These mucosal immune responses seem to underlie the success of the oral polio vaccine, which contains a weakened form of polio virus and has helped most of the world eradicate polio. When it debuted in the 1960s, the vaccine was considered, in many ways, an enormous improvement over its injected predecessor because it targeted the bodys immune response in the gut, where the virus thrives. Many people who took the oral vaccine seemed to quash infections even before they felt symptoms or passed the germ on to others.

It was a fabulous vaccine to stop the transmission of polio, said Dr. Anna Durbin, a vaccine expert at Johns Hopkins University. It helped induce herd immunity, she said, referring to the threshold of the population that needs to be immune to a pathogen to keep it from spreading.

Vaccines given through muscle are great for prompting the body to churn out antibodies in the bloodstream, like IgG. If a pathogen shows up, hordes of these on-call molecules will rush to meet it.

For many respiratory infections, thats good enough.

The majority of respiratory vaccines, like the measles vaccine, are given intramuscularly, and it works, Dr. Iwasaki said. If enough antibodies reach the right mucosal surface, it doesnt really matter how they were induced.

Still, relying on that strategy alone can be risky a bit like shoring up a banks security at every entrance except for the one a thief would most likely hit. Sentinels roving throughout the building could subdue the interloper after they trip the alarm. But by that point, some damage has probably already been done.

Updated July 15, 2020

Its mainly a timing issue, Dr. Bhattacharya said. If you have circulating cells and molecules, theyll eventually find the infection. But youd rather have a more immediate response.

Without a strong mucosal response, injected vaccines may be less likely to produce so-called sterilizing immunity, a phenomenon in which a pathogen is purged from the body before its able to infect cells, Dr. Durbin said. Vaccinated people might be protected from severe disease, but could still be infected, experience mild symptoms and occasionally pass small quantities of the germ onto others.

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Nasal Coronavirus Vaccines May Work Better Than Injected Ones, Researchers Say - The New York Times