Immunological changes in a cohort of COVID-19 survivors: Mansoura University experience

Introduction

COVID-19 is considered the most catastrophic pandemic in the 21^st century until now. Globally, as of 3 June 2022, 5:37pm CEST, there have been 528,816,317 confirmed cases of COVID-19, including 6,294,969 deaths, reported to WHO (WHO, 2022).

Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, initiates a humoral immune response that produces antibodies against specific viral antigens. These include anti-S protein antibodies that target the virus spike S1 protein subunit and receptor binding domain (RBD) (CDC, 2022).

Antibodies are detected in the blood of people who have been previously infected with a virus that causes a disease; they show the body’s efforts to fight off a specific virus. Antibodies may protect people from reinfection or becoming severely ill for some time afterward (Qu et al., 2020). However, antibodies wane over time and how quickly antibodies wane is different for each disease and each person.

The aim of this study was to identify the kinetics of IgM and IgG antibodies in a cohort of COVID-19 survivors over six months’ follow up and evaluate the longevity of immunity provided by COVID-19 infection.

Methods

Study participants

A convenience sample of 123 post-COVID-19 patients who attended the outpatient’s clinic were recruited. The follow up schedule was explained to them and they were free to leave the study at any time. Six participants were excluded because they refused follow up. Consequently, 117 post-COVID-19 participants were studied. Follow up was conducted at one month (first visit), three months (second visit) and six months (third visit) after recovery from COVID-19. Of 117 survivors who attended the first visit, only 98 attended the second visit and 76 completed the third visit (Figure 1). Demographics, comorbidities, severity of previous COVID-19 and laboratory data of the participants were recorded. Severity of previous COVID-19 infection was classified according to WHO severity definitions (WHO, 2021).

Figure 1. Flow chart of COVID 19 studied cases.

Results

Baseline characteristics of the cohort (117 post-COVID-19 participants) were as follow: mean age was 54 years old, about 54.7% of the participants were male, diabetes mellitus was detected in 38.5% while hypertension was detected in 34.5%. In addition, 46.2% and 47.9% of participants previously had non-severe and severe COVID-19 infection respectively (Underlying data (Khashaba, 2023)).

Of 117 participants, only 98 were enrolled in the second visit (three months post COVID-19) of which one IgM result was not recorded. Only 76 participants completed the third visit (six months post COVID-19) (Table 1).

Regarding the COVID-19 IgM antibody titer, there was significant reduction in IgM titer between the first and second visits (percentage change 68.8%; p<0.001). Moreover, a reduction in the IgM titer was observed between the second and third visit (percentage change 32.3%). After 6 months follow up (third visit) COVID-19 IgM titer declined by 78.8% from the first visit (Underlying data (Khashaba, 2023)).

A total of 88% (103 out of 117) of participants had a positive COVID-19 IgM in the first visit and after six months follow up 53.9% (41 out of 76) participants had a positive COVID-19 IgM and the difference was statistically significant (p<0.05) (Table 1).

For the COVID-19 IgG antibody titer, there was a significant reduction in IgG titer between the first and second visits (p<0.001). Also, a significant reduction in IgG titer was observed between the first and third visit. After six months follow up, at the third visit, the COVID-19 IgG titer had declined by 49.2% from the first visit (Table 1).

A total of 93.2% (109 out of 117) participants had positive COVID-19 IgG at the first visit and after six months follow up 60.5% (46 out of 76) of participants had positive COVID-19 IgG (Table 1).

The existence of a positive IgM test at the third visit was significantly associated with the age of the studied participants (mean age was 52 in participants with a positive test compared to 57.4 in participants with a negative test (p=0.04). Also, a positive test was more frequent among males than females (p=0.04). Moreover, positivity was significant among healthcare workers compared to non-healthcare workers (p=0.04). Although a non-significant association was detected between a positive IgM test and diabetes mellitus, a positive IgM test was frequent among non-diabetic participants (60.9% versus 43.3%). In addition, a positive IgM test was frequent among non-smokers and participants who had previously had severe and critical COVID-19 despite no statistical significance (Table 2).

On studying persistent IgG positivity at the third visit, it was slightly associated with older age of studied participants (mean age was 56.7 in participants with a positive test compared to 51.4 in participants with a negative test (p=0.05)). Also, a positive test was more frequent among males than females (p=0.008). A significant association was detected between a positive IgG test and diabetes mellitus as it was more frequent among diabetic than non-diabetic participants (83.3% versus 45.7%). Finally, a positive IgG test was more frequent among non-smokers, and participants previously having had severe and critical COVID-19 despite the absence of statistical significance (Table 3).

Based on logistic regression analysis, age, sex and occupation are significant predictors for positive IgM immunity; older age is associated with a decreased chance of positive IgM [OR (95%CI) 0.9 (0.8–0.9)]. However, males and health care workers had an increased chance of a positive IgM [OR (95%CI) 5.06 (1.6–16.2), 6.2 (1.0–35.5) respectively]. Regarding IgG immunity, males and diabetic participants had an increased chance of a positive IgG [OR (95%CI) [4.2 (1.4–12.3), 7.3 (2.2–24.3) respectively] (Table 4).

There was an overall reduction in the level of IgM from 10.9 units at one month to 3.4 units after three months and 2.3 units at six months (Figure 2). There was an overall reduction in the level of IgG from 6.7 units at one month to 4.3 units after three months and 3.4 units at six months (Figure 3).

Figure 2. IgM titer from one month to 6 months’ post COVID-19.

Figure 3. IgG titer from one month to 6 months’ post COVID-19.

Discussion

Understanding the persistence of neutralising antibodies in COVID-19 survivors is essential for developing effective vaccination programmes and COVID-19 pandemic control measures. Here, we looked into COVID-19 survivors’ natural neutralising antibody survival rates (Greaney et al., 2021; Sette and Crotty, 2021). We studied the changes of IgM and IgG to the spike protein of SARS-CoV-2 in 117 post COVID-19 participants. There was significant reduction in IgM and IgG antibody titer over time. By the end of six months follow up, IgM and IgG titer declined by 70.5% and 46.8% from the first visit respectively. Forty-one out of 76 participants remained IgM positive by the end of the sixth month. When relating this positivity with different factors, there was significant association with younger mean age of 52 years old and male sex. Although not statistically significant, there was persistently more positive IgM in non-smokers, non-diabetics, non-hypertensive and in more severe and critical cases. Forty-six out of 76 cases remained IgG positive by the end of the sixth month. Positive IgG immunity is found more frequently and statistically significant in older participants, male sex and diabetics. Although not statistically significant, there was persistently more positive IgG in non-smokers, hypertensive, and in more severe and critical cases. Based on logistic regression analysis, age, sex and occupation are significant predictors for positive IgM immunity; older age is associated with a decreased chance of positive IgM.

The persistence of IgG antibodies in diabetics compared to non-diabetic participants may be explained by more severe disease in diabetic patients. This is supported by several studies which stated that ICU patients showed a faster and increased neutralising antibody response compared to non-ICU patients (Lau et al., 2021; Liu et al., 2020; Qu et al., 2020; Williamson et al., 2020). The association between serum IgA, IgG, and IgM levels and glycated haemoglobin, an indicator of long-term diabetes control, fructosamine, and 111 healthy non-diabetic people was examined in 110 diabetic patients. While the concentration of IgM was considerably lower (by 46.7%, p<0.001) in diabetic patients compared to non-diabetic participants, there were significant increases in serum IgA (by 82.7%, p 0.001) and IgG (by 35.2%, p 0.001) concentrations (Ardawi et al., 1994).

The anti-SARS-CoV-2 antibody titers were statistically higher in older compared to younger participants (Lau et al., 2021; Ozgocer et al., 2022) and in men compared to women (Jin et al., 2020; Klein et al., 2020; Ozgocer et al., 2022). Indeed, clinical outcomes showed that males experience both a higher severity and fatality for COVID-19 infection than females (Mukherjee and Pahan, 2021; Peckham et al., 2020; Qu et al., 2020).

During the period of follow up, none of the studied participants had documented reinfection with COVID-19, which may be explained by protection provided by neutralizing antibodies. Although the onset of detection and duration of persistence of neutralizing IgM and IgG antibodies differ among studies, they were detectable in sera of most COVID-19 survivors (Alzaabi et al., 2021; Hou et al., 2020; Iyer et al., 2020; Lee et al., 2010; Li et al., 2003; Ogega et al., 2021; Petersen et al., 2021; Qu et al., 2020; Sariol and Perlman, 2020).

What amount of the neutralising antibody response could provide protection against infection or re-infection to properly time the vaccination strategy to sustain antibody-mediated protection against SARS-CoV-2 is an important question that must be answered by future studies. Although COVID-19 convalescents or vaccination recipients experienced re-infection, symptomatic re-infections and severe illnesses happened less frequently than in initial infections (Butt et al., 2021; Hacisuleyman et al., 2021; Leidi et al., 2022) and this is the rationale for studying antibody response against SARS-CoV-2 and implementation of vaccination strategies.

Our study was limited by a small number of participants, and we cannot detect a cutoff value for protecting antibody level. Further studies on larger numbers of participants are required to detect the level of the neutralizing antibody needed to protect against re-infection to correctly schedule the vaccination plan.

Conclusions

We concluded that the titer of neutralising IgM and IgG antibodies against COVID-19 virus waned over time until they became negative. In our study, we found a significant reduction in IgM and IgG titers over a period of six months follow up. Regarding younger age, male sex and health care workers, statistically significant persistence of IgM titer was noticed at six months follow up. Also, statistically significant persistent of IgG immunity was found in males and diabetic participants by the end of the six months follow up.

Consent

Written informed consent for publication of the participants’ details was obtained from the participants.