Left or right arm? Study reveals why location of injecting the vaccine matters

India Today

29-04-2025

Australian researchers have revealed that receiving a vaccine booster in the same arm as the original dose can trigger a quicker and more effective immune response.The findings of their study, published in the journal Cell, were led by the Garvan Institute of Medical Research and the Kirby Institute at UNSW Sydney.Scientists discovered that specialised immune cells, called macrophages, are 'primed' inside the lymph nodes nearest to the injection site after the first dose. These cells then help position memory B cells - a critical part of the immune system - to respond faster when a second dose is given in the same location.advertisement
"This is a fundamental discovery in how the immune system organises itself to respond better to external threats. Nature has come up with this brilliant system and we're just now beginning to understand it," said Tri Phan from the Garvan Institute.WHY VACCINE LOCATION MATTERSVaccines work by introducing a harmless form of a virus or bacteria into the body, teaching the immune system to recognise and fight it.Once the vaccine is administered, it travels to nearby lymph nodes - the body's immune training hubs. Memory B cells, which help produce antibodies when the body sees the same virus again, are often found in the lymph node closest to where the vaccine was injected.
Vaccines work by introducing a harmless form of a virus or bacteria into the body, teaching the immune system to recognise and fight it. (Phot: Getty Images)
advertisementUsing advanced imaging tools, researchers at Garvan found that these memory B cells move to the outer areas of the lymph node, where they interact with macrophages already trained by the first dose.When the booster is given in the same arm, the 'primed' macrophages quickly react and activate the memory B cells to produce stronger antibodies."Macrophages are known for clearing up infections, but here they're also organising the next immune response," said Rama Dhenni, co-author of the study.RESULTS FROM CLINICAL TRIALSTo see if the effect was the same in humans, researchers conducted a study with 30 participants receiving the Pfizer-BioNTech COVID-19 vaccine. Those who got both doses in the same arm had faster and more effective antibody responses, especially in the first week after the second shot."These early antibodies were also better at neutralising variants like Delta and Omicron," said Dr Mee Ling Munier from the Kirby Institute.While both groups had similar antibody levels by four weeks, scientists say that earlier protection could be critical during outbreaks."This simple strategy, using the same arm, might help build community-level protection faster," said Dr Munier.Researchers hope the findings will inform future vaccination strategies and possibly reduce the number of boosters needed.