Vaccine doses or dosages?

I think a dose is the amount of medicine I am given, but can also refer to me getting a single administration of a multiple-dose treatment. Dosage is the rate at which I am getting repeated administrations of that dose. Dosages would relate to the range of such rates that people of different ages, genders and body masses would be given a particular series of doses.

You are mostly correct about the definition of dosage. It could start out to mean a number of milligrams, but the complication comes in when you have a standard concentration of a given mass in a given amount of liquid. Let's think of it as a carrier fluid. For liquids administered for medical purposes, say with an over the counter cough syrup, the package would describe the recommended amount of capfuls, or fluid ounces--basically a volume description of that carrier liquid. That would be the dose, but it becomes the dosage, when they recommend that I do not exceed a certain number of such doses in a day. It is dependent on the age and presumed size of the individual, an adult getting a larger amount than a child under 12, for example, and infants none at all perhaps.

Yes people use them interchangeably, but I doubt that is the case among professionals, for whom these are terms of their jargon with very different meanings. However, if you have a case like the current vaccine where two doses comprise the dosage for an individual, it seems even easier to confuse the two terms, even if you are surrounded by professionals who presumably use the terms correctly. It is essentially a circumstance where the two shots become one dose and that dose is also the dosage--the entirety of the amount you are going to need--of that vaccine.

I am giving the Governor a pass here, since I think he is referring to a dosage as the pair of shots that an individual gets, therefore the capacity to meet the public's need is properly described in terms of the number of dosages needed for that population, while each person's dosage can also be described in the sense of a single dose, since one shot is not valuable without the other.

I would like to note that there is sometimes confusion between a similar set of nouns, sewer, sewage and sewerage. Of course a sewer is something you can enter, sewage is what flows through it, and sewerage is the collection of sewers in a system of part of a system. Yet I have encountered a false equivalency among the lay public, but never among civic engineers, where the liquid sewage is referred to as sewerage. It is easier to distinguish when you have all three terms in front of you, but if you have only one and someone describes a terrible amount of sewerage, you can easily understand the meaning intended, without being picky about the terminology. And in a certain sense you are right to do so, since at bottom the suffix -age indicates something related to the root word, so sewerage can be understood in lay English as "that which is associated with a sewer," that is, the liquid waste in it.

Thanks for a great and timely question! I'd love to hear from others if I have gotten close to the heart of the matter, especially from professionals in the medical field.

Dose is to use for the singular term however dosage is used for the plural term.

A single dose may have have multiple milligrams of any product, the dosage is identical for each dose.
Three different vaccines were formulated with different dosages of any particular to provide immunity to a human body against the same virus. The dosage of each vaccine may also refer to the number of required doses to complete the vaccination. If each dose of ibuprofen tablet is 200mg. You must take 4 doses ( or 4 pills ) to reach the same dosage of active ingredients found in a single dose of an 800mg ibuprofen tablet.

And that careless usage is why he had to resign. Among other things.

It's easy to remember: dosage, is a specific amount, number, and frequency of doses over a specified period of time.

Dose is a unit.
Dosage is a rate.