Study details

For the study, researchers looked at the immune responses of around 300 people who never had COVID-19 and received two doses of Pfizer and BioNTech's COVID-19 vaccine between March and September 2021.

The participants in the study were randomized into two groups — one that received both shots in the same arm and one that received their second dose in the opposite arm of the first dose.

Two weeks after they received their shots, the researchers found that certain immune cells called CD8+ T cells — commonly referred to as "killer T cells" — were detected in 67% of the people who received their injections in the same arm compared to just 43% in those who received injections in opposite arms.

However, co-author Martina Sester noted that antibody numbers in those who received shots in the same arm were not greater than those who received shots in opposite arms.

Discussion

Unlike killer T cells, antibodies don't immediately destroy a virus but instead dock onto a virus and prevent it from causing more harm, making it easier for cells called macrophages to find the virus and destroy it.

"What's interesting is that the antibodies in the ipsilaterally vaccinated subjects were better at binding to the viral spike protein," Sester said, which suggests the antibodies of those who were vaccinated in the same arm were more effective than those who were vaccinated in both arms.

The authors suggested it's possible that people respond better to shots in the same arm because the vaccines are targeting the same lymph nodes, which then makes those lymph nodes more active in producing immune cells and fighting off infections.

While the study is preliminary and small, it indicates the way people react to vaccines could go beyond age, sex, and medical conditions, said Ofer Levy, a pediatric infectious disease specialist and director of the Precision Vaccines Program at Boston Children's Hospital.

"This speaks to precision vaccination in the sense that everything matters," he said.

Levy added that more research is necessary, but the study could have implications beyond COVID-19 vaccines and could help standardize the way vaccinations are administered.

"We need to be precise about how we discover, develop, and deliver vaccines," Levy said. (Rodriguez, USA Today, 8/17; Saarland University press release, 8/14)