Mucous membranes of the nose and mouth might play a "significant role" in curbing the spread of COVID-19, say scientists who call for more studies to evaluate the role played by this arm of the immune system in asymptomatic and mild states of the coronavirus infection.
The analysis, published in the journal Frontiers in Immunology, noted that the mucosal immune system is the largest component of immunity, but 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 Michael W. Russell, a co-author 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.
Frequently Asked Questions
A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.
There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.
Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.
He said the initial focus of COVID-19 research was on cases of severe disease when the virus descends into the lower respiratory tract, especially the lungs.
In the lungs, the scientists said 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, they said the immune responses that are triggered there are of special interest.
The researchers believe the high rate of asymptomatic transmission of COVID-19 is another reason why mucosal immunity is so important.
"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.
The scientists called for more studies to determine the nature of mucosal secretory immunoglobulin A (SIgA) antibody responses over the course of infection -- including those which are asymptomatic or pre-symptomatic -- on both mild and moderate cases of COVID-19 disease.
They added that the mucosal immune responses may vary across different age groups and populations.
A focus on mucosal immunity might also make it possible to develop a type of vaccine, such as a nasal vaccine, that could be easier to store, transport and administer, the scientists noted.
Russell said these vaccines might not have special temperature requirements and might be more palatable for large swaths of the population, especially children, because they would not require an injection.
"The potential advantage of a mucosal vaccine is that it should induce immune responses, including SIgA antibodies, in the mucosal tracts, in this case especially the upper respiratory tract, where the coronavirus makes first contact," explained Russell, adding that injected vaccines usually do not do this.
The researchers believe more molecular studies on IgA antibodies and their relationship to the disease stage of COVID-19 could shed more light on the unknowns.
"As mucosal immunologists with several decades of experience behind us, we have been perturbed at the lack of attention to this, and we hope to draw attention to this glaring omission," said Russell.
"After all, the mucosal immune system is by far the largest component of the entire immune system, and it has evolved to protect the mucosal surfaces where the great majority of infections arise," he added.Follow our full coverage of the coronavirus pandemic here.