COVID-19 vaccination increases saliva antibody levels after asymptomatic infection

In a recent study posted to the medRxiv* preprint server, researchers at Ohio State University monitored individuals with asymptomatic coronavirus disease 2019 (COVID-19) in a large university in the United States between January 2021 and May 2022. The correlation between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) load, which varies according to each viral variant, and neutralizing capacity of all saliva immunoglobulins (Ig) isotypes was assessed.

Study: The emergence of SARS-CoV-2 lineages and associated antibody responses among asymptomatic individuals in a large university community. Image Credit: Andrey_Popov / Shutterstock.com

Background

B-cells in mucosa-associated lymphoid tissue (MALT) and those residing in nearby lymph nodes of the upper respiratory tract (URT) and oral cavity secrete IgM, IgA, and IgG for several days of following exposure to an antigen.

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These antibodies differ in their effector mechanism. For example, IgM appears first and uses a complement pathway to eliminate viruses.

However, in the case of SARS-CoV-2, IgA appears first at mucosal sites and sterically hinders antigen interaction with the epithelial surface. This antibody subsequently traps the attacking pathogen in the mucus and clears it through peristalsis.

During the immune response to SARS-CoV-2, IgG appears last but exhibits the most robust effector mechanism and highest durability. Moreover, B-cells producing IgG often persist in bone marrow-resident plasma cells to continually secrete this antibody for several months.

All therapeutic and vaccination strategies against SARS-CoV-2 have been developed based on the knowledge that these antibodies disrupt SARS-CoV-2 spike (S) and angiotensin-converting enzyme (ACE2) interactions. In fact, prior to mass testing for COVID-19 through the reverse-transcription polymerase chain reaction (RT-PCR) assay, anti-SARS-CoV-2-Ig in sera was the only biomarker for monitoring its prevalence at the population level.

The continuous emergence of new SARS-CoV-2 variants with multiple and diverse S mutations threatens the efficacy of all currently used COVID-19 therapies and vaccines. More than ever, it is crucial to better understand infection or vaccine-induced Ig levels and their inhibitory capacity. In this regard, monitoring PCR^POS asymptomatic individuals could provide critical insights, as they account for up to 65% of all COVID-19 cases.

About the study

In the present study, researchers at the Ohio State University developed a campus-wide plan to monitor the incidence of SARS-CoV-2 infection among its students, staff, and faculty. The control group comprised individuals testing negative for COVID-19 via RT-PCR, referred to as PCR^NEG; however, these individuals were previously diagnosed with COVID-19 or received the original COVID-19 vaccine. Therefore, the researchers also assessed SARS-CoV-2-specific Ig responses in these individuals.

Saliva samples were collected using a passive drool method from all individuals self-reporting asymptomatic cases of COVID-19 across all six Ohio university campuses. These samples were assessed for the presence of SARS-CoV-2 using quantitative RT-PCR (qRT-PCR).

Throughout this monitoring program, over 850,000 diagnostic RT-PCR tests were performed. A sample was considered PCR^POS if it had a cycle threshold (C_T) value less than or equal to 40.

Between January 2021, and June 2022, 11,989 PCR^POS individuals were identified. During six time periods, new PCR^POS case counts rose above the average overall period for three weeks in a row, which the researchers referred to as Waves 1 to 6.

Next-generation whole genome sequencing was used to identify the infecting SARS-CoV-2 variant. Subsequently, these data were correlated with Global Initiative on Sharing Avian Influenza Data (GISAID) reference sequences and subsequently submitted to GISAID in real-time.

Saliva antibody levels reflect immune status against SARS-CoV-2 variants

COVID-19 vaccines were found to generate adequate mucosal antibodies among people on the university campus when community viral loads were low. However, breakthrough Delta infections were caused by limited S binding activity by vaccine-elicited Ig when community viral loads surged.

The saliva of asymptomatic individuals had elevated SARS-CoV-2-specific IgM, IgA, and IgG, the latter of which was to a lesser extent, at the time of initial RT-PCR positivity monitoring. The degree of these antibody levels varied by SARS-CoV-2 lineage, with Delta being the most immunogenic of all lineages.

The saliva levels of SARS-CoV-2-specific IgG were markedly higher in Prior^POS individuals. Furthermore, most IGs were anti-S and its receptor-binding domain (RBD), with fewer antibodies targeting the nucleocapsid (N) protein. Saliva IgG^S levels were comparable between Delta-infected and uninfected vaccinees; however, other saliva antibody levels varied between these groups.

The saliva Ig response represents what occurs in the URT. Thus, weak neutralization capacity and higher SARS-CoV-2 viral loads during the Delta-dominant period also facilitated breakthrough infections.

The observation that the Delta variant was more concentrated in the saliva of asymptomatic individuals was consistent with previous studies reporting that those infected by Delta were more likely to transmit the virus before developing symptoms.

Nevertheless, the university community experienced the largest COVID-19 wave due to the Omicron variant, which caused rapid breakthrough infections in the highly vaccinated university population. The Ohio university testing program also detected Omicron subvariants BA.1 and BA.2 among the community.

The SARS-CoV-2 Omicron variant consists of over 30 amino acid substitutions in its S protein that enable it to bind to ACE2 with higher affinity. This may contribute to the ability of Omicron to evade anti-S-neutralizing antibodies elicited by infection or vaccination.

In the current study, Omicron infections caused less severe illness than Delta, which may be due to its higher presence in the URT, including the nasopharyngeal and oral cavities. Thus, antibodies in saliva could better inhibit Omicron transmission than antibodies in the blood plasma.

Conclusions

The study findings indicate that the Ig analysis of saliva samples, which are easier to collect than blood, is a rapid and efficient method to determine whether some individuals may have neutralizing antibodies against future SARS-CoV-2 variants.

Future studies should actively investigate the extent to which antibody-dependent enhancement (ADE) occurs after SARS-CoV-2 infection. To date, mouse and macaque models have not provided sufficient information on vaccine-enhanced disease (VED). Furthermore, monoclonal antibodies produced by B-cells from convalescent COVID-19 patients have not been shown to enhance SARS-CoV-2 infection in animal models.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.