One arm or two? How you get vaccinated can make a difference.

If you have been offered the same arm for each dose of a particular vaccine, you may want to reconsider. Alternate arms can trigger a more powerful immune responsesuggests a new study.

The researchers studied responses to the first two doses of Covid-19 vaccines. Those who alternated arms showed a small increase in immunity compared to those who received both doses in the same arm.

For individuals who respond poorly to vaccines due to age or health problems, even a small boost could prove significant, the researchers said. At this point in the pandemic, when most people have had multiple vaccine doses or infections, switching arms for Covid vaccines may not offer much benefit.

But if the results are confirmed by further research, they could have implications for all multiple-dose vaccines, including childhood immunizations.

“I'm not making any recommendations at this time because we need to understand this much better,” said Dr. Marcel E. Curlin, an infectious disease physician at Oregon Health & Science University who led the study.

But “all things being equal, we should consider switching weapons.”

The few studies comparing the two approaches have been small And have produced mixed results. And none of the studies showed a big difference in the field of immunity.

A mouse study found that a single lymph node can generate strong immunity after vaccination, says Jennifer Gommerman, chair of the University of Toronto's immunology department, who was not involved in the new research.

“This means the lymph nodes are really good at their job,” she said, and most vaccines will do well if they target one arm.

According to Dr. said Gommerman.

Still, it's worth studying all these strategies, because in people with weakened immune systems, “anything that helps their immune response is worth it,” she added.

In the new study, Dr. Curlin and his colleagues repeatedly measured antibody levels in 54 pairs of university employees matched for age, gender and time since vaccination.

The participants, part of a larger research project, were randomized to receive the second dose in the same arm as the first dose or in the opposite arm. The researchers excluded anyone who became infected with Covid during the study.

Swapping the arms increased antibody levels in the blood by as much as fourfold, the scientists found. The results have been published in The Journal of Clinical Investigation.

The immune response was stronger against both the original coronavirus and the Omicron variant, which emerged about a year after the first Covid vaccines were approved.

“It's a consistent, statistically significant effect; is quite extensive; and it seems to be quite durable,” said Dr. Curlin.

The results initially seem to contradict those of a German study last summer, which showed that rolling up the same sleeve every time could produce a better immune response. But in that study, antibody levels were measured just two weeks after the second dose.

The new research also found similar results during that period. But the pattern slowly changed over the following months to higher antibody levels in those who switched arms.

The results of the new research did not entirely surprise the German researchers.

“What they see is an option that I had in mind as a possibility, so in a way it's interesting that they actually see these kinds of changes in the effects,” says Martina Sester, an immunologist at Saarland University in Saarbrücken, Germany.

Switching arms with each dose could be “one part of many measures you can easily take to perhaps lead to a successful immune response,” said Dr. Sester.