Here’s the Arm You Should Get Your COVID Shot In, New Study Finds

When a pharmacist asks which arm you’d prefer a shot in, you might find yourself wondering if it really matters which one you pick for your COVID vaccine. Just as any gym-goer knows one side often takes the lead, research shows your immune system may play favorites, too. 

In 2023, a study published in The Lancet’s peer-reviewed journal eBioMedicine examined whether people should receive their second dose of a COVID-19 vaccine on the same side as their first (ipsilateral) or the opposite arm (contralateral). 

“Our study indicates that ipsilateral vaccinations generate a stronger immune response than contralateral vaccinations,” said lead author Laura Ziegler, a doctoral immunology student at Germany’s Saarland University. In other words, getting both shots on the same side might be to your advantage.

While the 2023 research focused on people receiving their second COVID vaccination, a new 2025 study published in Cell revisited the same question—this time looking more broadly at booster doses. The research confirms the 2023 findings, while offering new takeaways on why this matters. 

The COVID vaccine: which arm? Scientific research revealed the answer

In the 2023 study, researchers, including Ziegler and Martina Sester, PhD—chair of the Department of Transplant and Infection Immunology at Saarland University—tracked 303 previously unvaccinated people getting their first two COVID shots. About half (147) got both doses in the same arm, while the other half (156) switched sides.

Two weeks later, the scientists found that those who got both shots in the same arm had a stronger immune response. Specifically:

• Both groups had strong antibody levels, but the quality of these antibodies was stronger in the same-arm group—they bound more effectively to the viral spike protein. 
• Those who switched arms showed lower neutralizing activity, meaning their antibodies were less efficient at blocking the virus. 
• People who switched arms also had significantly lower levels of a type of “killer” T cell, which plays a role in destroying infected cells. 

Recent research shows the arm you choose still matters

The 2025 Cell research reinforces the original findings: getting a COVID booster shot in the same arm as your first dose, even years later, may offer a faster immune response. 

It explains that when you get a vaccine, your body creates long-lasting immune cells called memory B cells. A booster dose “wakes up” these immune cells, sending them into training mode in nearby lymph nodes to produce more effective virus-fighting antibodies. 

According to the study, the lymph nodes closest to your original vaccination site house stronger, faster-working memory B cells compared to the other side. So, getting a booster COVID shot in the same arm triggers a more efficient immune response.

That said, getting a COVID shot in the opposite arm still works. After about a month, antibody activity is about the same regardless of where you got your shot. The 2025 research emphasizes that the advantage of same-arm boosting appears to be faster immune activation—not necessarily stronger seasonal protection. But with a virus as contagious as COVID, that advanced start for your immune system could be a big benefit.   

What this means for you

“As dramatic as the pandemic was, it has provided us with reliable data that enables us to address questions of this kind,” explains Dr. Sester. However, it’s important to note that both studies acknowledge limitations and the need for more research. For instance, the 2025 Cell study relied heavily on animal models, with only a small sample size of human participants. 

Still, understanding the nuances of vaccine administration becomes crucial as the world continues to adapt and evolve its strategies against viruses like COVID. While these studies focus on a specific aspect of vaccination, they shed light on the importance of continual research and optimization.

For now, if you’re gearing up for your COVID booster, you might consider keeping it on the same side. If it seems tricky to remember which arm you received the first in, a general rule of thumb is that some clinicians administering vaccines will suggest your non-dominant hand to avoid interfering with your daily tasks as much as possible…a clever reminder to be armed with.

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