Vaccines are marvelous, and there are many well documented success stories, including rotavirus (RV) vaccines, where a live vaccine is administered to the gastrointestinal mucosa via oral drops. Antigens presented at the mucosal/epithelial surface not only induce systemic serum IgG – as do injectable vaccines – but also induce secretory IgA (sIgA), which is most helpful in diseases that directly affect the mucosa.
Mucosal vs. systemic immunity
Antibody being present on mucosal surfaces (point of initial pathogen contact) has a chance to neutralize the pathogen before it gains a foothold. Pathogen-specific mucosal lymphoid elements (e.g. in Peyer’s patches in the gut) also appear critical for optimal protection.1 The presence of both mucosal immune elements means that infection is severely limited or at times entirely prevented. So virus entering the GI tract causes minimal to no gut lining injury. Hence, there is no or mostly reduced vomiting/diarrhea. A downside of mucosally-administered live vaccines is that preexisting antibody to the vaccine antigens can reduce or block vaccine virus replication in the vaccinee, blunting or preventing protection. Note: Preexisting antibody also affects injectable live vaccines, such as the measles vaccine, similarly.
Classic injectable live or nonlive vaccines provide their most potent protection via systemic cellular responses antibody and/or antibodies in serum and extracellular fluid (ECF) where IgG and IgM are in highest concentrations. So even successful injectable vaccines still allow mucosal infection to start but then intercept further spread and prevent most of the downstream damage (think pertussis) or neutralize an infection-generated toxin (pertussis or tetanus). It usually is only after infection-induced damage occurs that systemic IgG and IgM gain better access to respiratory epithelial surfaces, but still only at a fraction of circulating concentrations. Indeed, pertussis vaccine–induced systemic immunity allows the pathogen to attack and replicate in/on host surface cells, causing toxin release and variable amounts of local mucosal injury/inflammation before vaccine-induced systemic immunity gains adequate access to the pathogen and/or to its toxin which may enter systemic circulation.
Live attenuated influenza vaccine (LAIV) induces mucosal immunity
Another “standard” vaccine that induces mucosal immunity – LAIV – was developed to improve on protection afforded by injectable influenza vaccines (IIVs), but LAIV has had hiccups in the United States. One example is several years of negligible protection against H1N1 disease. As long as LAIV’s vaccine strain had reasonably matched the circulating strains, LAIV worked at least as well as injectable influenza vaccine, and even offered some cross-protection against mildly mismatched strains. But after a number of years of LAIV use, vaccine effectiveness in the United States vs. H1N1 strains appeared to fade due to previously undetected but significant changes in the circulating H1N1 strain. The lesson is that mucosal immunity’s advantages are lost if too much change occurs in the pathogen target for sIgA and mucosally-associated lymphoid tissue cells (MALT)).
Other vaccines likely need to induce mucosal immunity
Protection at the mucosal level will likely be needed for success against norovirus, parainfluenza, respiratory syncytial virus (RSV), Neisseria gonorrhea, and chlamydia. Another helpful aspect of mucosal immunity is that immune cells and sIgA not only reside on the mucosa where the antigen was originally presented, but there is also a reasonable chance that these components will traffic to other mucosal surfaces.2
So intranasal vaccine could be expected to protect distant mucosal surfaces (urogenital, GI, and respiratory), leading to vaccine-induced systemic antibody plus mucosal immunity (sIGA and MALT responses) at each site.