I remember that I blew this off Friday evening. I must have been getting hungry, tired, or both
.
Your questions about
oral vaccines is a very good one, with a complex answer. One that will result in another reading assignment. See the Wikipedia pages for
Polio Vaccine and
Peyer's patch. Both oral and injected vaccines against Polio were developed in the 1950s. Both were tested, and one of the oral vaccines, the Sabin vaccine, was found to be the most effective. It was effective because it was an "attenuated" vaccine instead of a chemically inactivated vaccine as was the injected Salk vaccine. And also because the Sabin vaccine was taken orally, it was more effective and longer lasting. It resulted in two types of immunity, humoral (antibody-mediated which clears infectious viral particles from the blood stream) and cell-mediated which can be more effective with viral diseases because it kills host cells which are virally infected. The injected chemically inactived Salk vaccine resulted only in humoral immunity. Read the
Polio vaccine Wikipedia page for details.
Peyer's patches are important but small structures located in the small intestine. They play an important role in developing immunity and became the preferred route of vaccination, especially for polio which resides in the gastrointestinal (GI) tract for part of it's life cycle. They're elongated thickenings of the intestinal epithelium measuring a few centimeters long. About 100 Peyer's patches are found in humans. The lumen of the GI tract is exposed to the external environment. Much of it is populated with microorganisms, some are potentially pathogenic. Peyer's patches play an important role in the immune surveillance of the GI lumen and they facilitate production of immune responses.
Peyer's patches are the immune equivalent of standing flea markets for the immune system. Pathogenic microorganisms and other antigens entering the intestinal tract encounter a variety of immune cells, macrophages, dendritic cells, B-lymphocytes, and T-lymphocytes in Peyer's patches. Peyer's patches thus act for the GI system much as the tonsils act for the respiratory system, trapping foreign particles, surveilling them, destroying them, and starting a lasting immune response against them. A major reason why the oral Polio vaccine works better is because it sent the attenuated polio virus to the Peyer's patches. The injected Salk vaccine bypassed it.
The question about using a
non-replicating viral vector is more technical, but easier to answer. As you can understand, the FDA is highly concerned about a vaccine delivering a virus known to cause serious disease to human. They want to be certain the virus vaccine develops immunity but cannot cause the viral disease itself.
A modern method to achieve this is to package the genes for the viral envelope proteins of SARS-CoV-2, in another virus – a vector, such as Adenovirus type 5 (Ad-5) – that causes no disease on it's own in humans. The Ad-5 virus is also engineered to lack the necessary genes for replicate on it's own. It can deliver the genes needed to develop immunity against SARS-CoV-2, or any other virus, without any danger of introducing a viral infection, even in a patient who might be immune compromised. So no-replicating viral vectors are safer to use in vaccines.
