A recent study reveals that when you get your updated COVID-19 vaccine this fall, you might want to think carefully about which arm you choose. The immune response may be stronger if you select the same arm as your last COVID-19 shot, according to research published in the journal eBiomedicine.

“The question seems so banal, so trivial that nobody before has thought to ask it,” said study author Martina Sester, a biologist at the Institute of Infection Medicine at Saarland University Hospital in Germany.  According to CNN, the researchers analyzed the data of 303 people who received the mRNA vaccine as well as a booster as part of Germany’s vaccine campaign. The study subjects never had COVID-19 and received two doses of Pfizer-BioNTech’s COVID-19 vaccine between March and September 2021, says USA Today.

Two weeks after the booster shot was given, the number of killer T cells was much higher in those who had both shots in the same arm. Those killer cells were present in 67% of the same-arm cases and only 43% of people who had shots in the opposite arm, according to the study.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, called the study conclusions “absolutely fascinating.” He added “I can’t remember another study similar to this with other vaccines.” Schaffner says it makes sense that stimulating the immune cells in the same lymph nodes will elicit better or longer lasting protection.

“We wouldn’t know that unless we did a much larger clinical study with follow-up actual infections,” he said. The study authors acknowledge that while immune cells quickly destroy their target, antibodies are equally important because they prevent further harm, says CNN. The researchers did not find a larger number of antibodies in the same-arm subjects.

But the antibodies in the same-arm people were better at binding to the viral spike proteins, which are responsible for the coronavirus entering cells. Schaffner says that the study intrigued him, and he will consider using the results for his own purposes this fall when he gets inoculated for respiratory syncytial virus (RSV) and influenza, as well as the COVID-19 updated booster.

“I began thinking ‘Which arm am I going to get them in?’ And I think I’m going to get my COVID booster —on the basis of this study — on the same as the previous inoculations,” he said.