I understand that the vaccine-derived poliovirus originates from the live/attenuated virus used in the oral vaccine. (Or at least, first-gen versions of it! Does nOPV2 conclusively address these issues?)
Can anyone explain why it's rare for inactivated vaccines to be able to be delivered via the oral route? It would have been a lot more efficient/user-friendly to deliver the Coronavirus vaccinations in this manner, but almost all of the vaccines we seem to use nowadays appear to be intramuscular injections.
There are two main challenges to developing an effective oral vaccine:
* designing a formulation that can survive the harsh gastrointestinal environment. For example, COVID-19 vaccines, especially the mRNA-based ones, are especially fragile and degrade rapidly at room temperature even under normal conditions.
* Avoiding mucosal tolerance. Many antigens that would evoke an immediate immune response if encountered in the blood stream are ignored by mucus tissue, to avoid your body creating hundreds or thousands of spurious immune responses every day to antigens occurring in your food. For an oral vaccine to be effective it must get around this tolerance to provoke an effective immune response.
For respiratory viruses, another promising delivery method would be intranasal, and indeed there are some COVID-19 vaccines in development that use this route[1]. The bottom line is, developing vaccines is already complicated and costly, so unless there is a guaranteed large market demand pharmaceutical companies are unlikely to invest in delivery methods that add additional research challenges.
To wildly summarize...for the same reason you are not CONSTANTLY sick. your stomach is a pretty good, but not perfect, immunodefense - i.e., the stomach is a pretty tough environment for survival.
whether it is worth the trouble also depends on the method of infection. The live attenuated oral polio vaccine creates gut immunity, which is helpful because polio is passed via ingestion of infected materials. Gut immunity is less useful if a disease is passed via inhalation.
Can anyone explain why it's rare for inactivated vaccines to be able to be delivered via the oral route? It would have been a lot more efficient/user-friendly to deliver the Coronavirus vaccinations in this manner, but almost all of the vaccines we seem to use nowadays appear to be intramuscular injections.