Sandberg et al.
Safety and immunogenicity following co-administration of Yellow fever vaccine with Tick-borne encephalitis or Japanese encephalitis vaccines: Results from an open label, non-randomized clinical trial.
PLoS Negl Trop Dis. 2023;17(2):e0010616. doi:10.1371/journal.pntd.0010616

Co-administration of vaccines serves to provide protection against multiple pathogens in a short time span, saves time and visits to health care units, and may avoid missing vaccination. Various flavivirus vaccines are available to protect against infection by TBE virus, yellow fever virus (YFV) and Japanese encephalitis virus (JEV), but safety and immunogenicity when concomitantly applied have not appropriately been studied yet.

In an open label, non-randomized clinical trial, the safety and immunogenicity were analyzed in a total of 140 healthy subjects, and the following vaccines were concomitantly delivered: TBE and YFV, JEV and YFV, and in the control group only TBE, YFV or JEV. When concomitantly applied, the vaccines were given either in different upper arms or in the same upper arm. Blood samples were taken at day 0 and several times after the first and second and, when TBE vaccine was applied, also after the third injection.

Briefly, about 77% of all adverse events (AEs) across all cohorts were mild, 21% were moderate and 2% were severe. All cohorts in which TBE vaccine was given, had a greater number of total registered AEs compared to the other cohorts, ascribed to multiple TBE vaccination (n=3). Regarding immunogenicity (measuring neutralizing antibodies), there were no statistical differences between the cohorts (only one vaccine applied or two), also with respect to the application of the vaccines in the same or different arm(s).

In summary, these results show that co-administration of YFV vaccine and TBE vaccine or JEV vaccine is feasible without increased risk of adverse events or reduced development of neutralizing antibodies against the respective flaviviruses.

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