In this case control study of individuals with confirmed COVID-19 during the period September 2020, we found that 80% had reactive IgG antibodies to SARS-CoV-2 at one month follow-up. Male sex and Ct value ≤ 20 were associated with having reactive antibodies.
We found that nearly 80% of individuals had reactive antibodies to SARs-CoV-2 one month after recovery from COVID-19. By comparison, a study in Iceland, encompassing over a thousand qPCR-confirmed COVID-19 cases, reported that 90% of individuals as seropositive for anti-SARS-CoV-2 antibodies twenty-five days after their diagnosis (5). Another study reported a robust IgG seropositive rate of 92.3% within the first month after symptom onset, maintaining high levels during follow-up for the subsequent five months (22). Furthermore, a study conducted in Italy, revealed a 90% seroconversion rate two months post-recovery in unvaccinated adult COVID-19-affected individuals across all severity levels (23). Additionally, another investigation demonstrated up to a 97% seropositive rate among individuals studied after a median of 67 days since symptom onset (24). Our findings suggest a lower prevalence of antibody reactivity in this setting, perhaps due to the younger age of our participants, as younger age has been associated with a weaker acquired immunity to COVID-19 (25).
The male gender was significantly associated with 3-fold higher odds of reactive humoral immunity at the 1-month follow-up visit post-symptomatic onset compared to their female convalescent counterparts. This observation aligns with the outcomes of a longitudinal study focusing on the kinetics of IgM and IgG antibodies against SARS-CoV-2, which reported substantially higher persistence of IgG and IgM SARS-CoV-2 antibody titres in male patients (26). Additionally, a study in England examining SARS-CoV-2 antibody levels in 330 convalescent plasma donors found males to exhibit significantly higher antibody titres (27). These findings are noteworthy, considering that, in many infectious diseases, females typically manifest stronger immune responses than males (28). For example, elevated levels of 17β‑estradiol in females have been demonstrated to enhance humoral immunity and stimulate somatic hypermutation and class switch recombination in B cells by upregulating activation-induced deaminase (29). One explanation for our finding is that elevated antibody levels might be reflective of the heightened severity of SARS-CoV-2 infection in males, thereby stimulating B cell recruitment and subsequent antibody production (30). However, our analysis showed that the higher odds of reactive antibodies persisted even after accounting for symptomatic severity. The underlying factors influencing this observed gender-based difference in antibody responses remain unclear and warrant further exploration in future research endeavours.
We found that a Ct value ≤ 20 was associated with 2-fold higher odds of reactive antibodies, although with weak evidence against the null hypothesis, shown by the p-value. First, the weaker p-value can be explained by the lower power in this study given a highly prevalent outcome. Second, our finding of a two-fold higher odds of antibody reactivity is similar to a 12-month longitudinal investigation where an inverse relationship with peak N-IgG levels was observed, coupled with a marginally significant positive correlation with the cycle threshold value of RT-PCR (31). Our findings, taken together with existing literature, imply that a higher viral load at presentation may induce an augmented reactive humoral immune response.
Our study did not identify a significant association between BCG vaccination status and reactive humoral immunity. This is similar to findings from a multi-center, double-blinded, randomized controlled trial where only a borderline difference in IgG fold change between the BCG and placebo groups was also noted (32). In contrast, a multicentre randomized controlled trial, intracutaneous BCG vaccination demonstrated an association with stronger antibody responses following COVID-19 infection in elderly individuals aged over 60 (33). An epidemiological study, that examined countries with varying degrees of COVID-19 prevalence, revealed that nations adopting BCG vaccination to combat childhood TB may potentially experience cross-protection against COVID-19. The results suggested that countries like the United States, Italy, Spain, and France, where BCG vaccination for children has either ceased or is not universally implemented, demonstrate higher incidences of COVID-19 cases and fatalities compared to those with a comprehensive BCG immunization program. However, our results and data from other studies have shown that this could simply be ecological fallacy, with data showing that high numbers of cases and deaths were noted in China, the initial epicentre of the disease outbreak in December 2019, and India despite having a universal BCG strategy (39, 40). Further, many randomized controlled trials have since shown that either BCG vaccination or revaccination does not have benefit in acquired immunity to COVID-19 (34, 35), thereby concluding this debate, especially given that highly efficacious vaccines against COVID-19 are now available.
In this study, using the WHO symptomatic classification, there was no association with severe COVID-19 symptoms and reactive humoral response. On the contrary, a study showed that in more severe COVID-19 cases (those admitted to Intensive care unit ICU facilities, and those who died), the IgM, IgG, and IgA antibodies produced were higher and remained high as compared to outpatient COVID-19 cases and inpatient-non-ICU cases (36). This was further supported in another study where detectable antibodies (total immunoglobulin, IgG and IgA antibodies) were observed more in severe COVID-19 infection (hospitalized participants) as compared to mild infection (non-hospitalized participants) (37). Other studies showed similar results (38, 39). These results could be partly explained by the elevated viral load in severe disease, which allows for prolonged antigen presentation and more robust antibody response (40). Another study hypothesized that the increase in IgG levels observed in severe disease is due to the immune system ineffectively halting viral replication in cells likely due to defective IFN-I response, and as such, the immune system creates a burst of IgG antibodies to attempt to counteract this problem, which further exacerbates the inflammatory response and lessens the protective effects (39).
The study's limitations include the restricted assessment of antibody reactivity, limited to a single follow-up visit at 1-month post-symptomatic onset, impedes our ability to explore the temporal dynamics of reactive humoral immunity over an extended time period thereby limiting insights into sustained protective efficacy against reinfection. Conversely, a notable strength of the study is the inclusion of patients across all symptomatic severities, enhancing the generalizability of the results.