To characterize the epidemiological features including immunological response, viral transmission, and antibody seroconversion in asymptomatic SARS-CoV-2 carriers, we made a comprehensive comparison between asymptomatic carriers and COVID-19 patients in this study. Compared to symptomatic COVID-19 patients, asymptomatic SARS-CoV-2 carriers were younger and had higher levels of circulating WBC and lymphocyte and a lower level of CRP. These data indicate that asymptomatic carriers have a stronger antiviral immunity and a lower level of systemic inflammation. It has been proven that innate and adaptive lymphocytes and inflammatory factors were closely related to disease progression of COVID-19, from mild to severe [16, 17]. In a previous prospective study, we have demonstrated that lower circulating counts of T lymphocytes, CD4+ T cells, and CD8+ T cells as well as higher circulating levels of neutrophil proportion, neutrophil/lymphocyte ratio, interleukin-6, CRP, and procalcitonin facilitate the progression of COVID-19. Of those, CD8+ T cell exhaustion plays an important role in the pathogenesis of COVID-19 . Other studies have also shown that disease severity is negatively associated with NK cells and CD3+, CD4+, and CD8+ T lymphocyte levels, while intensive expansion of highly cytotoxic effector T cell subsets, such as CD4+ effector-granulysin, CD8+ effector-granulysin, and NKT CD160, is associated with convalescence of COVID-19 patients [19–21]. These evidences strongly indicate that damage of innate immunity and T cell-mediated immunity, which might be caused by proinflammatory factor-induced inflammation, play key roles in the development of COVID-19.
In this study, we also found that around 7% of asymptomatic carriers and COVID-19 patients during or after the incubation were seronegative for IgG against SARS-CoV-2, indicating SARS-CoV-2 infection might not induce sufficient humoral immunity against SARS-CoV-2. In the follow-up study, IgM seroconversion from positive to negative was much faster in asymptomatic carriers than in COVID-19 patients (P = 0.030). The overall rate of IgG seroconversion from positive to negative or weak positive was around 30% within 160 days after the diagnosis (Table 2), indicating that IgG against SARS-CoV-2 is not stable. Importantly, seroconversion of IgG against SARS-CoV-2 from positive to negative or weak positive occurred 53.4% in asymptomatic carriers and 25.5% in COVID-19 patients (P = 0.059), while consistently seropositive rate of IgG against SARS-CoV-2 was significantly higher in COVID-19 patients than in asymptomatic carriers (P = 0.037). The similar observations concerning rapid seroconversion of the antibody against SARS-CoV-2 or short-lived immune response after mild infection were also reported in the frontline health care personnel in the US and active workers in France [22, 23]. These data indicate that humoral immunity against SARS-CoV-2 was not efficiently aroused by a relative short exposure of SARS-CoV-2 in asymptomatic carriers or in those with a stronger innate and cell immunity. Long-term seropositive rate of antibody against SARS-CoV-2 in COVID-19 patients, which has been previously reported [24, 25], indicates the feasibility of antibody-generating vaccination in the worldwide prophylactic action. However, it might be possible that the antibody against SARS-CoV-2 take part in the pathogenic process of COVID-19.
Interestingly, compared to COVID-19 patients, asymptomatic carriers had a lower level of viral load and shorter viral shedding time (Table 1). Our finding is different from a study carried out in Chongqing that asymptomatic carriers had a significantly longer duration of viral shedding than the symptomatic patients , possibly because of small sample size. Lower viral load and shorter viral shedding duration in asymptomatic carriers should be unlikely caused by the neutralizing antibody, because the antibody, either IgM or IgG, was declining more rapidly in asymptomatic carriers than in COVID-19 patients. Innate immunity and cell-mediated immunity should play key roles in repressing viral replication in asymptomatic carriers . Lower viral load and shorter viral shedding time should be due to a relative stronger antiviral immunity, as a high viral load often predisposes adverse outcomes of COVID-19 [9, 28]. To develop effective vaccine against SARS-CoV-2, it is important to arouse the specific cell immunity, instead of focusing on humoral immunity.
In this study, we found that viral load increased from COVID-19 patients during the incubation to symptomatic COVID-19 patients (Table 1), indicating that infectivity is the highest at the stage of disease onset. Asymptomatic carriers had a lower level of viral load and shorter viral shedding duration, indicating that the transmissibility of asymptomatic carriers was relative weaker. In the 4 familial clusters, we found that asymptomatic carriers were mostly children and young adults, mild patients were young and middle-aged adults between 18 and 60 years, and severe cases were older than 60 years with underlying diseases. Family members were exposed to the same source of infection. However, they had diverse clinical manifestations: children were often asymptomatic whereas old members were very sick. This observation is quite in consistent with the findings of large epidemiological studies that children acquire SARS-CoV-2 infection mostly have mild respiratory symptoms or are asymptomatic, whereas elderly patients with COVID-19, especially male patients, are more likely to progress into severe-type and even die of this disease [29–33]. Thus, the host immunity and underlying inflammation, which is often affected by ageing, underlying diseases, and dysregulated macrophage response , should be the major determinants of disease severity of COVID-19. A family cluster occurred in Zhoushan is a suitable example to address this issue. Although asymptomatic carriers often acquire the infection from family members, they can transmit SARS-CoV-2 into family members and hospital centers, and eventually kill aged members. As a considerable percentage of SARS-CoV-2 infections may be asymptomatic or presymptomatic, enhanced testing approaches may be needed to detect those who transmit the virus.
Our study has some limitations. First, follow-up should be extended to observe the duration of SARS-CoV-2-specific antibodies. Second, sample size of asymptomatic carriers with SARS-CoV-2 infected was relatively small.