“Mucosal vaccines are the future of vaccines for respiratory infections,” asserted Jacco Boon, PhD, professor at Washington University School of Medicine in St. Louis. Following this prediction, Boon and his team have been working on developing a nasal vaccine for COVID-19. They presented data showing that nasal vaccines for respiratory illnesses both help reduce the viral load in infected individuals and stop transmission of the disease.
Boon’s research group studied the ability of a COVID-19 nasal vaccine to prevent the spread of infection compared to standard injected vaccines. This vaccine has recently been approved for emergency use in India as a booster and is licensed to Ocugen.
The study, published in Science Advances is titled, “Mucosal immunization with ChAd-SARS-CoV-2-S prevents sequential transmission of SARS-CoV-2 to unvaccinated hamsters.”
“To prevent transmission, you need to keep the amount of virus in the upper airways low,” Boon said. Next-generation vaccines, including nasal vaccines, are delivered directly to the location of initial infection. Using hamsters as a model for infection spread, the researchers vaccinated hamsters either with the nasal or an injectable COVID-19 vaccine. The hamsters were reared for a few weeks to allow for an immune response to settle, then were exposed to SARS-COV-2 via other infected hamsters. Eight hours following exposure, the exposed immunized and control hamsters were tested for COVID-19.
Researchers found that 86% of nasal vaccinated hamsters had detectable virus in their nose and lungs, while 94% of hamsters given injections had detectable virus. Most animals in both groups became infected, though hamsters vaccinated with the nasal vaccine had much lower viral titers, about 100- to 100,000-fold lower in the upper and lower respiratory tracts.
These titer levels are “sufficient to eliminate subsequent transmission to vaccinated and unvaccinated hamsters,” wrote the authors. Thus, they would not be able to spread infection to others. Conversely, those hamsters vaccinated with the injected vaccine had titer levels sufficient to spread the virus, showing the effectiveness of the nasal vaccine in preventing viral spread.
Next, researchers tested this assertion by using the vaccinated and exposed hamsters that developed infections to expose new groups of vaccinated and unvaccinated hamsters. They found that none of the new hamsters exposed to infectious hamsters that were nasally vaccinated developed an infection, while approximately half of the hamsters exposed to injection vaccinated infected hamsters developed their own infections. This demonstrates that nasal vaccination can halt the transmission and spread of the viral disease.
Boon explained, “The less virus that is there to begin with, the less likely you are to infect someone else if you cough or sneeze or even just breathe on them. This study shows that mucosal vaccines are superior to injected vaccines in terms of limiting viral replication in the upper airways and preventing spread to the next individual. In an epidemic or pandemic situation, this is the kind of vaccine you’re going to want.”
By limiting viral transmission, nasal vaccines may help prevent the spread of illness in a population. Injectable vaccines are effective for individuals, but typically take at least a week for the patient to develop an immune response. Further, the response is not specifically localized in the nose or mouth, where respiratory viruses typically enter the body or are expelled in infected individuals.
“Historically, developing such vaccines has been challenging. There’s still so much we don’t know about the kind of immune response we need and how to elicit it,” said Boon. “I think we’re going to see a lot of very exciting research in the next few years that could lead to big improvements in vaccines for respiratory infections.”
