To the Editor.
Since the SARS-CoV-2 emergence in December 2019, one of the major concerns is the
duration of immune protection after a first episode. This question is of paramount
importance for healthcare workers (HCWs) who are a highly exposed population and among
the first targets of vaccination programmes. To date, the persistence of SARS-CoV-2
antibodies in HCWs six months after disease onset (ADO) has not been studied with
both a virus neutralisation test and commercial assays.
HCWs who experienced COVID-19 during the early phase of the pandemic were included
in a prospective study conducted at the University Hospital of Lyon, France [1]. Serum
samples collected six months ADO were tested using three commercial assays: the Wantai
Ab assay that detects total antibodies against the receptor binding domain (RBD) of
the S protein, the bioMérieux Vidas assay that detects IgG to the RBD, and the Abbott
Architect assay that detects IgG to the N protein. The neutralising antibody (NAb)
titre was determined by a virus neutralisation assay (VNA) using live virus as previously
described [2].
A total of 296 HCWs were included; the median [interquartile range, IQR] age was 41
[32-51] years and 17.2% (51/296) were male. The median duration between symptom onset
and inclusion was 186 [180-196] days. Of note, 8/296 HCWs (2.7%) were asymptomatic
and the onset of disease was established on the basis of the median date of the RT-PCR
positive result of the ward cluster. All participants were tested positive for SARS-CoV-2
serology at least two weeks after disease onset. The SARS-CoV-2 infection was also
documented by RT-PCR test in 170 patients.
The positivity rate at six months ADO was 100% with the Wantai assay, 84.8% with the
Vidas assay, and 55.4% with the Architect assay. Only 51% of HCWs were positive for
the presence of NAb. Positive NAb titres ranged from 20 to 240. Only 27/296 (9.1%)
had a NAb ≥ 80 (Figure 1
A, raw data available in supplementary table). No difference in positivity rates with
any assay was observed between patients with a SARS-CoV-2 infection documented by
RT-PCR and the rest of the cohort.
Fig 1
A. Distribution of neutralisation antibody titres in convalescent subjects (n=296)
6 months after SARS-CoV-2 infection. B-D-F. Violin plots describing ODR according
to neutralising antibody titres. Dotted lines described positive threshold recommended
by each manufacturer. Comparisons was performed using the Kruskal Wallis test followed
by Dunn’s test. ***p<0.001, *p<0.05 C-E-G. ROC curves were built to estimate the performance
of Wantai (C), bioMérieux (E) and Abbott (G) assays for detecting the presence of
neutralising antibodies (PRNT50 ≥ 20-continuous line) and high neutralising antibody
titre (PRNT50 ≥ 80-dotted line). ODR-Optical Density Ratio, PRNT-Plaque Reduction
Neutralisation Titres.
Fig 1
Of the 296 HCWs, 6 (2.0%) developed a clinical form requiring hospitalisation; all
were positive with the three serological assays and for the presence of NAb with a
median titre of 40 (range: 30-160). By contrast, in asymptomatic HCWs, 8/8, 5/8, and
4/8 were positive with Wantai, Vidas, and Architect assays, respectively, and only
3/8 exhibited NAbs with low titres (range: 30-60).
The area under the ROC curve (AUC) was estimated for assessing the performance of
serological assays for two NAb titres (PRNT50 ≥ 20 or PRNT50 ≥ 80; (Figure 1C, E,
G). The highest AUCs were found with the Vidas assay: 0.85 (95% CI [0.81-0.89]) and
0.95 [0.92-0.97], respectively. The Wantai and Abbott assays had AUCs of, respectively,
0.73 [0.68-0.79] and 0.70 [0.64-0.76] for PRNT50 ≥ 20, and 0.71 [0.62-0.81], 0.75
[0.65-0.85] for PRNT50 ≥ 80. These results suggest that an optimised ratio with some
commercial serological assay could be found to maximize the positive predictive value
enabling to select individuals with a NAb titres ≥ 80. For instance, with the Vidas
assay, the median [IQR] ratio for samples with PRNT50 ≥ 80 was 15.4 [9.7-22.7] vs
5.9 [3.3-9.2] for samples with a titre between 20 and 80 and 1.8 [0.8-3.8] for samples
without NAb (Figure 1F). Among the 27 samples with NAb titre ≥ 80, all had a Vidas
ratio above 8 compared to 31.5% and 3.5% of the samples with a titre between 20 and
80 or without NAb, respectively.
The findings of the present study indicate that, six months after infection, NAbs
were no longer detected in about half of HCWs who presented mainly mild COVID-19.
Overall, the detection of SARS-CoV-2 Abs with commercial tests was higher despite
important heterogeneity between the assays evaluated herein. In a previous study [3],
about 40% of asymptomatic subjects became negative for IgG to the N protein within
3 to 6 months, which is consistent with that presented herein for the Architect assay.
This suggests that assays detecting only antibodies against the N protein must not
be used in long-term seroprevalence surveys. By contrast, the Wantai assay could be
very useful for epidemiological purposes as 100% of the HCWs were still positive at
6 months ADO. Although VNA should remain the gold standard to assess the protective
antibody response, the data presented herein suggest that some commercial assays could
be useful for first-line screening of long-term presence of NAb as previously reported
within 4 months ADO [2,4].
Despite these observations on the decrease of NAbs in patients with mild COVID-19,
it is important to note that they do not preclude the protective role of an anamnestic
antibody response in previously exposed subjects, nor that of the long-term cellular
immunity [5].
Ethics
Written informed consent was obtained from all participants; ethics approval was obtained
from the national review board for biomedical research in April 2020 (Comité de Protection
des Personnes Sud Méditerranée I, Marseille, France; ID RCB 2020-A00932-37), and the
study was registered on ClinicalTrials.gov (NCT04341142).
Conflict interests statement
Antonin Bal has received grant from bioMérieux and has served as consultant for bioMérieux
for work and research not related to this manuscript. Sophie Trouillet-Assant has
received research grant from bioMérieux concerning previous works not related to this
manuscript.
The other authors have no relevant affiliations or financial involvement with any
organisation or entity with a financial interest in or financial conflict with the
subject matter or materials discussed in the manuscript.