Health & Medicine
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You can get your COVID and flu vaccinations at the same time, but what’s best - one arm or two?
You can save time and money by getting two vaccinations at the same time, and new research finds that it doesn’t really matter which arm or arms the jab goes in
Published 6 May 2025
With cases surging, it looks like we are in for a severe flu season in Australia this year.
Everyone is talking about getting your Flu vaccine, but are you also up to date with the most recent COVID booster containing the JN.1 subvariant?
COVID and Flu are major causes of illness and death in Australia and around the world. Vaccines can prevent infection or make the illness less severe, but many people can’t find the time or motivation to stay up to date.
This ‘vaccine fatigue’ means that most Australians don’t have all the recommended vaccines.
It would be nice if there were an easy way to reduce the challenge of finding time to squeeze in extra health appointments into our busy lives.
And there is.
You can get both the COVID and influenza vaccines at the same appointment – the Australian Department of Health advises that it is safe to do this and doctors and pharmacists will now allow this.
If you’re eligible to have both vaccines around the same time, then that’s potentially a great way to add value to that one GP or Pharmacy visit and save some time as well.
Sounds like a win-win situation all around.
But you have an important decision to make – should you have the vaccines on one arm or two?
Usually that’s your choice, though sometimes the healthcare worker decides. But is there any evidence to help guide that choice – is it better to have both vaccines in the same arm or one in each?
And better how? Will it hurt less, or will the immune response be more effective?
Until recently, we didn’t really know.
Studies in mice suggested that giving the COVID and Flu vaccines in the same limb could improve immunity. This is because the COVID mRNA vaccines contain a built-in adjuvant (an immune boosting molecule) called the ‘ionizable lipid’.
These are fatty compounds that form the coat on the outside of the vaccine particles and protects the mRNA they contain inside.
Most Flu vaccines do not contain an adjuvant, but it’s possible that the built-in adjuvant in the COVID vaccine could also boost the immune response to the Flu vaccine – if both vaccines were given in the same arm.
On the flipside, getting two injections in the same arm could amplify the main side-effect of the vaccine, which is soreness in the arm.
To investigate this further in people, we conducted a randomised controlled trial at the University of Melbourne with 56 healthy adults to work out whether it was better to give the Moderna COVID mRNA vaccine and the CSL influenza vaccine in the same arm or opposite arms.
People were randomly assigned to one of two groups – both vaccines in the same arm or one vaccine in each arm. People in the ‘same arm’ group had both vaccines in their non-dominant arm – so in the left arm for right-handed people.
People in the ‘opposite arm’ group had the COVID vaccine in their non-dominant arm and the flu vaccine in their dominant arm.
This was because we do know that you can get a sorer arm from the COVID vaccine compared to the Flu vaccine. We collected blood, saliva and nose fluid and asked people about how they were feeling after getting the vaccines.
We found there were no important differences in key immune responses (neutralising antibodies) to either Flu or COVID, whether you had the two vaccines in the same arm or in opposite arms.
Some of our secondary analyses, however, did show slightly higher immune responses to COVID in the opposite arm group.
While nearly all people in the trial said they had a sore arm, people in the same arm group had more tenderness (reactions) in that one arm.
People in the opposite arm group had reactions in both arms, and as expected, those reactions were more common in the non-dominant arm that received the COVID vaccine.
Importantly, we showed that giving vaccines at the same time, in either the same arm or opposite arms, provides a good immune response to these important infections.
So, if you want to reduce your time at medical appointments by having both vaccines on the same day, it’s really up to you whether you have the vaccine in one or two arms.
Getting the vaccines in opposite arms may slightly improve immunity and reduce arm tenderness, but you’ll have two sore arms.
Almost all reactions are mild though and it’s definitely worth it to protect yourself from getting a severe infection.