Saliva-based testing as a method for monitoring population immunity to SARS-CoV-2 infection

By Sally Robertson, B.Sc.Mar 16 2021

Researchers in the United States have shown that saliva-based sampling could serve as an accurate and non-invasive alternative to blood sampling for monitoring people’s antibody responses following infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or vaccination against the virus.

The SARS-CoV-2 virus is the agent responsible for the coronavirus disease 2019 (COVID-19) pandemic that continues to pose a threat to global public health and has now caused more than 2.66 million deaths worldwide.

Christopher Heaney from Johns Hopkins University in Baltimore and colleagues assessed the durability of immunoglobulin G (IgG) antibodies against three SARS-CoV-2 antigens in saliva samples collected from more than 200 people up to 8 months after they tested positive for the virus.

The team found that the estimated half-life for IgG responses against the antigens was consistent with those previously reported for blood samples.

The results also demonstrate that IgG responses against two of the SARS-CoV-2 antigens are durable in saliva for up to 8 months following symptom onset.

“Saliva can serve as an alternative to blood to monitor humoral immune responses on a large scale following SARS-CoV-2 infection and vaccination for surveillance and assessment of population immunity,” write the researchers.  

A pre-print version of the research paper is available on the medRxiv* server, while the article undergoes peer review.

Study: Durability of SARS-CoV-2-specific IgG responses in saliva for up to 8 months after infection. Image Credit: JHDT Productions / Shutterstock

*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Robust techniques are needed to assess population immunity to SARS-CoV-2

Robust methods for the large-scale tracking of antibody responses following SARS-CoV-2 infection or vaccination against the virus are critical for accurate surveillance and assessment of population immunity.

Heaney and colleagues previously developed and validated a multiplex bead-based immunoassay for the detection of antibody responses against three SARS-CoV-2 antigens in saliva. The assay measures IgG responses specific to the nucleocapsid protein (involved in packaging of the viral genome), the spike protein (required for host cell binding), and the spike receptor-binding domain (RBD) – the main target of neutralizing antibodies following infection or vaccination.

What did the current study involve?

Now, the researchers have applied this assay to 531 saliva samples taken from 341 individuals up to 8 months after they tested PCR-positive for SARS-CoV-2 to evaluate the durability of IgG responses against the antigens.

Heaney and colleagues hypothesized that the durability of the antigen-specific IgGs in saliva would exceed at least six months and be comparable to those reported in blood.

The team found that the antigen-specific antibody levels peaked at around 30 days post symptom onset, after which decay of the IgGs was assessed in 268 saliva samples taken from 237 people.

Half-life estimates for salivary IgGs were comparable to those reported for blood

The estimated IgG half-life was 64 days for the nucleocapsid protein, 100 days for the RBD, and 148 days for the spike protein.

The researchers say these half-life estimates for the salivary IgGs are comparable to those previously reported for plasma IgGs: 67 days for the nucleocapsid protein, 83 days for the RBD, and 140 days for the spike.

The majority of saliva samples from individuals who developed COVID-19 were positive for IgGs specific to the nucleocapsid protein (92%), the RBD (95%), and the spike (93%) between 1 and 6 months after symptom onset.

The proportion of saliva samples that tested positive for these IgGs between 6 and 8 months after symptom onset was 32% for the nucleocapsid protein, 68% for the RBD and 59% for the spike.

“These results demonstrate that SARS-COV-2 RBD and spike IgG are durable in saliva for up to 8  months post symptom onset,” writes the team.

When saliva samples were tested for a combination of all three antigen-specific IgGs, positivity for SARS-CoV-2-specific IgG was 98% between 1 and 6 months after symptom onset and 73% between 6 and 8 months after onset.

Heaney and colleagues say this finding indicates that the saliva of most people with COVID-19 is positive for SARS-CoV-2-specific IgG up to 8 months following symptom onset and that incorporating the IgG response to multiple SARS-CoV-2 antigens can improve the sensitivity of saliva-based antibody testing.

SARS-CoV-2-specific IgG responses in saliva over time. (a) Log10 median fluorescence intensity (MFI) of SARS-CoV-2-specific IgG responses to nucleocapsid (N), receptor binding domain (RBD), spike (S), and Σ[S/CO] values among n=531 saliva samples (from n=341 COVID-19 cases) over time. The solid black line represents temporal kinetics (estimated by loess spline). Dashed blue line represents the estimated slope with half-life (t1/2) and 95% confidence interval using a linear fit model (n=268 saliva samples from 237 cases). Dashed gray lines indicate cut-off values. Hollow orange and gray circles represent saliva samples classified as positive or negative, respectively. (b) Heat map detailing the proportion of saliva samples classified as positive by days post symptom onset. Note. Log10 data is shown for all plots; Σ[S/CO], sum of signal to cut-off; NAC, Native Antigen Company; Gen, GenScript; Sino, SinoBiological.

The findings support the use of saliva-based testing for large-scale surveillance

“Overall, these results support the utility of saliva as an accurate and non-invasive alternative to blood for longitudinal SARS-CoV-2 antibody testing and large-scale surveillance,” says Heaney and the team.

“Ideally, this analysis would be repeated in a large, longitudinal sample set should it come available,” they advise.

*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.