Researchers have stressed that more Covid-19 studies should be devoted to how immunity to coronavirus emerges in the mucous membranes of the nose and mouth.

Anyone who has undergone a nasal swab or saliva test for Covid-19 knows that the virus is most easily detected in the nose and mouth, the study, published in the journal Frontiers in Immunology, reported.

Noting that the mucosal immune system is the immune system’s largest component, the researchers expressed concern that it hasn’t been a focus of much of the research on Covid-19 to date.

“We think it is a serious omission to ignore the mucosal immune response to SARS-CoV-2, given its initial sites of infection,” said study author Michael W Russell from the University at Buffalo in the US.

“Clearly the response of the systemic immunoglobulin G antibody (the most abundant circulating antibody) is important — we do not deny that — but on its own it is insufficient,” Russell added.

Russell noted that naturally, the initial focus of research on the disease was on cases of severe disease when the virus descends into the lower respiratory tract, especially the lungs, where the cellular immune responses exacerbate the inflammation rather than fight the infection.

But since the upper respiratory tract, including the nose, tonsils and adenoids are the initial point of infection for the SARS-CoV-2 virus, the immune responses that are triggered there are of special interest.

“Given that many infected people remain asymptomatic, and that a large number of those who develop symptoms suffer only mild to moderate disease, this suggests that something, somewhere, does a fairly good job of controlling the virus,” said Russell.

“Could it be that this is due to early mucosal immune responses that succeed in containing and eliminating the infection before it becomes serious? We will not know unless these questions are addressed,” he asked.

The paper recommends that studies are needed to determine the nature of mucosal secretory immunoglobulin A (SIgA) antibody responses over the course of infection, including asymptomatic or pre-symptomatic infection, and mild and moderate cases of Covid-19 disease.

In addition, the authors point out that the mucosal immune responses may vary depending on different age groups and populations.