Introduction
Outbreak pneumonia announced in Wuhan, China, in December 2019, had its causative factor classified as Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). The current CoronaVIrus Disease (COVID-19) pandemic is developing rapidly into a dramatically devastating public health crises in recent history. By April 2021, reported cases exceeded 147 million worldwide, with at least 3,144,381 deaths and 110.27 million people recovered. Molecular assays on nasopharyngeal swabs act as the confirmatory test in COVID-19 diagnosis. Despite massive efforts had been made, the high technicalities of nasopharyngeal sampling and molecular assays limit the testing capabilities [1]. The positive rate of RT-PCR RNA detection was 63% in nasal swabs and only 32% in pharyngeal swabs [2]. Serological assays play an important supporting role in COVID-19 clinical diagnosis. Generally, IgM and IgG-based assays are the gold standard for serological diagnosis in COVID-19 [3]. Nowadays, SARS-CoV-2 S1 and N antigens have been detected in the serum of SARS-CoV-infected patients [4], which might identify active infection and monitor disease progression in COVID-19 patients.
Currently, nasopharyngeal swabs are main recommended upper respiratory tract specimen types for COVID-19 diagnostic testing, while the use of saliva for the diagnosis of the disease has been recently suggested [5,6]. Saliva specimens could be conveniently obtained by telling patients to spit into a sterile container, which is non-painful and non-stressful for patients [7]. Also, the collection of saliva is non-invasive and greatly minimizes the exposure of healthcare workers to COVID-19. Some researchers have concluded that detection of SARS-CoV-2 salivary antibodies could serve as a non-invasive alternative to serological testing for monitoring of SARS-CoV-2 infection and seropositivity at population scale [8]. Saliva is secreted by salivary glands which is characteristic in abundant IgA. Usually, salivary IgG and IgM concentrations are much lower than in serum [9]. It has been hypothesized that both salivary IgG and IgM are derived from blood, whereas IgA is mainly produced by the salivary glands [10].
In a recent COVID-19 research, salivary IgA was associated with the presence of pneumonia, but not associated with serum immunoglobulins [11]. This might illuminate that salivary IgA was independent from serum immunoglobulins. In this study, we measured saliva specimens and serum specimens from 44 COVID-19 patients and 24 negative-control patients. The associations between saliva and serum immunoglobulins were described and the potential of saliva IgA in COVID-19 diagnosis was assessed.