These immune responses to the mucosa appear to underlie the success of the oral polio vaccine, which contains a weakened form of the polio virus and has helped most of the world to eradicate polio. When released in the 1960s, the vaccine was considered, in many ways, a huge improvement over its injected predecessor because it focused on the body’s immune response in the gut, where the virus thrives. Many people who took the oral vaccine seemed to suppress infections even before they felt symptoms, or to pass the germ on to others.
“It was a terrific 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 prevent it from spreading.
Vaccines administered through the muscle are great for causing the body to make antibodies in the bloodstream, such as IgG. If a pathogen appears, the hordes of these guard molecules will rush to find it.
For many respiratory infections, that’s good enough.
“Most respiratory vaccines, like the measles vaccine, are administered intramuscularly, and it works,” said Dr. Iwasaki. “If enough antibodies reach the correct mucosal surface, it really doesn’t matter how they were induced.”
Still, relying on that strategy alone can be risky, a bit like propping up a bank’s security on every entry, except one where a thief would likely hit. Sentinels roaming the entire building could subdue the intruder after they set off the alarm. But at that point, some damage has probably already been done.
“It is mainly a time problem,” said Dr. Bhattacharya. “If you have circulating cells and molecules, they will eventually find the infection. But I would 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 it can infect cells, Dr. Durbin said. Vaccinated people may be protected from serious illness, but they can still become infected, experience mild symptoms, and occasionally transmit small amounts of the germ to others.
