What concentrations of circulating SARS‑CoV‑2 Spike protein are observed in hospitalized versus mild COVID‑19 patients, and how do those levels compare to doses used in animal experiments?
Executive summary
Circulating SARS‑CoV‑2 Spike (S) protein—measured as soluble S or S1 antigen in plasma—has been reported in the low‑nanogram per milliliter range in patients, with investigators selecting experimental concentrations of ~2–20 ng/mL to reflect values seen in complicated COVID‑19 cases (authors citing Craddock et al.) [1]. By contrast, many in vitro and animal studies probing mechanistic effects of Spike employ microgram‑per‑milliliter concentrations (for example 10 μg/mL), a difference of three to six orders of magnitude compared with reported patient plasma levels [2] [1].
1. What clinical measurements show about Spike in hospitalized versus mild patients
Clinical and post‑acute studies detect Spike/S1 antigen in a substantial fraction of infected individuals and show a clear relationship with severity: one report found S1 antigen detectable in roughly two‑thirds of COVID‑19 patients and observed a significant correlation between higher plasma S1 concentration and disease progression [3], and cohort analyses report that patients who required hospitalization were nearly twice as likely to have viral antigens detected in the post‑COVID phase compared with non‑hospitalized cases [4]. Those findings support the notion that hospitalized patients are more likely to have persistent or higher circulating Spike antigen than mild cases [3] [4].
2. Absolute concentrations clinicians and bench scientists cite
When translating clinical observations into experimental design, some authors explicitly chose Spike concentrations of 2 and 20 ng/mL because those values “reflect the range observed in patients experiencing SARS‑CoV‑2 complications,” citing Craddock et al. as the empirical basis [1]. That places reported circulating Spike in the single‑digit to tens of ng/mL range in clinically significant cases [1]. Note that many human serological reports quantify anti‑Spike antibodies in binding antibody units per milliliter (BAU/mL), not Spike antigen, and show wide interindividual ranges (for example RBD‑IgG medians for mild versus severe disease: ~80.5 versus ~206.3 BAU/mL), which is a different measurement axis (antibodies, not antigen) and cannot be directly converted to ng/mL of protein without assay‑specific calibration [5].
3. How animal and in vitro experiments compare: orders of magnitude matter
Laboratory mechanistic studies frequently use recombinant Spike or S1 at concentrations far higher than patient plasma levels: for example, an endothelial barrier study treated cells or isolated arteries with 10 μg/mL Spike protein for 12 hours [2]. That single in vitro concentration is 10,000 ng/mL—about 500×–5,000× higher than the 2–20 ng/mL range cited as clinically observed [2] [1]. Animal vaccine or exposure studies usually report doses in terms of protein immunogen administered (μg of antigen per dose) or infectious viral dose (plaque‑forming units) rather than steady‑state circulating Spike concentrations, complicating direct comparisons between human antigenemia and animal dosing regimens [6] [7].
4. Why simple comparisons can mislead — assay, form, timing and compartment
Comparative statements must be qualified: “circulating Spike” in patients can be free soluble S1, full ectodomain, or vesicle/exosome‑associated peptides, and detection rates vary by assay sensitivity and timing after infection or vaccination [3] [4]. Hospitalized patients are more likely to retain detectable antigen over months, which motivated authors to use ng/mL concentrations in cell studies presumed to be clinically relevant [4] [1]. Conversely, in vitro research that uses μg/mL concentrations typically aims to amplify or reveal mechanistic effects under controlled conditions and does not imply those concentrations are present systemically in patients [2].
5. Balanced takeaways and limits of the reporting
The best available reporting provided here supports that clinically observed circulating Spike concentrations in severe or complicated human cases lie in the low‑ng/mL range (authors used 2 and 20 ng/mL as representative) and that hospitalized patients show higher likelihood of persistent antigen detection than mild cases [1] [3] [4]. However, many mechanistic in vitro and some animal studies deliberately use μg/mL doses—orders of magnitude greater—so extrapolation from those experiments to patient biology requires caution; direct numerical equivalence is not supported by the cited sources and comparisons are constrained by differing assay targets, units, and biological contexts [2] [6] [7]. Where assertions extend beyond these sources—such as precise plasma concentration distributions across large cohorts or direct toxicological thresholds in animals—this review does not claim evidence beyond the cited reports.