentrations less than 1.44 and 491 had concentrations 1.44 or more (Table). There were 98 documented BTIs: 65 from weekly saliva self-collection, 27 from clinical testing, and 6 participant reports of an outside result. Antibody concentrations (positive versus negative Roche anti-N, ≥144 versus <144 Roche anti-RBD, and ≥1.44 versus <1.44 Siemens anti-RBD) were not significantly associated with BTI in adjusted analyses (hazard ratio [95% CI] 0.66 [0.43–1.02], 0.49 [0.22–1.06], and 0.67 [0.29–1.56], respectively). In all 3 models, receipt of a COVID-19 booster was independently associated with a lower risk of BTI (Figures 1 through 3).

Figure 1.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Roche anti-nucleocapsid (anti-N) positive versus negative results.

Figure 1.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Roche anti-nucleocapsid (anti-N) positive versus negative results.

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Figure 2.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Roche anti–spike protein receptor-binding domain (anti-RBD) ≥144 versus <144.

Figure 2.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Roche anti–spike protein receptor-binding domain (anti-RBD) ≥144 versus <144.

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Figure 3.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Siemens anti–spike protein receptor-binding domain (anti-RBD) ≥1.44 versus <1.44.

Figure 3.

Adjusted hazard ratios (HRs) for SARS-CoV-2 breakthrough infection among participants with Siemens anti–spike protein receptor-binding domain (anti-RBD) ≥1.44 versus <1.44.

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Limitations of our study include variable timing of booster vaccination relative to serologic sample collection, suboptimal compliance with weekly saliva testing, and mild BTI that did not lead to clinical testing.5 

Contrary to our initial hypothesis and published suggestions, SARS-CoV-2 serology (anti-N and anti-RBD) did not identify HCWs at significantly increased risk for BTI.

Baseline Demographics and Clinical Characteristics Prior to Index Date, COVID-19 Vaccination, and SARS-CoV-2 Breakthrough Infection (BTI) by Anti-Nucleocapsid (Anti-N) and Anti–Spike Protein Receptor-Binding Domain (Anti-RBD) Serology Results

Baseline Demographics and Clinical Characteristics Prior to Index Date, COVID-19 Vaccination, and SARS-CoV-2 Breakthrough Infection (BTI) by Anti-Nucleocapsid (Anti-N) and Anti–Spike Protein Receptor-Binding Domain (Anti-RBD) Serology Results
Baseline Demographics and Clinical Characteristics Prior to Index Date, COVID-19 Vaccination, and SARS-CoV-2 Breakthrough Infection (BTI) by Anti-Nucleocapsid (Anti-N) and Anti–Spike Protein Receptor-Binding Domain (Anti-RBD) Serology Results

The authors thank the patients of Kaiser Permanente and their partnership with us to improve their health.

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Author notes

The study was funded by the Garfield Memorial Research Fund and the Kaiser Permanente Community Health Fund.

Supplemental digital content is available for this article at https://meridian.allenpress.com/aplm in the January 2024 table of contents.

The authors have no relevant financial interest in the products or companies described in this article.

Supplementary data