Researchers are always looking for ways to make vaccines more effective, but the latest find is a bit of an unusual one. According to a new study, alternating arms when receiving the first two doses of a COVID-19 vaccine can modestly improve the body’s immune response.

The potential impact of switching arms for multi-dose vaccines has long been a subject of research, albeit with mixed results. Researchers from the Oregon Health & Science University looked to build upon the limitations of these previous studies, recruiting a large sample of people and tracking their immune responses for longer.

After receiving two doses of a COVID-19 vaccine, the team followed 947 participants over the course of just over a year, collecting blood samples at various points and analyzing them for the level of antibodies to SARS-CoV-2. 

Those who had received the second shot in their other arm were found to have higher levels of antibodies in their blood compared to those who had received it in the same arm, with the effect increasing over time. The improved response was first made clear three weeks after the second dose, and by 14 months after vaccination, the increase was 1.4-fold.

The study also put 108 people into 54 pairs, matched based on age, gender, and time between vaccination. One person in the pair received the two doses in one arm, while the other received them in both. Although there didn’t appear to be much of a difference two weeks after the second dose, after three weeks, those who received the doses in alternate arms showed significantly higher SARS-CoV-2 antibody levels.

Quite why this happens is not entirely clear, though the researchers think it could have something to do with activating new immune responses in the different lymph nodes in each arm. “By switching arms, you basically have memory formation in two locations instead of one,” explained senior author Marcel Curlin in a statement.

The results of the study are in contrast with research published last year that suggested receiving the initial two vaccines and boosters in the same arm could be the most effective method. However, it should be noted that the earlier study only looked at the immune response at two weeks, not three which could explain why they may not have seen the same effect.

Though the effect seen in the current study was significant, there was a range of responses, all the way from 1.3 to 4-fold increase. However, Curlin said that “any incremental improvement might save a lot of lives.”

It’s hoped that the improved immune response seen in this particular study might also be seen in other multidose vaccines, though the team cautioned that further research would be required for both those and COVID-19 vaccines before any official clinical recommendations could be made.

“I’m not making recommendations at this point, because we need to understand this a lot better,” Curlin told The New York Times. “[But] all things being equal, we ought to consider switching up the arms.”

The study is published in the Journal of Clinical Investigation.