Which Arm You Get Vaccines in Can Change Effectiveness, Study Finds


Cold and flu season brings millions of Americans into pharmacies and doctor’s offices every year to get their flu vaccine; in more recent years, the flu shot has also been accompanied by a COVID shot for many people. When getting these dual shots, some people opt to get one in each arm, while others choose to get them both in the same arm. It turns out which one we choose when getting vaccines can affect just how effective the immunizations are. 

Researchers at Oregon Health and Science University decided to explore the question of arm choice early on in the COVID-19 pandemic. Nearly 1,000 OHSU employees were a part of the study, published in The Journal of Clinical Investigation, when COVID vaccines first became available; one random group received both doses in one arm, which the other received one dose in each arm. 

Both groups had similar antibody responses in the first two weeks, but in weeks three and four, the ones who had “contralateral” shots in different arms had a “substantially increase[d] antibody magnitude and breadth.” The immune response increased progressively over time up to four times more than the other group. What’s more, when the Omicron variant of COVID emerged in late 2021 a year after the second vaccine was given, the contralateral group’s immune systems had an even stronger response than it did with the original SARS-CoV-2 strain. 

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The scientists weren’t able to give an exact explanation for the phenomenon, though they believe it’s possible that injecting a vaccine in each arm creates separate immune responses in the corresponding lymph nodes on either side. 

“It turned out to be one of the more significant things we’ve found, and it’s probably not limited to just COVID vaccines. We may be seeing an important immunologic function,” study co-author Marcel Curlin told New Atlas. “By switching arms, you basically have [immune] memory formation in two locations instead of one.”

More research needs to be done to determine if the same immune response can be seen with other multidose vaccinations, so the scientists aren’t recommending changing how you get vaccines just yet. As always, consult your physician first if you have any questions. 

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