Study: Getting COVID-19 shots in the same arm might boost immune response 

A small study by German researchers published this month indicated there is a greater immune response when individuals receive both COVID-19 vaccine shots in the same arm, an intriguing premise that local physicians say should be studied further.

The study looked at 303 people who received the two-shot COVID-19 vaccine about five weeks apart: 147 individuals received the second dose in the same arm as the first and 156 received the second dose in the opposite arm.

The researchers found that the immune response was stronger in those who received both vaccine doses in the same arm, possibly because the lymph node that triggered the immune response on that side was already stimulated by the first shot. 

The study was published Aug. 11 in eBioMedicine, a peer-reviewed medical journal that's part of The Lancet family journals. The Lancet is the highly regarded British medical journal.

The authors of the study say that the "killer T-cells," which attack and destroy other cells, were present at 67% in those who received both COVID vaccine shots in the same arm, versus 43% who received the second shot in the opposite arm.

"Both responses are robust," said Dr. Matthew Harris, a pediatric emergency medicine physician and medical director of Northwell Health's COVID-19 vaccine program.

Harris added that the study was a "small, but interesting" one. "It sort of asks a simple question that hasn't really been addressed before."

The authors noted the study's limitations, but said: "The choice of arm for the second vaccination represents a previously unappreciated factor that may contribute to overall vaccine effectiveness on a population level."

Harris said there were two things to consider about the study: "They don't have any long-term follow-up," which he said the study authors also noted. Second, he said they don't address whether giving the shot in the same arm makes the vaccine last longer.

Similarly, Dr. Susan Donelan, medical director of health care epidemiology at Stony Brook Medicine, saw some drawbacks to the study.

"In brief, this is a small study which raises some interesting points that would have to be studied further in a larger group," Donelan said in a statement.

She noted the weeks' time span between the two doses "is relatively small compared to getting a booster months to more than a year after the last shot. Would that 'advantage' still hold up for people who are four months, six months or 12 months out from their last vaccine? Still to be determined."

Harris said that while individuals' COVID-19 vaccine card doesn't indicate in which arm previous vaccine shots were given, it was "common practice" for most medical practitioners "to document the site of a vaccine," which he said should be on record when individuals return to the same place for their vaccines.

Despite unanswered questions in the study, Harris said it was, at the very least, "timely," noting an updated COVID-19 vaccine is expected to come on the market by the end of September.

"Folks should be having these discussions," he said. "This study is a way to generate conversations again about the COVID-19 vaccine." 

By Olivia Winslow

olivia.winslow@newsday.com@newsollie