Getting a COVID-19 booster shot in the same arm as the initial vaccine dose may significantly speed up the body’s immune response, according to new research from Sydney, Australia, published in the journal Cell.

The study, led by scientists at Sydney’s Garvan Institute of Medical Research and the Kirby Institute at UNSW Sydney, reveals how immune cells in lymph nodes “remember” the first dose, enabling faster protection during critical early stages of a pandemic.

“When a vaccine is administered, specialized immune cells called macrophages in nearby lymph nodes become primed to recognize the antigen. These macrophages then guide memory B cells-critical for long-term immunity-to strategic positions within the lymph nodes. If a booster is given in the same arm, these pre-alerted macrophages rapidly capture the antigen and activate memory B cells, leading to quicker antibody production,” a  press release put out by the Garvan Institute explains. 

“Location matters because the macrophages in the lymph node closest to the injection site are already on high alert,” said Dr. Rama Dhenni, co-first author of the study. “They orchestrate a faster and more effective response when the booster arrives in the same place”.

Same arm vaccine shots also effective against Delta and Omicron variants

In a clinical study of 30 Pfizer-BioNTech COVID-19 vaccine recipients, researchers found that participants who received both doses in the same arm produced neutralizing antibodies against SARS-CoV-2 twice as fast as those who switched arms. These early antibodies were also more effective against Delta and Omicron variants. 

“Those first weeks of protection are crucial for curbing transmission in a pandemic,” said Dr. Mee Ling Munier, co-senior author and vaccine immunogenomics expert. “A same-arm strategy might help achieve herd immunity faster, especially against rapidly evolving viruses”.

The findings, validated in both mice and humans, challenge conventional vaccination practices, which often alternate arms for convenience. While the long-term protection remains similar regardless of injection site, the study highlights how small adjustments to vaccine protocols could enhance early outbreak control.

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