People taking multi-dose vaccines may have a better immune response if they alternate their arms while taking each shot, a recent study suggests.

In the past, healthcare practitioners believed that the specific arm chosen for vaccine administration did not have any notable effect. However, researchers at the Oregon Health & Science University now found a four-fold increase in immune response when people alternate from one arm to the other while getting vaccinated against the SARS-CoV-2 virus.

After COVID-19 vaccines were made available in 2020, the researchers decided to test if it made a difference if they alternated arms in the two-dose regimen.

"This question hasn't really been extensively studied, so we decided to check it out. 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," said senior author Dr. Marcel Curlin.

The study evaluated the antibody response of 947 participants who received two-dose COVID-19 vaccinations during the early stages of the pandemic. The results were published in the Journal of Clinical Investigation.

"The new study tested serum samples collected at various times after vaccination. They found a substantial increase in the magnitude and breadth of the antibody response among people who had "contralateral" — or a shot in each arm — boosting compared with those who did not," the news release said.

The improved immune response was seen three weeks after the second booster and the effects lasted beyond 13 months after taking the booster shot.

"Investigators found heightened immunity to the original SARS-CoV-2 strain and an even stronger immune response to the omicron variant that emerged roughly a year after arm alternation," the news release said.

The study noted that immune response progressively increased over four weeks from 1.3-fold to as much as a 4-fold increase against the omicron variant of the virus.

The study has not investigated the reason behind the better immune response by alternating the arms. However, researchers speculate that giving a shot in each arm activates new immune responses in different lymph nodes in each arm. "By switching arms, you basically have memory formation in two locations instead of one," Curlin said.

Although the study focused on immune response against COVID-19, researchers expect similar results for other multi-dose vaccinations.

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