A COVID-19 vaccine designed to be taken orally not only protects against the disease but also decreases the airborne spread of the SARS-CoV-2 virus to other close contacts, according to a study.

The research, published in the journal Science Translational Medicine, demonstrates the potential of the vaccine to work through the mucosal tissue to neutralise SARS-CoV-2, limiting infections and the spread of the active virus in airborne particles.

"Considering most of the world is under-immunised -- and this is especially true of children -- the possibility that a vaccinated person with a breakthrough infection can spread COVID to unimmunised family or community members poses a public health risk," said Stephanie N Langel from Duke University Medical Center in the US.

"There would be a substantial benefit to developing vaccines that not only protect against disease but also reduce transmission to unvaccinated people," Langel said in a statement. The researchers -- including teams from the US vaccine developer, Vaxart, and clinical research non-profit, Lovelace Biomedical Research Institute -- tested the vaccine that uses an adenovirus as a vector to express the spike protein of the virus.

The spike protein is used by SARS-CoV-2 to enter and infect the human cells. The human vaccine is designed to be taken as a pill, they said. In studies using the experimental models, the vaccine elicited a robust antibody response in blood and the lungs.

When the experimental models were exposed to the SARS-CoV-2 virus at high levels, prompting breakthrough infections, they were less symptomatic than non-vaccinated ones and had lower amounts of infectious virus in the nose and lungs.

Because of this, they did not shed as much virus through normal airborne exposures, according to the researchers. Unlike vaccines that are injected into the muscle, they said, mucosal immunisations increase the production of immunoglobulin A (IgA) -- the immune system's first line of defence against pathogens -- in the nose and lungs.

These mucosal ports of entry are then protected, making it less likely that those who are vaccinated will transmit the infectious virus during a sneeze or cough, the researchers said. "Our data demonstrate that mucosal immunisation is a viable strategy to decrease the spread of COVID through airborne transmission," Langel said.

The researchers noted that the study focused on the original SARS-CoV-2 virus, and new studies will be designed to test the vaccine against Omicron variants.