Randomised trial of same vs opposite arm co-administration of inactivated influenza and SARS-CoV-2 mRNA vaccines.

Background: The immunogenicity of current influenza vaccines need improvement. Inactivated influenza and COVID-19 mRNA vaccines can be co-administered but randomized controlled trial data is lacking on whether the two vaccines are more immunogenic if given in the same or opposite arms. Murine studies suggest mRNA vaccines can adjuvant influenza vaccines when co-formulated and delivered together.

Methods: We randomly assigned 56 adults to receive the Afluria quadrivalent inactivated influenza and Moderna monovalent SARS-CoV-2 XBB.1.5 mRNA vaccines, either in opposite arms or both in the same arm at the same site. The primary endpoint was the difference in median combined serum haemagglutination inhibition titre to the H1, H3 and B-Vic vaccine influenza strains after vaccination.

Results: We found no significant difference in haemagglutination inhibition antibody levels between the groups (p = 0.30), with the same arm group having a 1.26-fold higher titre than the opposite arm group. There was no difference in analyses of antibodies to individual influenza strains, nor in nasal or saliva antibody levels. While both binding and neutralising antibody titres against SARS-CoV-2 were not significantly different between groups post-vaccination, there was a higher fold-change in BA.5 and ancestral strain neutralising antibodies in the opposite arm group.

Conclusion: Influenza vaccination is equivalently immunogenic if given in same or opposite arms as the SARS-CoV-2 vaccine, but it may be preferable to administer the SARS-CoV-2 vaccine at a different site to influenza vaccines.

Trial Registration: Australian New Zealand Clinical Trials Registry ACTRN12624000445572.

Funding: Australian National Health and Medical Research Council and Medical Research Future Fund.