Which Arm Should You Use for Vaccines? Science Says: Stick to the Same Side

When booking your next vaccine appointment, you probably focus on the type of vaccine—not the arm it goes into. But it turns out, that choice might matter more than you think.

A recent study published in Cell suggests that receiving your first and second vaccine doses in the same arm could trigger a faster and stronger immune response than switching arms.

The Immune System’s Memory Has a Favorite Spot

Researchers from Sydney’s Garvan Institute of Medical Research and the Kirby Institute at UNSW conducted both mouse experiments and human clinical trials to examine this phenomenon. Their work centers around memory B cells—immune cells that “remember” a pathogen and respond rapidly if it returns.

In mice, scientists used advanced imaging techniques to observe how these memory B cells respond to vaccines. They discovered that when a vaccine is injected, memory B cells travel to the nearest lymph node—the one that “drains” the injection site. There, they interact with macrophages, immune cells that help process the vaccine antigen and kickstart the production of protective antibodies.

When the second dose is given in the same arm, these nearby macrophages are already primed to recognize the antigen. This leads to quicker reactivation of memory B cells and a more robust antibody response.

Does This Apply to Humans? Yes—And Fast

To test the theory in humans, the researchers gave 30 volunteers COVID-19 mRNA vaccines. Twenty received both doses in the same arm, while ten had them in opposite arms. The result?

Within just one week of the second dose, same-arm participants showed significantly higher levels of neutralizing antibodies, including against variants like Delta and Omicron. Although both groups eventually reached similar antibody levels by week four, the early advantage could be crucial during a pandemic, where speed of immunity can save lives.

Why This Matters in Real Life

“This study shows that location matters,” explains Dr. Rama Dhenni, co-lead author. By choosing the same arm for subsequent doses, your immune system leverages its existing “training ground,” speeding up its defensive response.

So while switching arms won’t hurt you in the long run, if you’re looking for maximum protection in the shortest time, especially during outbreak periods, sticking to the same arm could give you a vital head start.

Bottom Line: Pick a Side—and Stay There

Whether it’s your left or right arm, the science suggests that consistency counts when it comes to vaccine efficacy. Until future vaccines are developed to work with fewer doses, this small detail could make a big difference.