Numbers – How to Say ‘1273’ in the mRNA-1273 Vaccine Aloud?

If you want to pronounce it the way the experts in the industry would, including the scientists and clinicians who developed, researched, and actually named it, then it's "twelve seventy-three".

From my personal experience, though I haven't been personally involved in the development of this vaccine, I work in clinical trials of experimental drugs with similar designations, and four-digit drug designations like this are almost universally pronounced, by native English speakers, as pairs of two-digit numbers (except if the first or third digit is a zero, like 0103).

There is one exception. If an expert was introducing the designation to an audience for the first time (e.g., at the beginning of a talk), he or she might spell it out as "one two seven three" the first time it was used, just for clarity. I'm quite certain that no expert would ever read the number out in full as "one thousand two hundred and seventy-three".

In this specific case, there is compelling video evidence for the "twelve seventy-three" pronunciation. You can find a video in NEJM's twitter feed where this pronunciation is used at the 0:16 mark. Even more convincingly, you can watch the FDA hearing on YouTube for the emergency use authorization of the vaccine and hear Dr. Fink (Deputy Director of the vaccines division at FDA) around 26:50, and Dr. Zaks (Chief Medical Officer at Moderna) around 1:45:29, both use the "twelve seventy-three" pronunciation.

As @JohnMontgomery mentioned in a comment, this convention isn't exclusive to designations of experimental drugs. English speakers will often pronounce four-digit years this way, like "nineteen eighty-four" and "twenty twenty-one". Other four-digit identifiers like model numbers might also be pronounced this way.

However, it would usually be wrong to pronounce four-digit amounts this way, especially if the unit (e.g., "dollars") is part of the phrase being spoken. If you have $1273 in your pocket, it's either "one thousand two hundred and seventy-three dollars" or "twelve hundred and seventy-three dollars", but not "twelve seventy-three dollars".