Why Arm Choice Matters in Vaccination
Researchers from the Garvan Institute of Medical Research and the Kirby Institute at UNSW Sydney led a study—published in the journal Cell—to explore how the site of vaccine administration affects immune response. They found that administering a booster shot in the same arm as the initial dose can lead to a more effective and quicker immune response. Specialized immune cells called macrophages become ‘primed’ in the lymph nodes near the injection site after the first dose. These macrophages then guide memory B cells, helping them respond more efficiently when the booster enters the same location.
Study Findings
In a clinical trial involving 30 participants who received the Pfizer-BioNTech COVID-19 mRNA vaccine, 20 individuals got both doses in the same arm, while 10 received the second dose in the opposite arm. Participants who used the same arm for both doses produced neutralizing antibodies against SARS-CoV-2 significantly faster—within the first week after the second dose. Moreover, these antibodies showed greater effectiveness against variants like Delta and Omicron.
Further supporting these findings, researchers at the University of Melbourne found that mice developed protective antibodies more quickly when they received both doses in the same limb, compared to those vaccinated in different limbs. This evidence suggests that repeating the injection in the same arm could offer faster protection during outbreaks.
Practical Implications
Although no official guidelines require using the same arm for booster shots, the evidence suggests that doing so may enhance vaccine effectiveness. Both healthcare providers and individuals can adopt this simple strategy to potentially strengthen immune responses—especially when rapid protection is critical.
