Moderna put out a press release today with initial data from their booster trial.
They took 40 volunteers who had gotten the Moderna vaccine 6-8 months ago in the trial and gave them either a half dose (50 mcg) of the original vaccine, or one altered to match the spike protein of the B.1.351 (South Africa) variant. Three of the 40 no longer had detectable antibodies from the initial two shots, so apparently it can wear off within 8 months in a minority of people. Half of the 40 did not have antibodies that could neutralize either the SA or Brazil variants.
Two weeks after either booster, antibody levels to all three variants (original, SA, Brazil) were similar to or higher than what they had been after the initial two shots. Unsurprisingly, those that got the variant booster had higher neutralizing antibodies against the variants. The variant booster also boosted the response to the original variant. That's to be expected, as there is a lot of overlap between the two, and the immune system "remembers" and revives an older immune response when it finds something similar.
Side effects were similar to the second shot of Moderna. Interestingly, the more bothersome side effects were less frequent with the SA variant booster than with the original booster.
Moderna is also testing a third booster, with a 50-50 mix of the original and SA variants. This data is not out yet, but should be soon.
Here is more data from that study. In particular, we can see that at least at two weeks, the difference between the regular and variant boosters is pretty small. It's possible that it will diverge over time, though.
The most interesting thing that wasn't in the press release is a bit more information about the three people (out of 40) who lost their antibodies after 6-8 months. I was a bit worried that there would be some people who had antibodies but at levels so low as to be ineffective. However, it looks like those three are outliers. Those that had antibodies at 6-8 months have levels that are likely protective against the original virus. One of the three boosted to medium levels relative to everyone else. The other two were at the low end after boosting. Probably some individuals are just genetically predisposed to not being able to make good antibodies to this particular protein. This is something that has been seen with other vaccines, so not particularly surprising.
Link to study : https://www.medrxiv.org/content/10.1101/2021.05.05.21256716v1.full.pdf