Salivary IgG antibody response against SARS-CoV-2 differs between vaccinated children and adults

Studies comparing antibody responses against SARS-CoV-2 between children and adults show conflicting results in blood and are limited in the mucosa. Furthermore, as the results of studies comparing systemic and salivary immune responses are inconsistent, it is unclear whether determination of salivary antibodies against SARS-CoV-2 could be a non-invasive approach to evaluate the humoral immune response.

In this work, by studying and comparing systemic and salivary IgG antibody responses in vaccinated adults, we observed that the salivary immune response against SARS-CoV-2 in vaccinated adults largely reflects that observed at systemic levels. Higher salivary and systemic antibody concentrations were observed in adults who had schedules that included mRNA-based vaccines, a greater number of exposures, a shorter interval time between last exposure and saliva collection and had been exposed to SARS-CoV-2. Detection of salivary antibodies was associated with schedules that included mRNA-based vaccines, time between last exposure and sample collection, and systemic antibody concentrations. This suggests that salivary antibody detection might be compromised in subjects with lower systemic antibody levels.

Comparison of salivary antibody levels between vaccinated children and adults showed a stronger salivary antibody response against SARS-CoV-2 in vaccinated children. This also remained when antibody levels were compared between children and adults who had similar vaccination schedules or the same number of exposures or interval time between last exposure and saliva collection. A multivariable linear regression analysis confirmed these results showing that age, symptomatic exposure, number of vaccine doses and schedules that included mRNA-based vaccines associated with salivary anti-SARS-CoV-2 IgG antibody levels.

Determination of salivary antibodies against SARS-CoV-2 could be a non-invasive approach to evaluate the short-term immune response in children and adults with multiple exposures, especially in settings where blood sampling cannot be fulfilled. The higher levels of salivary anti-SARS-CoV- 2 IgG antibodies observed in children align with studies reporting stronger systemic antibody responses in children, suggesting their adaptive immune responses might be more efficient both locally and systemically. Further studies are required to characterize the salivary microenvironment for a better understanding of the difference observed in the local humoral immune response between children and adults.