Researchers in the US have developed an inhalable, self-administered COVID-19 vaccine that is shelf-stable at room temperature for up to three months.

The team found that the delivery mechanism for this vaccine a lung-derived exosome called LSC-Exo is more effective at evading the lung's lining than the lipid-based nanoparticles currently in use. Exosomes are extracellular vesicles generated by all cells and they carry nucleic acids, proteins, lipids, and metabolites.

The research paper, published in the journal Nature Biomedical Engineering, describes the development of the vaccine prototype from proof-of-concept to experimental model studies, which can be used effectively with protein-based vaccines.

"An inhalable vaccine will confer both mucosal and systemic immunity, it is more convenient to store and distribute, and could be self-administered on a large scale," said Ke Cheng from the University of North Carolina at Chapel Hill, US.

"So while there are still challenges associated with scaling up production, we believe that this is a promising vaccine worthy of further research and development," he added. There are several challenges associated with vaccine delivery that the researchers wanted to address.

First, taking the vaccine via intramuscular shot is less efficient at getting it into the lung system, and so can limit its efficacy, the researchers said; adding that inhaled vaccines would increase their benefit against COVID-19. Second, mRNA vaccines in their current formulation require cold storage and trained medical personnel to deliver them, they said.

A vaccine that is stable at room temperature and that could be self-administered would greatly reduce wait times for patients as well as stress on the medical profession during a pandemic, according to the researchers. However, reformulating the delivery mechanism is necessary for it to work through inhalation, they said.

The researchers created and tested an inhalable, protein-based, virus-like particle (VLP) vaccine by decorating the exterior of LSC-Exo with a portion of the spike protein -- known as the receptor-binding domain, or RBD -- from the SARS-CoV-2 virus. RBD is a short fragment from a virus that binds to a specific receptor to gain entry into host cells.

"Vaccines can work through various means. For example, mRNA vaccines deliver a script to your cell that instructs it to produce antibodies to the spike protein (which helps the virus to gain entry into a cell), Cheng said.

"This VLP vaccine, on the other hand, introduces a portion of the spike protein to the body, triggering the immune system to produce antibodies to the spike protein, the scientist added.