Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is very infectious, primarily infecting the respiratory tract mucosal surfaces (Gorbalenya et al., 2020; Zhu et al., 2020; Hu et al., 2021). Mucosal immunity in the upper airways and nasal passages is particularly important as the first defensive barrier, affecting the initial viral spread (Mazanec et al., 1995; See et al., 2006). Among antibody isotypes, secretory immunoglobulin A (sIgA) at mucosal surfaces plays a crucial role in protecting against respiratory virus infection (Mazanec et al., 1995; Terauchi et al., 2018). A previous study by Sterlin et al. (2021) reported that both serum IgA and mucosal IgA could effectively neutralize SARS-CoV-2 and dominated the neutralizing antibody response to SARS-CoV-2 in the early phase of infection. Recent studies have reported that dimeric IgA, the secretory form of IgA in the mucosa, was a more potent neutralizer than IgG and serum IgA monomers against authentic SARS-CoV-2 (Ejemel et al., 2020; Wang et al., 2021). These results suggest that SARS-CoV-2 induces specific sIgA and strong mucosal immunity within the respiratory system effectively against virus infection.

It was already known that vaccines delivered by inhalation can elicit IgA response in both mucosal surfaces and serum, whereas vaccines delivered intramuscularly primarily elicit serum IgG (Torrieri-Dramard et al., 2011; Ku et al., 2021; Lund and Randall, 2021). van Doremalen et al. (2021) recently reported that intranasal ChAdOx1 nCoV-19/AZD1222 vaccination, an approved adenovirus-vectored vaccine, reduced shedding of SARS-CoV-2 from the upper respiratory tract in vaccinated macaques and hamsters, whereas intramuscular vaccination protected against lung inflammation and pathology but did not reduce shedding. A similar result was also reported by Hassan et al. (2020) adenovirus-vectored vaccine ChAd-SARS-CoV-2-S delivered by inhalation could provide more effective sterilizing protection than intramuscular delivery, and promote systemic and mucosal IgA response when intramuscular delivery failed to induce IgA.

Although IgA response to SARS-CoV-2 in respiratory tract is particularly important in neutralizing the virus and affecting the initial viral spread, long time longitudinal studies of mucosal IgA kinetics were relatively rarely reported. In this study, we established an indirect ELISA method for IgA detection using nucleocapsid protein (NP) and detected COVID-19 samples of patients collected at different time points within 6 months and throat swabs of 55 volunteers who have received at least one dose of inactivated SARS-CoV-2 vaccines. Our results showed the kinetics of sIgA to SARS-CoV-2 in sputum and throat swabs and non-secretory IgA in the blood sample, which could improve our understanding of the mucosal immune response of the virus and provide new ideas for immunological evaluation of pandemic prevention and control.

As a mucosal targeted virus, SARS-CoV-2 can generate sIgA and induce strong mucosal immunity (Arakawa et al., 2019; Yu et al., 2020). Our study reports the kinetics of sIgA response to SARS-CoV-2 in the mucosa of the respiratory system and non-secretory IgA in the blood. We found that IgA could be detected at the early stage of virus infection in sputum and throat swabs, and some patients even on the first day after the onset. The median conversion time for IgA in sputum and throat swabs was less than 4 days, 6–7 days earlier than in serum. Moreover, IgA in sputum could be all detected among patients within 1 week after the onset of the disease. Other studies have shown that the first seroconversion day of IgM and IgG in serum was 5 days after onset. The median conversion time for IgM and IgG was 14 and 14–15 days, respectively (Wang et al., 2020; Yu et al., 2020). The proportion of patients with positive virus-specific IgG reached 100%, approximately 17–19 days after symptom onset, while the proportion of patients with positive virus-specific IgM reached a peak of 94.1%, approximately 20–22 days after symptom onset (Long et al., 2020; Zhou et al., 2021). Our study demonstrated that IgA response to SARS-CoV-2 in the respiratory mucosa was much earlier than IgG and IgM. Other studies have reported that mucosal IgA had more efficient neutralization potential than IgG and was dominant in early SARS-CoV-2 neutralizing antibody (Sterlin et al., 2021; Wang et al., 2021). These results indicated particularly the important role of mucosal sIgA against the initial SARS-CoV-2 viral spread.

Meantime, our results suggested that there is a particular value of mucosal sIgA detection in COVID-19 diagnosis. Our SARS-CoV-2 NP IgA ELISA method showed excellent specificity in the detection of SARS-CoV-2-infected patients. There was no cross-reaction when it was used to detect other non-SARS-CoV-2-infected patients, such as HB, HC, HIV, UF, or H1 influenza. Our IgA ELISA assay could be useful when the suspected cases are repeatedly negative by RT-qPCR in the diagnosis of acute phase. Detection of sIgA can aid diagnosis as soon as possible, which is important for effective intervention and isolation of patients to prevent further spread of infection.

Moreover, according to our results, sIgA, compared with non-secretory IgA, would rapidly turn negative during the recovery period. NP IgA in the sputum and throat swab of the patient had turned negative partially at 1 month after onset and more than half by 2 months after onset. A previous study of Isho et al. (2020) detected anti-spike and anti-RBD IgA levels in the saliva of the patient, and the results also showed significant decreases from 1 month and barely detectable IgA level on day 100. These results indicated that production of sIgA in the lung epithelium is time limited. Although reinfection cases of SARS-CoV-2 was very uncommon, it occurred in 1.39% health-care workers at a median of 7 months after the onset of the first episode in a large, multicenter, prospective cohort study (Hall et al., 2021). Another report reviewed 60 cases of reinfection with viral sequencing, and episodes of infection were separated by a median of 116 days (Qureshi et al., 2021). As one crucial part of mucosal immunity barrier against virus invasion, the short duration of sIgA and insufficient level at an indicated time may contribute to the occurrence of reinfection.

Given that intramuscular vaccine primarily elicits serum IgG and IgM, the role of IgM and IgG neutralizing antibodies attract most attention of scientists. However, SARS-CoV-2 viremia is associated with COVID-19 severity, and viremia in patients generally occurred in severe illness (Jacobs et al., 2021; Li et al., 2021). Elicited IgM and IgG antibodies seem to mainly protect vaccinated people from severe pathology. Their antiviral effect on respiratory mucosal epithelial cells is not as important as secreted IgA, or it even has not much effect (Mazanec et al., 1995). Our study suggested that there is insufficient mucosal immunity and IgA response in respiratory tract in vaccinated people with intramuscular delivery. Despite various intramuscular delivery vaccines have shown high efficacy against SARS-CoV-2 infection, rare breakthrough infections have been reported, and most of them were mild or asymptomatic (Bergwerk et al., 2021). This may also be a problem that current vaccinators have to consider because even asymptomatic infected people could spread the virus and bring great challenge to the society. Interestingly, recent studies of two adenovirus-vectored vaccines showed that intranasal vaccination could reduce shedding of virus and induce higher levels of neutralizing antibodies, particularly systemic and mucosal IgA than intramuscular delivery (Hassan et al., 2020; van Doremalen et al., 2021). Therefore, to further improve the immune effect of the vaccine, multiple dosage forms of vaccine immunization strategies (injection and spray) should be considered by scientists and government decision-making departments.

A limitation of our study is that our results do not directly imply the protection of mucosal immunity against SARS-CoV-2, given that the NP antibody is not a neutralizing antibody. NP protein is a suitable candidate for diagnosis due to its high immunogenicity. Previous studies have shown closely similar patterns between anti-NP antibodies and anti-spike and anti-RBD IgA/IgG responses (Isho et al., 2020; Sterlin et al., 2021). Our study of kinetics of sIgA suggested that anti-NP IgA detection of sputum and throat swabs may have some diagnostic value for pandemic prevention and control.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by the Ethics Committee of Anhui Medical University. The patients/participants provided their written informed consent to participate in this study.

CR, YG, and CZha carried out the experiment and drafted the manuscript. CZho, YH, MQ, ZZ, YD, LY, BL, and SW participated in the design and coordination of the study. YL, YS, and MH were the project coordinators, responsible for the project design, implementation, and oversaw all aspects of case definition, fieldwork, laboratory activities, and data analysis. All authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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