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COVID-19 Testing

Presence of Antibodies Among Unvaccinated Individuals Following COVID-19 Diagnosis

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BACKGROUND AND PURPOSE:

  • Among the unvaccinated, previous SARS-CoV-2 infection may provide some immunity against subsequent infection
    • Persistence of antibodies beyond 6 months remains unknown
  • Alejo et al. (JAMA, 2022) characterized natural immunity and long-term durability among unvaccinated individuals using anti–spike antibodies

METHODS:

  • Cross-sectional study
  • Participants
    • Healthy adults
    • Unvaccinated against COVID-19
  • Exposures
    • COVID confirmed: Test-confirmed COVID-19 infection
    • COVID unconfirmed: Suspected COVID-19 but not tested
    • No COVID: No suspected COVID-19 | Never tested positive
  • Study design
    • Participants recruited via social media
    • Participant-completed questionnaire: Demographics | COVID-19 status | Mask use
    • Participants placed into an exposure group using weighted random sampling (relative weights based on the estimated unvaccinated US population by age, race and ethnicity, and education)
    • Antibody testing: Invited to get tested over a month 
      • Qualitative detection of antibodies against the SARS-CoV-2 antinucleocapsid (N) protein | Positive cutoff index ≥1.0
      • Semiquantitative detection of antibodies against the SARS-CoV-2 spike protein receptor-binding domain (RBD) | Positive cutoff ≥0.8 U/mL
  • Primary outcome
    • Association between antibody titers and previous infection status

RESULTS:

  • COVID confirmed: 295 | COVID unconfirmed: 275 | No COVID: 246
    • 1580 individuals invited to undergo serologic testing: 52% accepted (n=816)
    • Mean age 48.0 years | 52% women | 82% White
    • Reported routine mask use in public: 14%
  • Among COVID confirmed participants
    • Median months passed since diagnosis: 8.7 (IQR, 1.9 to 12.9)
    • Tested positive for anti-RBD: 99%
    • Median anti-RBD level: 205 (IQR, 61 to 535) U/mL
    • There was no evidence of association between time after infection and antibody titer
      • 0.8% increase per month (95% CI, –2.4% to 4.2%); P=0.62
  • Among COVID unconfirmed participants
    • Tested positive for anti-RBD: 55%
    • Median anti-RBD level: 131 (IQR, 35 to 402) U/mL
  • Among no COVID participants
    • Tested positive for anti-RBD: 11%
    • Median anti-RBD level: 82 (IQR, 19 to 172) U/mL

CONCLUSION:

  • Among those participants who received a positive COVID-19 test, 99% had anti-RBD antibodies with persistence up to 20 months
  • Among those who thought that they had COVID-19, only approximately half of the cohort had detectable antibodies
  • Whether these antibodies are associated with protection against future SARS-CoV-2 encounters is still unclear
  • Authors acknowledge limitations including selection bias and the fact that antibody levels alone do not directly correlate with immunity
  • The authors state

Although evidence of natural immunity in unvaccinated healthy US adults up to 20 months after confirmed COVID-19 infection is encouraging, it is unclear how these antibody levels correlate with protection against future SARS-CoV-2 infections, particularly with emerging variants

Learn More – Primary Sources:

Prevalence and Durability of SARS-CoV-2 Antibodies Among Unvaccinated US Adults by History of COVID-19

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Related ObG Topics:

How Long Does It Take for COVID-19 Patients to Develop Antibodies?
What is the Seroprevalence of SARS-CoV2-2 Antibodies in Patients on Labor and Delivery?
How Common is SARS-CoV-2 Reinfection among Healthcare Workers with Antibodies?

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