COVID-19 Vaccines More Effective When First, Second Shots Are Injected in Different Arms [Study]

A slight change when getting a COVID-19 booster could make the vaccine more effective. The process involves switching arms so that both are injected with the vaccine.

Slight Change That Could Boost COVID-19 Vaccines' Effectiveness

Throughout all that time, relatively few specialists have looked at the topic of whether vaccination order matters. According to researchers at Oregon Health & Science University (OHSU), it just might.

In a new study, experts at OHSU were asked by healthcare professionals taking part in trials on immunological response to the new COVID-19 vaccines if they should switch arms between the first and second dose. The group was unsure of what counsel to offer. Most scientists had just thought that it didn't matter up to that point.

"This question hasn't really been extensively studied, so we decided to check it out," said infectious disease specialist Marcel Curlin.

Researchers at OHSU examined the antibody levels of 947 individuals who received a two-dose COVID-19 immunization. Randomization was used to assign half of the cohort to receive the second dose in the same arm as the first dose and the other half to receive the staggered injections in different arms. Serum antibodies specific to SARS-CoV-2 were 1.4 times greater in individuals who received the vaccine in a separate arm four weeks following their second dose.

A subset of 108 individuals were matched into 54 pairs according to their age, gender, and immunization history, and their blood work was compared.

Serum samples taken in the weeks and months following immunization demonstrated distinct variations between the groups. Serum antibodies specific to SARS-CoV-2 were up to four times higher in those who had injections in both arms four weeks after the second injection than in those who only received injections in one arm.

Furthermore, the enhanced immune response persisted for over a year when the booster was given.

"It turned out to be one of the more significant things we've found, and it's probably not limited to just COVID vaccines," Curlin hypothesizes. "We may be seeing an important immunologic function."

Injections in both arms started to perform better than the same arm only after three weeks, according to OHSU researchers. These benefits increased gradually, peaked at week four, and continued for several months.

Why Administering COVID-19 Vaccines in Both Arms Make It More Effective?

Curlin and his associates have a notion but are unsure of the specifics of that unique function's operation. Muscle immune cells recognize the antigens in a vaccine and 'handcuff' the invaders before transporting them to lymph nodes for additional interrogation. This sends out sought signals of the invader, priming the immune system against this specific antigen.

Because the body's lymph nodes are on opposite sides, inducing an immune response on both sides may make the body more vigilant.

"By switching arms, you basically have memory formation in two locations instead of one," explains Curlin.

However, another study suggested otherwise. Research conducted in 2023 discovered that administering the COVID-19 immunization in the same arm instead of various arms resulted in greater immune responses.

Even though more research is needed to weigh the benefits and drawbacks of using different arms, Curlin says he won't think twice about switching up his booster when it comes time for another shot.

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