Which Arm Gets the Booster?

A burning question for everybody who’s gotten a booster, or wants one: the same arm, or the other arm?

Right or Left?

Every time you get a vaccination, the person giving it will ask you which arm. Your humble Weekend Editor always picks the left arm, to sleep on the right side comfortably. The Weekend Editrix also picks the left arm, so she can use her right arm more freely and without pain.

But the question everybody, apparently, wants to know: should we get the booster(s) in the same arm as the original, or the other arm?

Somebody’s done a study!

The Same/Opposite Limb Study

Delthia Ricks first brought it to our attention:

That pointed us at a summary article in Medical Xpress ^[1], which dropped enough hints that we could find the actual research paper in Science Immunology. ^[2]

Some of the high points:

• This was a mouse study, not human. The vaccination was for flu, not COVID-19.
• Mice were injected in either the ipsilateral or contralateral footpad for initial and booster doses (those being painfully fancy-pants words meaning “same side” or “opposite side”).
• Some things were pretty much the same either way, like magnitude of serum antibody responses.
• But a couple things were different:
  • Germinal centers (GCs) are places where your body responds to infection and generates memory B cells for long-term immunity. Typically they’re associated with lymph nodes.
  • Same-side boosters generated better quality GCs with more avidity for the viral antigen, higher immunoglobulin mutations (varieties of stuff that gloms onto invading viruses), and increaseed recall of B cells for the virus.

So basically, hitting the same GC in the same limb reactivated local memory B cells and got a superior response. Slightly. It didn’t seem to be a big effect.

So that’s what they said. Only with graphs. And statistics.

I’m being a little funny here, because most of what they measured was not statistically significant, and it makes me wonder if they just kept hunting until the found something that was, regardless of the experiment design. (See p-hacking, upon which we generally frown!)

The Weekend Conclusion

Still, the results are pretty consistent with what we expect: it mostly doesn’t matter until you look really closely. And even then, maybe not. So it’s a nice little paper on a nice little issue. In terms of actually catching COVID-19 vs not, it probably makes no difference.

The main thing that does make a difference: get vaccinated, then get boosted.

All else is commentary, to quote R Hillel in another (wonderful) context.

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Notes & References

1: B Yirka, “Booster immunization in same limb as the first shot yielded stronger adaptive immunity in mice”, Medical Xpress, 2022-May-09. ↩

2: M Kuraoka, et al., “Recall of B cell memory depends on relative locations of prime and boost immunization”, Sci Immunol 7:1, 2022-May-06. DOI: 10.1126/sciimmunol.abn5311. ↩