Common vaccines are synonymous with injections and grant you systemic immunity. But the injection isn’t the only way to develop immunity. And considering how most pathogens, including coronaviruses, get into our bodies, injections might not even be the best way.
SARS-CoV-2 often enters through the nose, where it encounters a specific protein, ACE2, which is found in abundance in the nasal passage. ACE2 is the virus’s doorway into our cells. In fact, the mucosal membrane that lines your airways is often the frontier of your body to face SARS-CoV-2. Immune cells reside underneath your mucosal membranes, creating a front line of defense against invaders and preventing infection from taking root. Your mucosal immune cells produce a special class of antibodies, immunoglobulin A (IgA), that are constantly secreted from the mucosae to protect the nose, gut, and other vulnerable sites from pathogens we’ve seen in the past. This is the mucosal immune system and it is hard to study. Secreted IgA is tricky to measure. “It is a lot easier to measure things from the blood than it is to measure from mucosa,” says Michael Barry, who has developed an experimental intranasal vaccine for COVID-19 at the Mayo Clinic. To determine how much IgA an animal has produced after vaccination typically requires killing the animal to wash IgA off its lungs. In humans, IgA can be measured by collecting nasal fluid or saliva, but the IgA levels will vary depending on how the sample is collected.