Advocating mucosal immunization: A global need in a viewpoint from China
Human mucosal immunization is expected to afford protection against infection and reduce transmission by generating anti-infective immunity at the mucosal entry site of viruses and bacteria. Nasal or oral administration has the advantage of being needle free and self-administered, thereby improving compliance and coverage of large populations. In China, the experience of COVID-19 has promoted substantial efforts in the development of nasal vaccinations in the general health protection strategy. The hurdles we are facing in the development of mucosal vaccines, however, come from the still limited knowledge of the mechanisms controlling mucosal immunity in different anatomical locations and in response to different pathogens/vaccines. Identifying and filling the knowledge gaps in order to develop effective and safe mucosal immunization strategies requires global collaboration, not only at the scientific level but, most importantly, by engaging public and private health organizations, governments, and regulatory authorities. We have highlighted here some of the crucial issues in mucosal immunization and provided suggestions for the way forward toward a global preparedness effort to prevent infectious diseases and ensure vaccine equity.
Introduction: Why do we need mucosal vaccines?
Vaccination is an effective tool for preventing infectious diseases and is also becoming particularly important because of new health emergencies caused by antimicrobial resistance and pandemic infections.
The first immunization practice in the world was by a mucosal route, the old and widely used variolation in China, dating back over 1,000 years, performed by blowing powdered smallpox scabs into the nostrils. The vaccines currently on the market are, however, almost all injectable and are based on the activation of systemic immunity. The application of new technologies (mRNA and adenoviral vectors) was fast forwarded by the global pandemic emergency of SARS-CoV-2. Vaccines were generated worldwide using both new and conventional technologies (e.g., inactivated and protein subunit vaccines) and rapidly approved for emergency use by regulatory authorities. All first-generation COVID-19 vaccines were administered via intramuscular injection and were effective against severe disease, but none were effective in preventing infection and transmission. Thus, the World Health Organization (WHO) established a new target product profile for COVID-19 vaccines that included the desirability of mucosal vaccines able to block infection and transmission.
Mucosal immunization has the advantage of easy administration, even self-administration at home. Avoiding the use of needles also increases acceptance and decreases vaccine hesitancy. The WHO, the Gates Foundation, and the Program for Appropriate Technology in Health (PATH) are all pushing for the development of mucosal immunization strategies that can be applied in a rapid fashion to a large population in the effort for preparedness and use in less developed countries. In China, about 50 companies have vaccines on the market, and several of them have started the development of mucosal vaccines. China needs to protect a very large population distributed across different geographical and economic/social contexts, and mucosal vaccines may become the preferred goal for domestic vaccination programs. Problems and difficulties in developing highly effective and fully safe mucosal vaccines and immunization protocols come from the fact that immunologists are still engaged in attaining a better understanding of the peculiarities of mucosal immunity.
