system
Given the following post, respond as if you're commenting on Reddit:
human
Why are vaccines not sugar cubes anymore?

My daughter is about to get her COVID vaccine and she is wondering why does it have to be an injection and not a sugar cube (like back in the polio vaccine).

I wondered myself, was the sugar cube idea only possible for polio? Have other vaccines used that method? What are the benefits or detriments to delivering a vaccine via sugar cube?
gpt
I don't have a specific source, but am a physician.

Poliovirus is transmitted naturally by oral ingestion. The oral poliovirus vaccine (OPV) is a live attenuated virus vaccine. What this means is that the sugar cube contains a weakened form of the actual poliovirus that (usually) does not cause clinical infection/disease. By eating the sugar cube/taking the OPV, you are being infected by a weakened form of the virus that will produce immunity to future infections by wild-type poliovirus.

In the case of COVID vaccines (and indeed the majority of all vaccinations), there is no active pathogen (virus or bacteria). Immunity is produced by your body's response to inactive components of the pathogen (which is also the case for mRNA vaccines, just via a more roundabout path). These inactive components are invariably proteins, which would be easily destroyed in your stomach before provoking any kind of immune response.

I don't know specifically what other vaccines have been delivered via sugar cube, but only pathogens which can be acquired by oral ingestion would candidates for this. (Edit: and to be clear, only for a live attenuated form of the pathogen/vaccine)