Whelp, to answer my own question, this is above the fold on the NYT right now.
The World Health Organization called on Wednesday for a moratorium on coronavirus vaccine booster shots until the end of September to help all countries vaccinate at least 10 percent of their populations, appealing to the world’s wealthiest nations to address dramatic disparities in global vaccination rates.
...Scientists have still not come to a consensus on whether booster shots are necessary. But as worries emerge about continuing pandemic waves and future lockdowns, an increasing number of countries like Germany, Israel and France are preparing to give part of their population booster doses, or have already started.
Studies have indicated that the immunity generated by the Pfizer-BioNTech and Moderna vaccines is long-lasting. Researchers are still working to understand recent Israeli data suggesting that the Pfizer-BioNTech vaccine declined in efficacy months after inoculation.
(Granted, the WHO is not the CDC.)
But is there any evidence that the US will actually start exporting doses if they're unused, or will they just sit around until they expire?
Britain and Germany are already planning to do this, to some extent, as is Israel. https://www.timesofisrael.com/britain-germany-to-begin-giving-3rd-doses-of-coronavirus-vaccine/
I keep hearing about vaccines expiring in the US. If it’s useful, better to use it than to have it get thrown out.
Some people are suggesting to get a third of a different vaccine than you got before (e.g. if you got Moderna get Pfizer or J&J)… by “some” I mean I heard it on the All-in Podcast. It was either on the July 15th episode or the July 22nd episode. https://podcasts.apple.com/us/podcast/all-in-with-chamath-jason-sacks-friedberg/id1502871393?i=1000529038600
This doesn't exactly answer your question, but from personal experience, when I got my jabs from Walmart, the website didn't seem to differentiate from first and second doses. That is, from the perspective of the appointment scheduling system, I seemed to get two "first" doses. And I didn't even get a vaccination card until I specifically asked for one and they went out of their way to look me up in the system. So I don't think there's anything stopping me from getting another jab if I wanted it, in practice.
Similar experience - I got a J&J from a mass vaccination site in April, then a Pfizer from Walgreens a few months later. Walgreens didn't ask me anything about whether I'd been vaccinated before.
Not an answer, but just my epistemic status: I heard that they're good to get 6 months after the 2nd dose, so I'm planning to get another dose in ~2 months.
Meta comment: You can leave comments on questions as well as answer, so since this is not an answer you could move it into a comment.
People seem to think that getting a third mRNA vaccine dose, or "booster shot", substantially increases immunity after 4ish months from your second dose.