If you have presented the same arm for each dose of a particular vaccine, you may want to reconsider. Alternating arms may produce a stronger immune response, according to a new study.
The researchers studied the responses to the first two doses of the Covid-19 vaccines. Those who switched arms showed a small increase in immunity compared to those who received both doses in the same arm.
For people who respond poorly to vaccines because of age or health conditions, even a small boost can prove important, the researchers said. At this point in the pandemic, with most people having had multiple vaccine doses or infections, switching hands for Covid vaccines may not offer much benefit.
However, if confirmed by further study, the results could have implications for all multi-dose vaccines, including childhood vaccinations.
“I’m not making recommendations at this point, 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 work.
But “all things being equal, we should consider changing hands.”
The few studies that compared the two approaches were small and had mixed results. And none of the studies showed much difference in immunity.
A study in mice found that a single lymph node can generate strong immunity after vaccination, said Jennifer Gomerman, chair of the department of immunology at the University of Toronto, who was not involved in the new research.
“That means the lymph nodes are really good at their job,” he said, and most vaccines will do well targeting a single arm.
For most people, extending the interval between doses — by three to four months, as has been done in Canada for Covid vaccines, rather than the three to four weeks recommended in the United States — may provide more benefit than switching arms, said the Dr. Gomerman said.
Still, all of these strategies are worth studying, because in people who are immunocompromised, “anything that helps their immune responses is worth doing,” he added.
In the new study, Dr. Kerlin and his colleagues repeatedly measured antibody levels in 54 pairs of university employees matched for age, sex, and time since vaccination.
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.
Switching hands increased blood antibody levels up to fourfold, the scientists found. The results were published in The Journal of Clinical Investigation.
The immune response was stronger against both the original coronavirus and the Omicron variant, which appeared about a year after the first Covid vaccines were approved.
“It’s a consistent, statistically significant effect. is it big enough? and it appears to be quite durable,” Dr. Kerlin said.
The results initially appear to contradict those of a German study last summer that showed rolling up the same sleeve over and over again might elicit a better immune response. But this study only measured antibody levels two weeks after the second dose.
In that period, the new study also found similar results. But the pattern slowly shifted over the following months to higher antibody levels in those switching arms.
The results of the new study did not completely surprise the German researchers.
“What they’re seeing is an option that I had in mind as a possibility, so in a way it’s interesting that they’re actually seeing this kind of change in results,” said Martina Sester, an immunologist at Saarland University in Saarbrücken, Germany.
Changing arms with each dose could be “one part of many measures that you could easily adopt to perhaps lead to a successful immune response,” Dr. Sester said.
