Getting vaccines in different arms improves effectiveness, study finds

Researchers have found that getting a vaccine booster shot in a different arm from the one you got the first in may improve the body’s immune response up to four-fold. While they exclusively studied responses to the COVID-19 vaccine, they suspect this effect may be seen with other multidose vaccines.

When you get a vaccine, do you have a ‘preferred arm’ that you present to the health professional administering it? Many opt to have vaccines, including booster shots, injected into the same arm, usually to prevent the muscle ache or heaviness that follows from affecting the use of their dominant arm or hand.

Historically, the choice of arm was not thought to matter to vaccine effectiveness. However, a new study by researchers at Oregon Health and Science University (OHSU) has found that when it comes to multidose vaccines, people who have one dose in one arm and the second in the other have an improved immune response.

“This question hasn’t really been extensively studied, so we decided to check it out,” said Marcel Curlin, the study’s corresponding author. “It turned out to be one of the more significant things we’ve found, and it’s probably not limited to just COVID vaccines. We may be seeing an important immunologic function.”

The researchers recruited 947 OHSU employees who agreed to receive two-dose vaccinations against the SARS-CoV-2 virus early in the pandemic. They were randomized to get the second (booster) dose in the same or the opposite arm as the first dose. Blood samples were collected at various times after boosting and tested for antibody response.

While both groups had a similar antibody response at week two, those who’d had ‘contralateral’ shots – that is, a shot in each arm – showed a substantial increase in “the magnitude and breadth” of their antibody response by weeks three and four that progressively increased over time to up to a four-fold increase.

Additionally, the researchers found a heightened immune response to the original SARS-CoV-2 strain and an even stronger response to the Omicron variant that emerged approximately a year after the second vaccine was given.

Although the researchers can’t explain this effect, they speculate that injecting a vaccine into each arm produces a new immune response in the corresponding lymph nodes.

“By switching arms, you basically have [immune] memory formation in two locations instead of one,” Curlin said.

Although the current study focused on SARS-CoV-2, the researchers say that the improved immune response seen with contralateral injections might apply to other multidose vaccinations. Further research will determine if that’s the case, especially in children. But at this stage, they’re not advocating for a change in practice based on the results of this study.

The study was published in The Journal of Clinical Investigation.

Source: OHSU

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