The emerging COVID-19 pandemic caused by SARS-CoV-2 infection has created a global crisis. Under the circumference of no effective treatment or vaccine, the Chinese government has implemented multifaceted measures of social distancing, home isolation, and centralized quarantine, which achieved a remarkable result of controlling the COVID-19 outbreak [
1]. However, the personal, psychological, economic, and societal consequences of the shutdown and physical distancing make it difficult to sustain these public health interventions for a long time [
2]. To find a new balance between curbing the pandemic and minimizing the indirect effects on society, a better understanding of adaptive immunity in response to SARS-CoV-2 infection is required. Monitoring B cell and T cell immunological memory activated by SARS-CoV-2 over a prolonged period is essential in anticipating durable protection after infection and in developing vaccines. If maintained at sufficiently high levels, the immune response could effectively block re-infection, which might confer long-lived protection [
3,
4]. Even though, the case report of re-infection with completely different SARS-CoV-2 strains from the first episode [
5] raised widespread public concern for the “immune passport” and virus mutation. Despite the urgent need to answer these crucial scientific questions, limited studies have systemically evaluated the long-term humoral and cellular immunity.
Therefore, the study by Tan
et al. [
6] has great importance in filling the knowledge gap (Table 1 provides summaries of studies on the dynamics of antibody response after SARS-CoV-2 infection). The study reported that the IgG antibody of 17 COVID-19 patients were detectable at 6
-7 months after diagnosis, although the concentrations were slightly lower compared to results in the early 2 weeks to 2 months. This is the longest observation of antibody dynamics to our best of knowledge so far. Another novel observation from this study was that 14 samples showed durable neutralizing activities in a pseudovirus assay, with no difference in blocking the cell-entry of the 614D and 614G variants of SARS-CoV-2 [
6]. Moreover, the study [
6] provided compelling evidence that both interferon
g-producing CD4
+ and CD8
+ T cells were increased in response to SARS-CoV-2 antigen stimulation as compared with non-stimulated samples at 6
-7 months post-infection. Taken together, this study has provided the most updated evidence for the persistence of humoral and cellular immunity over a relatively longer period, and susceptibility to second infection for mutant coronavirus among convalescent patients. As the level of neutralizing antibodies against the SARS-CoV-2 spike protein (to block viral entry) is the key to evaluate the protection against re-infection, these findings would inform therapeutic strategies and guide public health intervention.
The study by Tan
et al. [
6] has its limitation of small sample size and not using the plasma samples from the same patients in different periods. A recent serological study in Iceland [
7] evaluated the longitudinal changes in antibody levels among 487 recovered patients with two or more serum samples and found that the antiviral antibodies against SARS-CoV-2 did not decline within 4 months after diagnosis by RNA test [
7]. On the contrary, some reports [
8–
10] observed decay in IgG or neutralizing antibodies among the recovered patients during 2–3 months post-infection, particularly among the asymptomatic participants [
9,
10]. The potential interpretations for the disparity may include the inherent difference of humoral immune responses for asymptomatic and symptomatic infections [
11] (time course and duration), and the relatively short observation period of antibody dynamics (typically less than 4 months) in prior investigations [
8]. The majority of the plasmablasts (B cells) are short-lived, which may contribute to the decay of antibody levels after the acute phase of infection [
4]. Meanwhile, the effector CD8
+ T cell response exhibited a similar pattern as the B cell [
3,
4]. After the early decay phase, serological memory is maintained by a smaller number of longer-lived plasma cells that provide sustained immunity in the absence of antigen [
3,
4]. Therefore, samples collected during the early recovery phases may reflect a transient waning process [
3]. By contrast, observations over a prolonged period would lead to more accurate modeling of the immune response [
3]. The findings would ease the public apprehension regarding the decline of neutralizing antibody levels and the possibility of subsequent infection. However, it remains largely unclear how long the antibodies will persist at the needed protection threshold, and more studies are still needed.
The outbreak of COVID-19 in China during early 2020 was mainly caused by the 614D SARS-CoV-2 variant. However, the emerging 614G SARS-CoV-2 variant occurs more frequently among the recently infected individuals, which was reported to be more infectious than the original 614D variant
in vitro and animal studies [
12]. In Tan
et al.’s study, although most convalescent patients were infected by the 614D variant, the neutralizing activities in blocking the cell-entry of the 614D and 614G variants at 6 months post-infection showed no significant differences [
6]. The results indicated that vaccines developed for the 614D variant might also confer protection for the 614G variant. B cells and T cells both play an important role in the immune protection against SARS-CoV-2. The antibodies are generated by B cells, which would recognize and bind to the coronavirus and prevent cell entry. On the other hand, T cells mainly function as a target for killing the infected cells [
13,
14]. Most existing literature regarding immune protection against SARS-CoV-2 discussed the sustainability of antibody response [
7,
15–
18], whereas less evidence were available for T cells durability [
14,
19]. Tan
et al.’s study highlighted that interferon
g-producing T cells (CD4
+ and CD8
+ cells) were increased in response to SARS-CoV-2 antigen stimulation as compared to non-stimulated samples during 6–7 months post-infection [
6]. The finding has important implications for ongoing vaccine development.
Elucidating the persistence of humoral and cellular immunity would provide us a powerful tool for modeling individual immune protection and developing vaccine distribution plans. However, many of the critical variables of the immune response (e.g., neutralizing potency for mutant coronavirus and minimally needed protection threshold) still remain largely unknown. Although the whole world is now waiting for the results from phase 3 vaccine trials, the study by Tan
et al. [
6] has its unique contribution to help understand the durability of the antibody levels from nature infection. More studies with larger samples size and repeated measures are urgently needed to validate the findings.
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