PORTLAND, OR -- Researchers at OHSU say multi-dose vaccines could be more effective if given in alternating arms. 

Dr. Marcel Curlin studied blood tests from about 2,000 OHSU staffers who agreed to take part in the project, in the early days of the COVID vaccine roll-out. About half took both doses in the same arm, the other half alternated arms. "If you got your second dose in the opposite arm from the first dose, you had something like a 1.5 to two-fold increase in your antibody levels that were directed against COVID, the strain that the vaccine was made of," says Dr. Curlin.

And, when tested against future variants, like Omicron, there was a four-fold increase in the antibody response, and for a longer timeframe, "When you switch arms, you get an increase in the absolute size of the response. But you’re also getting a broader response, that’s more able to cope with slightly different variations and changes in the virus as they come in; so, new threats."

Dr. Curlin says they’re learning more about the human immune system, "When you get exposed in one place in your body, the lymphatic system on that side is what’s learning to respond. And so, if you switch arms, you’re now asking a new lymphatic center to respond. So, you may be getting two for one."

More study is needed to determine whether the results hold true with other immunizations or when administered in a leg, like for infants. But Curlin hopes it'll lead to better outcomes with other multi-dose vaccines, "If you have a marginal vaccine that’s close to maybe 50% effective, and you can be smart about how you vaccinate and inch up the efficacy by even 10 or 15%, suddenly you have something that actually works."

Overall, he's encouraged by the results, "If you’re getting vaccinated for the first time against something and you’re going to get more than one dose, it does make sense to consider alternating arms because we think there’s a chance that it could boost the immune response that you get from that."