Specificity of SARS-CoV-2 Antibody Assays

Both assays measuring pan-Ig antibodies had low numbers of false positives among samples collected in 2017: there were 0 and 1 false positives for the two assays among 472 samples, results that compared favorably with those obtained with the single IgM anti-N and IgG anti-N assays (Table S3). Because of the low prevalence of SARS-CoV-2 infection in Iceland, we required positive results from both pan-Ig antibody assays for a sample to be considered seropositive (see Supplementary Methods in Supplementary Appendix 1). None of the samples collected in early 2020 group were seropositive, which indicates that the virus had not spread widely in Iceland before February 2020.

SARS-CoV-2 Antibodies among qPCR-Positive Persons

Antibody Prevalence and Titers among qPCR-Positive Cases as a Function of Time since Diagnosis by qPCR.

Shown are the percentages of samples positive for both pan-Ig antibody assays and the antibody titers. Red denotes the count or percentage of samples among persons during their hospitalization (249 samples from 48 persons), and blue denotes the count or percentage of samples among persons after they were declared recovered (1853 samples from 1215 persons). Vertical bars denote 95% confidence intervals. The dashed lines indicated the thresholds for a test to be declared positive. OD denotes optical density, and RBD receptor binding domain.

Prevalence of SARS-CoV-2 Antibodies by Sample Collection as Measured by Two Pan-Ig Antibody Assays.

Twenty-five days after diagnosis by qPCR, more than 90% of samples from recovered persons tested positive with both pan-Ig antibody assays, and the percentage of persons testing positive remained stable thereafter (Figure 2 and Fig. S2). Hospitalized persons seroconverted more frequently and quickly after qPCR diagnosis than did nonhospitalized persons (Figure 2 and Fig. S3). Of 1215 persons who had recovered (on the basis of results for the most recently obtained sample from persons for whom we had multiple samples), 1107 were seropositive (91.1%; 95% confidence interval [CI], 89.4 to 92.6) (Table 1 and Table S4). Since some diagnoses may have been made on the basis of false positive qPCR results, we determined that 91.1% represents the lower bound of sensitivity of the combined pan-Ig tests for the detection of SARS-CoV-2 antibodies among recovered persons.

Results of Repeated Pan-Ig Antibody Tests among Recovered qPCR-Diagnosed Persons.

Among the 487 recovered persons with two or more samples, 19 (4%) had different pan-Ig antibody test results at different time points (Table 2 and Fig. S4). It is notable that of the 22 persons with an early sample that tested negative for both pan-Ig antibodies, 19 remained negative at the most recent test date (again, for both antibodies). One person tested positive for both pan-Ig antibodies in the first test and negative for both in the most recent test.

The longitudinal changes in antibody levels among recovered persons were consistent with the cross-sectional results (Fig. S5); antibody levels were higher in the last sample than in the first sample when the antibodies were measured with the two pan-Ig assays, slightly lower than in the first sample when measured with IgG anti-N and IgG anti-S1 assays, and substantially lower than in the first sample when measured with IgM anti-N and IgA anti-S1 assays.

IgG anti-N, IgM anti-N, IgG anti-S1, and IgA anti-S1 antibody levels were correlated among the qPCR-positive persons (Figs. S5 and S6 and Table S5). Antibody levels measured with both pan-Ig antibody assays increased over the first 2 months after qPCR diagnosis and remained at a plateau over the next 2 months of the study. IgM anti-N antibody levels increased rapidly soon after diagnosis and then fell rapidly and were generally not detected after 2 months. IgA anti-S1 antibodies decreased 1 month after diagnosis and remained detectable thereafter. IgG anti-N and anti-S1 antibody levels increased during the first 6 weeks after diagnosis and then decreased slightly.

SARS-CoV-2 Infection in Quarantine

SARS-CoV-2 Infection among Quarantined Persons According to Exposure Type and Presence of Symptoms.

Of the 1797 qPCR-positive Icelanders, 1088 (61%) were in quarantine when SARS-CoV-2 infection was diagnosed by qPCR. We tested for antibodies among 4222 quarantined persons who had not tested qPCR-positive (they had received a negative result by qPCR or had simply not been tested). Of those 4222 quarantined persons, 97 (2.3%; 95% CI, 1.9 to 2.8) were seropositive (Table 1). Those with household exposure were 5.2 (95% CI, 3.3 to 8.0) times more likely to be seropositive than those with other types of exposure (Table 3); similarly, a positive result by qPCR for those with household exposure was 5.2 (95% CI, 4.5 to 6.1) times more likely than for those with other types of exposure. When these two sets of results (qPCR-positive and seropositive) were combined, we calculated that 26.6% of quarantined persons with household exposure and 5.0% of quarantined persons without household exposure were infected. Those who had symptoms during quarantine were 3.2 (95% CI, 1.7 to 6.2) times more likely to be seropositive and 18.2 times (95% CI, 14.8 to 22.4) more likely to test positive with qPCR than those without symptoms.

We also tested persons in two regions of Iceland affected by cluster outbreaks. In a SARS-CoV-2 cluster in Vestfirdir, 1.4% of residents were qPCR-positive and 10% of residents were quarantined. We found that none of the 326 persons outside quarantine who had not been tested by qPCR (or who tested negative) were seropositive. In a cluster in Vestmannaeyjar, 2.3% of residents were qPCR-positive and 13% of residents were quarantined. Of the 447 quarantined persons who had not received a qPCR-positive result, 4 were seropositive (0.9%; 95% CI, 0.3 to 2.1). Of the 663 outside quarantine in Vestmannaeyjar, 3 were seropositive (0.5%; 95% CI, 0.1 to 0.2%).

SARS-CoV-2 Seroprevalence in Iceland

None of the serum samples collected from 470 healthy Icelanders between February 18 and March 9, 2020, tested positive for both pan-Ig antibodies, although four were positive for the pan-Ig anti-N assay (0.9%), a finding that suggests that the virus had not spread widely in Iceland before March 9.

Of the 18,609 persons tested for SARS-CoV-2 antibodies through contact with the Icelandic health care system for reasons other than Covid-19, 39 were positive for both pan-Ig antibody assays (estimated seroprevalence by weighting the sample on the basis of residence, sex, and 10-year age category, 0.3%; 95% CI, 0.2 to 0.4). There were regional differences in the percentages of qPCR-positive persons across Iceland that were roughly proportional to the percentage of people quarantined (Table S6). However, after exclusion of the qPCR-positive and quarantined persons, the percentage of persons who tested positive for SARS-CoV-2 antibodies did not correlate with the percentage of those who tested positive by qPCR. The estimated seroprevalence in the random sample collection from Reykjavik (0.4%; 95% CI, 0.3 to 0.6) was similar to that in the Health Care group (0.3%; 95% CI, 0.2 to 0.4) (Table S6).

We calculate that 0.5% of the residents of Iceland have tested positive with qPCR. The 2.3% with SARS-CoV-2 seroconversion among persons in quarantine extrapolates to 0.1% of Icelandic residents. On the basis of this finding and the seroprevalence from the Health Care group, we estimate that 0.9% (95% CI, 0.8 to 0.9) of the population of Iceland has been infected by SARS-CoV-2. Approximately 56% of all SARS-CoV-2 infections were therefore diagnosed by qPCR, 14% occurred in quarantine without having been diagnosed with qPCR, and the remaining 30% of infections occurred outside quarantine and were not detected by qPCR.

Deaths from Covid-19 in Iceland

In Iceland, 10 deaths have been attributed to Covid-19, which corresponds to 3 deaths per 100,000 nationwide. Among the qPCR-positive cases, 0.6% (95% CI, 0.3 to 1.0) were fatal. Using the 0.9% prevalence of SARS-CoV-2 infection in Iceland as the denominator, however, we calculate an infection fatality risk of 0.3% (95% CI, 0.2 to 0.6). Stratified by age, the infection fatality risk was substantially lower in those 70 years old or younger (0.1%; 95% CI, 0.0 to 0.3) than in those over 70 years of age (4.4%; 95% CI, 1.9 to 8.4) (Table S7).

Age, Sex, Clinical Characteristics, and Antibody Levels

Association of Existing Conditions and Covid-19 Severity with SARS-CoV-2 Antibody Levels among Recovered Persons.

SARS-CoV-2 antibody levels were higher in older people and in those who were hospitalized (Table 4, and Table S8 [described in Supplementary Appendix 1 and available in Supplementary Appendix 2]). Pan-Ig anti–S1-RBD and IgA anti-S1 levels were lower in female persons. Of the preexisting conditions, and after adjustment for multiple testing, we found that body-mass index, smoking status, and use of antiinflammatory medication were associated with SARS-CoV-2 antibody levels. Body-mass index correlated positively with antibody levels; smokers and users of antiinflammatory medication had lower antibody levels. With respect to clinical characteristics, antibody levels were most strongly associated with hospitalization and clinical severity, followed by clinical symptoms such as fever, maximum temperature reading, cough, and loss of appetite. Severity of these individual symptoms, with the exception of loss of energy, was associated with higher antibody levels.

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