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Letter to the Editor| Volume 83, ISSUE 5, P607-635, November 2021

Long-lasting immune response to a mild course of PCR-confirmed SARS-CoV-2 infection: A cohort study

Open AccessPublished:August 21, 2021DOI:https://doi.org/10.1016/j.jinf.2021.08.030
      Dear Editor
      We read with great interest the paper by Wells et al..
      • Wells P.M.
      • Doores K.J.
      • Couvreur S.
      • Nunez R.M.
      • Seow J.
      • Graham C.
      • et al.
      Estimates of the rate of infection and asymptomatic COVID-19 disease in a population sample from SE England.
      Wells et al. investigated seroprevalence of SARS-CoV-2 antibodies in a cohort of 431 non-hospitalized UK twins (mean age = 48.38, SD = 28; 85.1% female), who systematically reported the presence or absence of COVID-19 symptoms on many occasions through time via an ad-hoc study app. Their results indicated that 51 of the 431 individuals (11.8%) were seropositive with IgG response to both N and S proteins 4-fold above the background of the assay. Within the group of seropositive individuals with complete symptom data (n = 48), there were 35 participants (72.9%) with core symptoms (defined as anosmia, cough and fever) and 13 (27.1%) without core symptoms. Among these 13, 9 (18.7%) participants were fully asymptomatic. This finding is particularly relevant, as it shows that also participants with an asymptomatic course of the disease developed antibodies against SARS-CoV-2. While the longitudinal assessment of symptoms with the app allowed to track the clinical course of the disease, a major limitation of the study by Wells et al.
      • Wells P.M.
      • Doores K.J.
      • Couvreur S.
      • Nunez R.M.
      • Seow J.
      • Graham C.
      • et al.
      Estimates of the rate of infection and asymptomatic COVID-19 disease in a population sample from SE England.
      is that SARS-CoV-2 infection was not confirmed by real-time polymerase chain reaction (RT-PCR) at symptom onset. Therefore, the time interval between infection and sample collection was unknown. This is an important source of variability that likely affected the estimated percentage of seropositive individuals. Moreover, the cohort comprised related individuals (twins), whose immune response to exposure to SARS-CoV-2 was probably correlated to some extent.
      • Brodin P.
      • Jojic V.
      • Gao T.
      • Bhattacharya S.
      • Angel C.J.
      • Furman D.
      • et al.
      Variation in the human immune system is largely driven by non-heritable influences.
      It follows that observations were not independent and this probably affected results. Nevertheless, the idea of studying the immune response against SARS-CoV-2 in individuals who underwent a mild course of COVID-19 is of pivotal importance, because mild cases represent the most typical manifestation of this disease and many infections are even asymptomatic.
      • Tian S.
      • Hu N.
      • Lou J.
      • Chen K.
      • Kang X.
      • Xiang Z.
      • et al.
      Characteristics of COVID-19 infection in Beijing.
      ,
      • Wu Z.
      • McGoogan J.M.
      Characteristics of and important lessons from the coronavirus disease 2019 (COVID-19) outbreak in China: summary of a report of 72314 cases from the chinese center for disease control and prevention.
      Previous studies described the longitudinal stability of immunity against SARS-CoV-2 mostly in hospitalized COVID-19 patients with mixed clinical profiles (mild, moderate or severe symptoms), using different laboratory methods for the analysis of antibodies (see for example
      • Seow J.
      • Graham C.
      • Merrick B.
      • Acors S.
      • Pickering S.
      • Steel K.J.A.
      • et al.
      Longitudinal observation and decline of neutralizing antibody responses in the three months following SARS-CoV-2 infection in humans.
      • Secchi M.
      • Bazzigaluppi E.
      • Brigatti C.
      • Marzinotto I.
      • Tresoldi C.
      • Rovere-Querini P.
      • et al.
      COVID-19 survival associates with the immunoglobulin response to the SARS-CoV-2 spike receptor binding domain.
      • Long Q.X.
      • Liu B.Z.
      • Deng H.J.
      • Wu G.C.
      • Deng K.
      • Chen Y.K.
      • et al.
      Antibody responses to SARS-CoV-2 in patients with COVID-19.
      • Dehgani-Mobaraki P.
      • Kamber Zaidi A.
      • Porreca A.
      • Floridi A.
      • Floridi E.
      • Monti M.
      • et al.
      Antibody persistency and trend post-SARS-CoV-2 infection at eight months.
      • Dispinseri S.
      • Secchi M.
      • Pirillo M.F.
      • Tolazzi M.
      • Borghi M.
      • Brigatti C.
      • et al.
      Neutralizing antibody responses to SARS-CoV-2 in symptomatic COVID-19 is persistent and critical for survival.
      ). Therefore, knowledge about the longitudinal stability of the immune response in mild and asymptomatic SARS-CoV-2 infections is limited.
      We present recent findings from a cohort of COVID-19 convalescent individuals that partly replicate and extend the evidence by Wells et al.
      • Wells P.M.
      • Doores K.J.
      • Couvreur S.
      • Nunez R.M.
      • Seow J.
      • Graham C.
      • et al.
      Estimates of the rate of infection and asymptomatic COVID-19 disease in a population sample from SE England.
      We collected blood plasma samples to determine pan-IG antibody titre from a cohort of 326 non-hospitalized volunteers (median age = 42 years, IQR = 31–52; 61.7% females) tested positive for SARS-CoV-2 by RT-PCR between February 2020 and January 2021. Samples of serum, lithium heparin plasma, sodium citrate plasma, EDTA plasma and EDTA buffy coat were collected at the baseline visit after remission of SARS-CoV-2 infection (median = 66 days, IQR = 42.0–111.8 between infection and baseline visit) and 1, 2, 5 and 12 months after the baseline visit. Clinical symptoms of COVID-19, demographic characteristics, lifestyle, and comorbidities were collected by study physicians via an electronic questionnaire at the baseline visit. Antibody levels were determined with the Elecsys® Anti-SARS-CoV-2 (pan-Ig qualitative test against the viral nucleocapside protein, positive if ≥ 1 COI) and the Elecsys®Anti-SARS-CoV-2 S assays (pan-Ig quantitative test against the receptor-binding domain of the viral spike protein S1-ab, positive if ≥ 0.8 U/ml) (Roche Diagnostics, Rotkreuz, Switzerland). Data and sample collection for late timepoints is still ongoing. Samples collected after the date of first vaccination were censored.
      Overall, 300 (92.0%) participants experienced COVID-19 core symptoms (at least one among fever, cough, dyspnea, ageusia and anosmia during the course of the disease), 26 (8.0%) did not; 88 participants (27.0%) presented with comorbidities and they were equally distributed among individuals with/without core symptoms. A clinically relevant finding is the high proportion of individuals with long-lasting symptoms: After a median time of 38 days post infection (IQR = 14–84 days), 218 participants (66.9%) still had symptoms. The most common were fatigue (N = 134, 41.1%), dysgeusia (N = 77, 23.6%), headache (N = 76, 23.3%), anosmia (N = 65, 19.9%), nasal congestion (N = 56, 17.2%) and dyspnea (N = 54, 16.6%).
      Results of antibody tests revealed very high positivity rates (qualitative assay at baseline visit positive in 303 of 321
      1Five observations were censored due to vaccination already at baseline (visit 1).
      Five observations were censored due to vaccination already at baseline (visit 1).
      individuals, 94.4%; quantitative antibody assay at baseline visit positive in 310 of 320
      2The value of the quantitative antibody assay of the first visit was missing for one participant.
      The value of the quantitative antibody assay of the first visit was missing for one participant.
      individuals, 96.9%) and these increased through time, reaching 96.7% and 100% positivity rates by visit 4 for the qualitative and the quantitative assays, respectively.
      To account for variability in the time lag between the first PCR test and the baseline visit (visit 1), participants were subdivided into 2 groups: Individuals with baseline visit within 42 days since the first PCR test (early baseline visit group) and individuals with baseline visit occurring more than 42 days since the first PCR test (late baseline visit group). Table 1 shows median (IQR) of the quantitative antibody assay in the whole cohort (see also Fig. 1) and in the different subgroups. We modeled the longitudinal variation of quantitative antibody titres through time with linear mixed effects models, adding a random intercept and a random slope to account for the random variation of individual time trajectories. Results revealed a significant positive association between log-transformed antibody quantitative titre and time since the first PCR test (estimate = 1.07, p < 0.001). The effect of the baseline visit group was also significant: The late group generally had higher antibody levels than the early group (estimate = 1.38, p < 0.001). The presence of core symptoms (estimate = 0.52, p < 0.001) and of comorbidities (estimate = 0.30, p < 0.001) was related to higher antibody levels. The interaction between time and the early/late baseline visit group was significant (estimate = −0.61, p < 0.001), indicating that the increase in the antibody levels was steeper in the early vs. late baseline group.
      Table 1.Results of the quantitative antibody assay (U/ml) in the whole cohort and in each subgroup.
      Baseline (Visit 1)Visit 2Visit 3Visit 4
      NMedian (Q1, Q3)NMedian (Q1, Q3)NMedian (Q1, Q3)NMedian (Q1, Q3)
      Whole cohort320*48.7 (14.9, 161.0)24168.8 (27.7, 189.0)14694.7 (37.6, 311.5)90131.5 (51.3, 421.5)
      Participants with early baseline visit8411.4 (3.0, 35.8)7436.5 (12.8, 89.5)3537.3 (16.8, 95.8)242.2 (41.0, 43.5)
      Participants with late baseline visit23672.2 (27.5, 213.0)16789.2 (37.8, 300.0)111115.0 (49.9, 381.5)88132.5 (59.9, 437.0)
      Participants with core symptoms29650.0 (16.5, 173.8)22370.5 (29.2, 209.5)13794.9 (38.4, 314.0)86144.0 (50.0, 467.0)
      Participants without core symptoms2427.6 (2.1, 83.0)1845.8 (7.2, 83.9)956.1 (28.4, 95.7)476.5 (71.3, 90.5)
      Participants without comorbidities23539.4 (12.2, 111.5)17650.4 (22.1, 146.8)10777.1 (32.4, 250.0)61117.0 (42.7, 415.0)
      Participants with comorbidities8594.1 (42.6, 232.0)65122.0 (55.8, 319.0)39177.0 (80.3, 434.5)29221.0 (79.9, 579.0)
      Note. Visits 2, 3, 4 occurred 1, 2, 5 months after the baseline (visit 1), respectively. * Five observations were censored due to vaccination already at baseline (visit 1). The value of the quantitative antibody assay of the first visit was missing for one participant.
      Fig 1
      Fig. 1Kinetics of the antibody response as a function of time since the first positive PCR test. Antibody levels are shown on a log10-scale on the y-axis. The blue line (with a gray band for the 95% confidence interval) is the output of Local Polynomial Regression Fitting (LOESS). Please note that, to improve data visualization, one observation at day 360 was excluded from the plot.
      Taken together, our results indicate a strong and persistent immune response against SARS-CoV-2 infection in individuals who recovered from a mild course of COVID-19 for up to 8 months post infection. Importantly, our findings are in line with a recent study
      • L’Huillier A.G.
      • Meyer B.
      • Andrey D.O.
      • Arm-Vernez I.
      • Baggio S.
      • Didierlaurent A.
      • et al.
      Antibody persistence in the first 6 months following SARS-CoV-2 infection among hospital workers: a prospective longitudinal study.
      that used the very same quantitative anti-RBD antibody assay as we did and reported a robust and persistent antibody response after six months post infection. The titre at the baseline visit and its change through time was modulated by the time lag between infection and sampling. In line with Wells et al., we found that individuals without core symptoms developed immunity against COVID-19, although to a lower degree than individuals showing core symptoms, and it persisted through time.
      In the framework of this ongoing pandemic, our study highlights the importance of high quality research data that arise from patient and sample collections of biorepositories.

      Declaration of Competing Interest

      The authors declare no conflict of interest.

      Ethics approval

      The study was performed in accordance with the latest version of the Declaration of Helsinki and was approved by the local ethics committee of the Medical University of Graz (ethics vote: 32–423 ex 19/20) All participants signed a Biobank-informed consent and a study-specific informed consent.

      Funding

      This work was supported by the Austrian BMBFW project BBMRI.at [BMBFW 10.470/0010-V/3c/2018 (2018–2023)], cultural office of the city of Graz and MEFOgraz. Antibody test kits were kindly provided by Roche Diagnostics.

      Acknowledgments

      The samples/data used for this project have been provided by Biobank Graz of the Medical University of Graz, Austria. Cohort 5003_20, COVID-19 Convalescent Cohort.

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