Australian researchers find same arm for both shots boosts vaccine response

The findings could help improve vaccine strategies and may eventually lead to vaccines that need fewer boosters, Xinhua news agency reported.

The study, which was led by the Garvan Institute of Medical Research and the Kirby Institute at the University of New South Wales (UNSW) Sydney in Australia finds that the immune system responds more quickly when both doses are given in the same arm.

That's because immune cells in nearby lymph nodes, which are the body's infection-fighting hubs, become "primed" after the first shot. When the booster arrives in the same spot, these cells spring into action and help produce stronger antibodies, the team explained in the paper, published in the journal Cell.

Researchers discovered this effect first in mice, then confirmed it in a clinical study involving 30 people who received the Pfizer Covid-19 vaccine. Those who had both doses in the same arm developed faster and more effective protection, especially against Covid-19 variants like Delta and Omicron.

"This is a fundamental discovery in how the immune system organizes itself to respond better to external threats," said Tri Phan, director of the Precision Immunology Programme at the Garvan Institute.

While both groups ended up with similar antibody levels after four weeks, the same-arm group gained protection more quickly, a potential game-changer during pandemic outbreaks.

"If you've had your Covid jabs in different arms, don't worry, our research shows that over time the difference in protection diminishes," said the study's co-senior author Mee Ling Munier from the Kirby Institute.

But during a pandemic, even a few days can make a big difference, said Munier