SINGAPORE - Covid-19 vaccines that are administered through the nose or mouth could potentially be more effective in blocking the virus at the upper respiratory tract, and thus reduce its transmission, said infectious disease experts in Singapore.
But the safety and efficacy of such vaccines still have to be proven in clinical trials, they stressed.
The experts were commenting on Covid-19 vaccines delivered through alternative ways besides injection, after China's drug authority last week approved an inhaled vaccine co-developed by CanSino Biologics for clinical trials.
Inhaled or intranasal vaccines that are sprayed into the nose mimic the way respiratory viruses usually infect a person, by entering the respiratory tract.
These vaccines are expected to induce a strong local immune response in the upper respiratory tract, where the Sars-CoV-2 virus takes hold.
"This local immune response is therefore potentially more effective at blocking the pathogen immediately when the virus is inhaled or comes into contact with the nose," said Associate Professor Sylvie Alonso, co-director of the Infectious Diseases Translational Research Programme at the NUS Yong Loo Lin School of Medicine.
There may be a higher chance of reducing human-to-human transmission when the pathogen is effectively neutralised immediately or soon after it starts replicating, she added.
Injectable Covid-19 vaccines that have been approved so far - such as the Pfizer-BioNTech and Moderna vaccines - have been shown to prevent disease, but have yet to show conclusively that they can reduce transmission.
Professor Ooi Eng Eong from the emerging infectious diseases programme at Duke-NUS Medical School said the theoretical benefits of intranasal and inhaled Covid-19 vaccines are uncertain without efficacy trials.
These vaccines are in the exploratory stages of testing, noted Prof Ooi, the co-developer of an injectable Covid-19 vaccine that is to undergo phase three trials in the second quarter of this year.
He added that before intranasal and oral vaccines can be used, clinical trials will be needed to show they are safe and to compare their efficacy with vaccines that have been approved.
Prof Alonso noted that vaccines administered orally, nasally or vaginally have been under development for decades, but face unique scientific challenges.
She added that increased support for basic research, such as for understanding the immunity at those mucosal sites, is needed.
Intranasal Covid-19 vaccines are also being studied elsewhere besides China.
Last Thursday, Oxford University said it is launching a study of immune responses of a nasal administration of its vaccine developed with AstraZeneca.
In Singapore, home-grown biotech company Esco Aster is working with US company Vivaldi Biosciences to develop a vaccine which will be delivered as a nasal spray. The chimeric vaccine - made by merging proteins from different viruses - is currently undergoing animal studies, said the firm's chief executive Lin Xiangliang.
On safety, Prof Alonso said that as the nasal cavity is close to the brain, developers must ensure that the components of the vaccine cannot reach the brain.
Prof Ooi pointed out a challenge where some who receive the vaccine through the nose may sneeze immediately after, causing some of the vaccine to be expelled, thus affecting dosing.
Among the advantages of the intranasal route is that trained personnel may not be needed to administer the vaccine as needles are not used, said Prof Alonso. These vaccines may also be cheaper and could help accelerate mass vaccination, especially in low- and middle-income countries, she added.
There are currently no licensed nasally delivered vaccines in Singapore, said Prof Ooi. But there are licensed oral vaccines, such as the liquid polio vaccine and typhoid vaccine that comes in pill form, he added.