How long does spike protein from mRNA COVID vaccines remain detectable in blood by PCR or antigen tests?
Executive summary
Typical clinical and regulatory interpretations say spike protein or its mRNA from intramuscular mRNA COVID vaccines is detectable in blood for days to a few weeks after vaccination under routine assays, but highly sensitive research methods and reports in special clinical groups have found detectable signal for much longer in rare cases, with substantial methodological and interpretive caveats [1] [2] [3] [4].
1. What the question really asks — protein vs. mRNA and test type matter
The core question conflates two different targets and technologies: PCR detects vaccine mRNA sequences (not the protein), while antigen tests or proteomic assays detect spike protein fragments or full-length spike; each has different sensitivity, kinetics and interpretation, so any answer must separate mRNA persistence from protein persistence and note which assay is meant [2] [3].
2. Typical detection windows reported in mainstream clinical sources
Multiple clinical summaries and infectious-disease guidance report that vaccine-derived spike protein and the mRNA that encodes it are generally transient: spike protein is expected to last up to a few weeks and mRNA is fragile and usually cleared in days to a couple of weeks in most people, consistent with IDSA and hospital information summaries [1] [5].
3. Peer‑reviewed and high‑sensitivity studies that found longer detection
Targeted studies using highly sensitive methods have extended that window: some RT‑qPCR and immunoassay work detected vaccine mRNA in leukocytes up to about 6 days and in plasma up to 15 days after vaccination, while single‑molecule immunoassays detected spike protein from Moderna vaccine in plasma up to ~28 days in many subjects [2]. Proteomic mass‑spectrometry approaches can specifically distinguish vaccine‑derived spike peptides and have reported detection of recombinant spike in blood samples beyond the timeframe of routine antigen tests [3].
4. Exceptional findings in special clinical cohorts
Select clinical cohorts show much longer persistence in specific contexts: adolescents and young adults with post‑mRNA‑vaccine myocarditis had circulating full‑length spike unbound by antibodies, a signal not seen in all vaccinated people and interpreted in the study as possibly related to pathogenesis in that subgroup [6]. Investigators studying “post‑vaccination syndrome” reported rare participants with measurable spike levels hundreds of days after vaccination (greater than 700 days in some cases), but those findings derive from small, emerging cohorts and require replication and careful exclusion of intervening infection [4].
5. Persistence after infection vs. vaccination — different patterns
By contrast, SARS‑CoV‑2 infection itself can seed tissues and release viral spike fragments for prolonged periods; long‑COVID studies have detected spike fragments in plasma up to roughly a year after infection in some patients, underlining that infection and vaccination are not identical in biodistribution or durability of detectable antigen [7] [8].
6. Why results vary so much — assays, definitions and selection bias
Differences in reported durations stem from: what is being measured (mRNA vs S1 subunit vs full spike), assay sensitivity (routine antigen tests vs single‑molecule assays or mass spectrometry), whether the cohort had clinical syndromes that alter clearance, and small sample sizes or publication bias in extreme findings; critics caution that low‑abundance signals can be overinterpreted without clear clinical correlation [3] [9].
7. Practical takeaways and limits of current evidence
For the overwhelming majority of vaccinated people, routine PCR or antigen tests will detect vaccine mRNA or spike protein only for days to a few weeks after vaccination; specialized, highly sensitive research assays can detect trace vaccine‑derived material longer and rare clinical groups have shown prolonged signals, but those are exceptions with unresolved causal interpretation and require more rigorous, larger studies to determine prevalence and significance [1] [2] [4] [6].