Higher level of antibodies when influenza vaccine and COVID-19 booster are administered intermittently Concurrent administration of both vaccines is safe, but the quantitative and functional antibody responses against SARS-CoV-2 were marginally lower compared to booster vaccination alone

Elderly have more antibodies against COVID-19 when they receive a COVID-19 booster and influenza vaccination subsequently than when they receive them at the same time. This possibly means that concurrent administration of the COVID-19 booster and influenza vaccine provides lower protection against COVID-19, but follow-up studies are necessary to confirm. No additional side effects occur with concurrent administration. 

Every fall, people over 60 and people in frail health receive an invitation for an influenza vaccination. In recent years, people have also been invited for a COVID-19 booster during this period. In view of a possible resurgence of SARS-CoV-2 in the winter season, the question arose whether concurrent administration of both vaccines is possible, or whether it would be better to have a few weeks in between. Radboud university medical center initiated a study into this. 

The results of this study showed that people who first received the COVID-19 booster and three weeks later the influenza vaccination produced more antibodies than people who received both vaccinations concurrently. In addition the antibodies of that first group, were able to neutralize the virus better than the antibodies of the people who got them combined.

Research leader Mihai Netea, professor of internal medicine at Radboudumc: ‘We see a strong immune response against the coronavirus in both groups. However, the immune response was lower in people who received the vaccines combined. Whether this actually leads to lower protection cannot be determined with certainty. There may be clinical implications. For that, follow-up studies are needed, in which we follow the participants for longer periods of time.’

He does stress that it is an important aspect to take into account in future vaccination campaigns for the elderly: 'The findings of this study underline the need for more research into the effects on the immune system. Now we are talking about a combination of the influenza vaccine and COVID-19 booster, but it also applies to other vaccine combinations.'

People who received the COVID-19 booster and influenza vaccine at the same time had no more side effects than those who received them subsequently. Physician-researcher and first author Elisabeth Dulfer: ‘This means that concurrent administration of the vaccines is safe. For policy makers, these results provide important information when planning a new vaccination campaign in the fall.’

About the study design

154 participants  aged 60 years and older were given an influenza vaccine and COVID-19 booster at different times and in different sequences. Their blood samples were used to examine how their immune systems responded to these vaccinations. Participants also kept track of how much they suffered from any side effects. The participants were followed up to six weeks after the first vaccination. ZonMw subsidized this research, the TACTIC study. The researchers have offered their results to the Ministry of Health, Welfare and Sport.

About this publication

This article appeared in The Lancet Regional Health - Europe: Timing and sequence of vaccination against COVID-19 and influenza (TACTIC): a single-blind, placebo-controlled randomized clinical trial - Elisabeth A. Dulfer, Büsra Geckin, Esther J.M. Taks, Corine H. Geurts-van Kessel, Helga Dijkstra, Liesbeth van Emst, Christa E. van der Gaast-de Jongh, Djenolan van Mourik, Petra C. Koopmans, Jorge Domínguez-Andrés, Reinout van Crevel, Josephine S. van de Maat, Marien I. de Jonge, Mihai G. Netea.

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