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By the end of May 2020, Oregon had confirmed 4,243 cases of COVID-19 by nucleic acid (PCR) testing — about 0.1% of the state. That count misses everyone who was infected but never tested: people with mild or no symptoms, people who chose not to be tested, and people who could not get a test. An antibody (seroprevalence) survey finds some of them, because past infection leaves antibodies in the blood. Other U.S. states and areas had measured seroprevalence between 1.0% and 6.9%; Oregon had no estimate yet.
How the samples were gathered
The Oregon Health Authority followed the World Health Organization's protocol for seroepidemiologic investigations.
| Step | What was done |
|---|---|
| Facilities | 86 clinics and hospitals in CDC's influenza-like illness network and Oregon's ESSENCE early-warning system were put in random order and approached one by one, aiming for 18 facilities × 50 samples |
| Samples | random, de-identified leftover blood from patients of any age in outpatient, emergency or inpatient care, with collection date and date of birth |
| Test | Abbott Architect SARS-CoV-2 IgG assay at the Oregon State Public Health Laboratory — reported sensitivity 96.8% from 14 days after a positive PCR, specificity 99.1–100% |
| Result | 19 facilities (one could send only 15 samples, so another was added), spread roughly like Oregon's population; 898 samples from May 11 to June 15, one lost to a laboratory error — 897 tested |
What they found
Antibodies were found in 9 of 897 samples: an unadjusted seroprevalence of 1.0% (95% confidence interval 0.2–1.8%). None of the 29 children and teenagers had antibodies, and the rate generally rose with age (test for trend, p = 0.049).
| Age (years) | Tested | Positive | % (95% CI) |
|---|---|---|---|
| 0–4 | 5 | 0 | 0 (0–52) |
| 5–17 | 24 | 0 | 0 (1–14) |
| 18–49 | 274 | 1 | 0.4 (0–2.0) |
| 50–64 | 211 | 1 | 0.5 (0–2.6) |
| 65–74 | 178 | 3 | 1.7 (0.3–4.8) |
| 75–84 | 144 | 3 | 2.1 (0.4–6.0) |
| 85 and over | 61 | 1 | 1.6 (0–8.8) |
| All | 897 | 9 | 1.0 (0.2–1.8) |
What it means
- About ten times the reported cases. Roughly 1% with antibodies against 0.1% confirmed — the same tenfold gap seen in seven other U.S. states and areas.
- Most infections were never diagnosed, and most Oregonians were still susceptible.
- Older people more often positive, a pattern also reported in neighbouring Idaho, though the sample was small.
Limits
- A convenience sample of people already using health care is not necessarily representative of the whole state.
- Where infection is rare, as in Oregon, false positives weigh more heavily.
- Some infected people never make antibodies, and in others the antibodies fade below detection.
Oregon planned follow-up surveys to track cumulative infection as the pandemic went on.
Sources
Written from "Notes from the Field: Seroprevalence Estimates of SARS-CoV-2 Infection in Convenience Sample — Oregon, May 11–June 15, 2020," by Melissa Sutton, Paul Cieslak and Meghan Linder, Public Health Division, Oregon Health Authority, Morbidity and Mortality Weekly Report 69(32), CDC; a work of the United States government in the public domain. It cites Bryan and others (2020, Boise, Idaho), Menachemi and others (2020, Indiana), Biggs and others (2020, Georgia) and Havers and others (2020, ten U.S. sites).
Licenza: CC0 1.0 (pubblico dominio) · Tratto da www.cdc.gov
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