Study suggests that which arm you use for vaccinations may make a difference

Vaccines that require two or more doses, like those used for the COVID-19 vaccine, could be more effective if they are given in the same arm, a new study claims.

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According to the research, if you are given a dose of a two-dose vaccine in one arm and then get the second dose in the other arm, the immune response is not as high.

The research published in EBioMedicine, a peer-reviewed journal from The Lancet Discovery Science, offers one suggestion as to why that would be the case.

According to the study’s authors, when two doses of the vaccination are moved through the body by the same lymph nodes, it appears the body is more active in producing immune cells to fight off infections.

“Our study indicates that ipsilateral vaccinations generate a stronger immune response than contralateral vaccinations,” explained Laura Ziegler, a doctoral student at Saarland University and one of the study’s authors.

In an ipsilateral vaccination, the vaccine is injected twice into the same arm. In a contralateral vaccination, the primary vaccination is delivered in one arm while the booster is injected into the other.

The study looked at 303 people who never had COVID-19 and received two doses of Pfizer-BioNTech’s COVID-19 vaccine between March and September 2021.

According to the results, two weeks after the booster shot, a higher number of cytotoxic CD8+ T cells, often referred to as “killer T cells,” was found in 67% of those who had both shots in one arm.

For those who had shots in both their arms, only 43% saw an increase of killer T cells.

The number of antibodies a person generated did not increase when shots were given in the same arm. While killer T cells attack the virus outright, antibodies settle into the virus and either stop it from doing more harm or help their mechanisms in the body destroy it.

According to Professor Martina Sester, another of the study’s authors, “the antibodies in the ipsilaterally vaccinated subjects were better at binding to the viral spike protein” of the virus.

The study shows how the reason why some people react differently to vaccines could go further than just age, sex and medical conditions, Dr. Ofer Levy, a pediatric infectious disease specialist and director of the Precision Vaccines Program at Boston Children’s Hospital, told USA Today.

“This speaks to precision vaccination in the sense that everything matters,” said Levy, who is not affiliated with the study.

“We need to be precise about how we discover, develop and deliver vaccines,” Levy said.