A recent study published in the European Journal of Immunology indicates a high seroprevalence of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) among individuals working in a Paris-based institute. However, the study findings indicate that the antibody response has a short half-life.
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Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative pathogen of coronavirus disease 2019 (COVID-19), has emerged first in China in December 2019. Since then, the virus has spread exponentially across the world and so far, caused more than 60 million infections globally.
In Paris, the trajectory of the COVID-19 pandemic peaked in March/April, and a national-wide lockdown has been announced by the French government to curb the viral spread. However, to date, the exact prevalence of SARS-CoV-2 infection is not properly known for the French population.
Among various diagnostic methods, polymerase chain reaction (PCR)-based assays, which determine the presence of viral RNA in respiratory samples, provide more accurate information about the present status of SARS-CoV-2 infection. In contrast, serological antibody assays determine the presence of SARS-CoV-2-specific antibodies in the blood, and thus, provide information about past infections. Hence, estimating the seroprevalence of SARS-CoV-2-specific antibodies is considered to be a more efficient approach to monitor the actual viral spread in a given population.
In the current study, the scientists conducted high-throughput assays to determine the seroprevalence of viral spike protein- and nucleoprotein-specific antibodies in individuals working in a Paris Institute.
Current study design
The scientists conducted the study on 1847 participants who work at an oncology hospital and research center located in 3 different regions (Paris, Saint Cloud, and Orsay). The study participants who live in the Paris conurbation were marginally in contact with COVID-19 patients; hence, representing an urban population of healthy adults.
Two bioluminescence-based serological assays were developed by the scientists for the assessment of IgG-specific antibodies against the viral spike protein and nucleoprotein. Moreover, the virus-neutralizing ability of antibodies was assessed using a pseudo-neutralization test.
Important observations
In the study population, the seroprevalence of SARS-CoV-2-specific IgG antibodies was 11%. About 9.5% of all serum samples tested showed neutralizing ability against SARS-CoV-2. Considering the seropositivity of individual assays, a seroprevalence of 11.6% was observed (215 of 1847 participants).
The web-based survey conducted by the scientists revealed that about 54% of participants had at least one symptom, and the seropositivity was higher in symptomatic participants than asymptomatic participants. Symptomatic participants exhibited higher levels of anti-nucleoprotein antibodies than asymptomatic participants.
However, no difference was observed for anti-spike protein antibodies and neutralizing antibodies, suggesting that mild SARS-CoV-2 infection may trigger the production of nucleoprotein-specific antibodies.
About 99% of all participants who tested positive in PCR assays were symptomatic; however, only 44% of them were seropositive. Interestingly, in 5% of PCR-positive participants, no IgG antibodies were detected in the blood. This indicates that having an active infection does not always specify the presence of systemic antibodies.
Two COVID-19 related symptoms, anosmia (loss of smell) and ageusia (loss of taste), occurred in 52% and 3% of seropositive and seronegative participants, respectively. In contrast, about 30% of participants with anosmia/ageusia showed seronegative results. Moreover, a total of 48 seronegative participants with anosmia/ageusia also exhibited characteristic COVID-19 symptoms. This indicates that an additional 48 infected participants are present in the study population who are seronegative.
By assuming that the incidence of anosmia/ageusia is similar in seropositive and seronegative participants, the scientists believe that the actual prevalence of SARS-CoV-2 infection would be 16.6% (215 seropositive participants + 48 additional SARS-CoV-2 positive participants), which is higher than the estimated prevalence (11.6%; 215 seropositive participants).
The scientists collected the second set of blood samples 4-8 weeks after the first one to evaluate the long-term prevalence of the antibody response. They observed a sharp reduction in antibody levels and virus-neutralizing ability. Specifically, they estimated that the half-lives of anti-nucleoprotein IgG, anti-spike protein IgG, and neutralizing antibodies were 35, 87, and 28 days, respectively. This indicates that the timing of the test is an important factor for serological estimation of infection prevalence.
Taken together, the study findings indicate a high seroprevalence (11.6% - 16.6%) of SARS-CoV-2 antibodies in a population that represents urban healthy adults. Importantly, the short lifespan of SARS-CoV-specific antibodies observed in the study indicates that serological antibody assays conducted long after the infection onset can provide an underrated prevalence.
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