A recent animal study has found that a nasal COVID-19 vaccine effectively stops the transmission of the virus. This suggests that vaccines administered through the nose or mouth could be vital in controlling the spread of respiratory infections.
The rapid creation of COVID-19 vaccines within months of the virus’s emergence showcased the capabilities of modern science and saved countless lives. However, despite their success in reducing morbidity and mortality, these vaccines fell short in eliminating the pandemic due to a significant limitation: they did not prevent the virus’s spread.
Researchers from Washington University School of Medicine in St. Louis have conducted a new study indicating that advanced vaccines targeting the virus’s entry points—the nose and mouth—might accomplish what conventional injections have failed to do: curb the spread of respiratory viruses. Utilizing a nasal COVID-19 vaccine based on technology from Washington University, which is licensed for use in India and further development in the U.S. through Ocugen, researchers found that hamsters vaccinated with this nasal form did not transmit the virus even when infected, thus breaking the transmission cycle. In contrast, the standard injectable COVID-19 vaccine did not prevent viral spread.
The results, published on July 31 in Science Advances, highlight the potential of mucosal vaccines, which are delivered into the nose or mouth, to help control respiratory diseases such as COVID-19 and influenza, which still pose a significant health risk.
Senior author Jacco Boon, PhD, a professor specializing in medicine and microbiology, stated, “To halt transmission, it is crucial to keep the viral load in the upper respiratory tract low. The lower the amount of virus present, the lower the chances of infecting others through coughing, sneezing, or even just breathing. This study shows that mucosal vaccines are more effective than injectable ones at limiting viral replication in the upper airways and preventing its spread to others. In scenarios of epidemics or pandemics, this is the type of vaccine that is most desirable.”
Creating vaccines that can manage virus levels in the nasal passages has been challenging. Viruses like influenza, SARS-CoV-2 (the cause of COVID-19), and respiratory syncytial virus (RSV) can increase quickly in the nose and spread swiftly after initial contact. Traditional injectable vaccines prompt immune responses that take about a week to fully develop and are typically less effective in the nasal area compared to the bloodstream, leaving this area vulnerable to rapidly multiplying viruses.
Theoretically, a vaccine administered directly into the nose or mouth could limit viral reproduction and reduce transmission by triggering an immune response right where it’s needed. However, demonstrating that mucosal vaccines indeed lower transmission has been challenging due to the complexity of tracking person-to-person transmission and establishing reliable animal transmission models.
In their study, Boon and colleagues created and validated a model for community transmission using hamsters, which are naturally susceptible to SARS-CoV-2, making them ideal for this type of research. The team immunized different groups of hamsters with lab versions of approved COVID-19 vaccines: the nasal iNCOVACC used in India and the injected Pfizer vaccine, along with a control group that received no vaccine. After allowing time for the immune responses to develop, they infected other hamsters with SARS-CoV-2 and placed them together with the vaccinated hamsters for eight hours, simulating exposure to a COVID-positive person.
After this eight-hour interaction with infected hamsters, most of the vaccinated ones became infected. Viral presence was detected in the noses and lungs of 12 out of 14 (86%) hamsters given the nasal vaccine, and 15 out of 16 (94%) with the injected vaccine. However, while both groups showed infections, the severity differed significantly. Hamsters who received the nasal vaccine exhibited viral loads in their airways that were 100 to 100,000 times lower than those vaccinated by injection or those unvaccinated. Although the study did not examine the overall health of the animals, prior research indicates that both vaccines lower the risks of severe illness and mortality from COVID-19.
The next part of the experiment produced even more impressive findings. The researchers took vaccinated hamsters that developed infections and introduced them to healthy hamsters (both vaccinated and unvaccinated) for eight hours to assess virus transmission from vaccinated to other animals.
Miraculously, none of the hamsters exposed to those vaccinated nasally became infected, regardless of their own vaccination status. In contrast, about half of the hamsters that interacted with those vaccinated by injection ended up infected, again irrespective of whether they had been vaccinated. This indicates that nasal vaccination—unlike injection—successfully disrupts transmission.
Boon noted that these findings could be significant as the world braces for potential avian influenza outbreaks that could affect humans. An injectable vaccine for avian influenza is already in existence, and researchers at Washington University are developing a nasal vaccine for it, including Boon and co-author Michael S. Diamond, MD, PhD, the Herbert S. Gasser Professor of Medicine and one of the innovators of the nasal vaccine technology used in this study.
“Mucosal vaccines represent the future of vaccination strategies for respiratory infections,” Boon remarked. “Historically, creating such vaccines has posed challenges, and much remains unclear about the immune responses required to achieve efficacy. We can anticipate exciting research in the coming years that may significantly enhance the vaccines available for respiratory infections.”
