Little is known about the nature and durability of the humoral immune response to infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
We measured antibodies in serum samples from 30,576 persons in Iceland, using six assays (including two pan-immunoglobulin [pan-Ig] assays), and we determined that the appropriate measure of seropositivity was a positive result with both pan-Ig assays. We tested 2102 samples collected from 1237 persons up to 4 months after diagnosis by a quantitative polymerase-chain-reaction (qPCR) assay. We measured antibodies in 4222 quarantined persons who had been exposed to SARS-CoV-2 and in 23,452 persons not known to have been exposed.
Of the 1797 persons who had recovered from SARS-CoV-2 infection, 1107 of the 1215 who were tested (91.1%) were seropositive; antiviral antibody titers assayed by two pan-Ig assays increased during 2 months after diagnosis by qPCR and remained on a plateau for the remainder of the study. Of quarantined persons, 2.3% were seropositive; of those with unknown exposure, 0.3% were positive. We estimate that 0.9% of Icelanders were infected with SARS-CoV-2 and that the infection was fatal in 0.3%. We also estimate that 56% of all SARS-CoV-2 infections in Iceland had been diagnosed with qPCR, 14% had occurred in quarantined persons who had not been tested with qPCR (or who had not received a positive result, if tested), and 30% had occurred in persons outside quarantine and not tested with qPCR.
Our results indicate that antiviral antibodies against SARS-CoV-2 did not decline within 4 months after diagnosis. We estimate that the risk of death from infection was 0.3% and that 44% of persons infected with SARS-CoV-2 in Iceland were not diagnosed by qPCR.
We estimate that during the first wave of the SARS-CoV-2 pandemic, the incidence of infection in Iceland was 0.9% (95% CI, 0.8 to 0.9) and the infection fatality risk was 0.3% (95% CI, 0.2 to 0.6). Our estimate of the infection fatality risk is lower than but consistent with estimates described by others.6-8 We estimate that of the infected persons, 56% had cases previously diagnosed by qPCR, 14% had been in quarantine (but either had not been qPCR-tested or had tested negative), and 30% neither were known to be qPCR-positive nor had been placed in quarantine. We therefore conclude that, despite extensive screening by qPCR, a substantial fraction of infections were not detected, which indicates that many infected persons did not have substantial symptoms.
The case fatality risk is straightforward to estimate but may differ across countries and over time. An accurate calculation of infection fatality risk requires an accurate estimate of the number of infections, both diagnosed and undiagnosed. In Iceland, the high percentage of infections identified through qPCR (56%) as compared with that of other countries (for example, approximately 9% in Spain4) renders a commensurately accurate estimate of the total number of infections.
Each of the pan-Ig SARS-CoV-2 antibody assays that we used has high specificity (99.8%, according to the manufacturers’ literature), which raises the question of whether using a single pan-Ig assay would have sufficed. One sample obtained in 2017 was positive on only one pan-Ig antibody assay, a finding that supports the use of two separate assays to determine seroprevalence, if the infection rate is below 1%, as in Iceland.
By April 30, a total of 20,766 Icelanders had been placed in quarantine. Of the 1797 Icelanders who tested positive by qPCR, 1088 (61%) were in quarantine when tested. Despite substantial qPCR testing of persons in quarantine, 2.3% of persons in quarantine who did not receive qPCR-positive result (i.e., a diagnosis of infection) developed SARS-CoV-2 antibodies. Household exposure was more likely to lead to infection than other types of exposure, which suggests that people who share a household with an infected person should not have contact during quarantine and that contacts of household members should be quarantined. Seroprevalence in the two regional hot spots (Vestfirdir and Vestmannaeyjar) was absent or low outside quarantine, which indicates that most infections were detected by qPCR screening and that quarantine, social distancing, contact tracing, and limits on public gatherings were effective in limiting spread.
Over 90% of qPCR-positive persons tested positive with both pan-Ig SARS-CoV-2 antibody assays and remained seropositive 120 days after diagnosis, with no decrease of antibody levels as detected by the two pan-Ig assays. We observed some diminution of antibody titer with some of the single-Ig assays. Previous smaller studies reported reduction of IgG antibodies against the N protein and a peptide representing the S protein within 21 to 28 days5 and against trimeric S protein within 56 days14 after a positive test by qPCR. These discrepancies may be explained partly by differences in the specificity and sensitivity of the assays used as well as differences in the design and performance of the semiquantitative assays used, including the antigen targeted and the analytic sensitivity and range, as well as differences in the study populations. For example, because of widespread qPCR testing and screening, it is likely that the Icelandic qPCR-positive persons were healthy, as compared with the participants in other studies. Repeated SARS-CoV-2 exposure is unlikely to affect the persistence of antibody levels in Iceland, given the low prevalence of infection. Comparative studies using validated quantitative SARS-CoV-2 antibody assays are needed; those described in the published literature are based on small sample sizes.9-12
Of the 22 recovered persons who had a negative result (using the combined pan-Ig antibody tests) for an early sample and who had another sample tested at least a month later, 19 (86%) received a second negative result. Thus, either some persons infected by SARS-CoV-2 produce no antibodies or undetectable levels of antibodies reactive to the S1 and N proteins, even 3 months after infection, or some qPCR delivered false positive results.
Among recovered persons, antibody levels are higher in older persons and in those more severely affected by SARS-CoV-2 infection. Women, who tend to become less sick than men, had lower antibody levels in two spike protein antibody assays. SARS-CoV-2 antibody levels were lower in smokers. Smoking increases the probability of severe Covid-19 illness among young adults,15 and smoking has been reported to increase the expression of ACE2,16 the receptor for cellular entry of the SARS-CoV-2 virus.
The humoral immune response is critical for the clearance of cytopathic viruses and is generally important for the prevention of viral reinfection.17 A relationship between a humoral immune response to SARS-CoV-2 infection and protection against reinfection by this virus has been shown in rhesus macaques18 but has yet to be established in humans. Regardless of the relationship or lack thereof between seropositivity against SARS-CoV-2 and protection against reinfection, the low SARS-CoV-2 antibody seroprevalence in Iceland indicates that the Icelandic population is vulnerable to a second wave of infection.
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This article was published on September 1, 2020, at NEJM.org.