Age and disease severity:
There was no correlation between age and disease severity (k = 0.09) or neutralizing antibodies and disease severity (k = -0,0951), nor could we find significant predominance of severe cases in older age groups or male patients (p = 0.53).
The mean age of severe cases was 46 (sd 15.1, n = 5) versus 49 years (sd 16.8; p = 0.36, not significant) in cases of other severity categories. All patients with severe infections had neutralizing antibodies at T1. However, only 2 (40%) developed constantly neutralizing antibodies, and 60% in this group showed low titers (<1:8) 5 months post infection.
DISCUSSION
Specific neutralizing antibodies were detected in 90.0% of 34 laboratory confirmed cases of SARS-CoV-2 infected patients in the early convalescent phase ~21 days post infection and stayed constant in 85.3% of the patients in the convalescent stage five months later, confirming recently published data of high prevalence of neutralizing antibodies in most SARS-CoV-2 afflicted individuals
In our study, neutralization activity was correlated with disease severity. 86.7% of the symptomatic patients (21 of 31) as well as 66.7% of the asymptomatic patients (1 of 3) maintained neutralizing antibodies over the observation period of 5 months. Investigations of IgG levels and neutralizing antibodies in early phase of convalescence gave a similar result (. 60% of the study members who suffered severe infections (3/5) developed low neutralizing antibody titers, which may allow the speculation that impaired immune responses may contribute to severe disease manifestation or an unevently observed B cell lymphopenia in COVID-19 (Melenotte et al., 2020; Ni et al., 2020; Q. X. Long et al., 2020; Wu et al., 2020; Qin et al., 2020). Although, concomitant treatments e. g. with glucocorticoids or other immunosuppressants recommended for COVID-19 may mitigate the generation of an efficient humoral immune response (Bamoulid et al., 2015; Thaunat et al., 2016).
In accordance with another study , the IgG levels in the symptomatic group (n=31) were significantly higher than those in the asymptomatic group (n=3; p = 0.008).
Two patients were negative in the early stage of the convalescent phase four weeks post infection (T1), one - a patient with a mild course of disease - stayed negative in all tests, whereas another one with an asymptomatic infection developed a weak neutralization response in the late convalescent phase (T5). This also fits well with published data, where milder courses of disease may cause longer periods to generate specific antibodies and in a low number of cases, patients did not seroconvert at all after infection with SARS-CoV-2
A recently published study (Wajnberg et al., 2020) investigated the persistence of neutralizing antibodies and is in accordance with our results of relatively stable maintenance of neutralizing antibodies in convalescent COVID-19 patients over a study period of 5 months.
The comparison of different serologic methods shows that the IgG CLIA reflects the results of the neutralization assay well with concordances of 95%, giving overall 5.9% false-positive and 3% false-negative results. The discrepancies between the two methods may be found in the coating with Spike S1 and S2, but the Receptor Binding Domaine (RBD), which is expressed in the NT results, is of great significance, too. It is also discussed by others (Suthar et al., 2020).
Many commercially available SARS-CoV-2 ELISAs are coated with Spike 1, 2 or both, but neutralizing antibodies are directed against the RBD or other targets and more unspecific antibodies tend to decline in the convalescent phase of an infection (Suthar et al., 2020). A high ELNA titer does not necessarily mean a high CLIA titer and vice versa.
The CLIA IgG is a fast and easy to handle diagnostic tool, which is known to have limited specificity. The high number of positives at the first time point might be due to the low specificity of the method and suggests a high degree auf cross-reactivity, presumably with endemic nonSARS-hCoVs like OC43.
Our results show that a high proportion of convalescent SARS-CoV-2 afflicted patients maintain constant neutralizing antibody titers at least five months post-infection, high enough to give hope for a protective and longer-lasting immunity after wild-type SARS-CoV-2 infection, which may also be relevant to estimate the long-term efficacy of a vaccination. Furthermore, our data reveal that individuals who have recovered from symptomatic COVID-19 generated more robust neutralizing antibody responses than those with asymptomatic infections.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support of the Österreichische Forschungsförderungsgesellschaft (FFG), grant no. 35863961. The authors report no potential conflict of interest. We also thank DDI Martin Lamprecht, Stefanie Sonnleitner and Mag. Eva Hinterbichler for excellent technical assistance and valuable suggestions.
ETHICAL STATEMENT
The authors confirm that the ethical policies of the journal, as noted on the journal’s author guidelines page, have been adhered to and the appropriate ethical review committee approval has been received (Ethics Committee of the University Hospital Wuerzburg no. 20201105_01).
CONFLICT OF INTEREST
The authors have no conflict of interest to declare.
REFERENCES
Table 1: Comparison of different serologic methods for antibody detection in 34 COVID-19 patients one respectively five months post infection.