Abstract:
Tick-borne encephalitis (TBE) is a flavivirus infection transmitted by Ixodes ticks across Eurasia. Recently, rare indigenous cases of TBE have been reported in the United Kingdom, although most documented traveller infections come from Central Europe. Populations at risk are those undertaking forest activities in the summer, which include some Australian Defence Force (ADF) personnel participating in the training mission Operation (OP) Kudu in the UK. A killed vaccine has been available in Europe for decades and has recently been registered in the USA. Despite the lack of any known military cases during the 1990s Bosnian deployments, selected USA and ADF military members are now receiving immunisation against TBE in conjunction with deployment on OP Kudu. The three-dose (0, 1, 6 months) killed vaccine was generally very well-tolerated in the US military during Bosnian deployments. TBE immunisation is similar to Japanese encephalitis in that a well-established vaccine is used for a very low-risk, high-consequence pathogen.
Key words: tick-borne encephalitis, military, travel medicine, immunisation
Clinicians should be aware of the possibility of TBE when the cause for encephalitis is not identified, even in the absence of travel to previously identified endemic regions.1
Encephalitis caused by viral pathogens is, fortunately, rare but potentially very consequential. Arboviruses spread by mosquitoes that cause encephalitis in Australia include Murray Valley Encephalitis virus (MVE), Japanese Encephalitis virus (JEV) and Kunjin Virus (KUNV). Tick-borne encephalitis (TBE) is considered confined to Eurasia and is rarely seen among travellers in Australia.2 In 2025, it was decided that TBE immunisation would be appropriate for Australian Defence Force (ADF) members sent to Europe on OP Kudu, a training mission to the Ukrainian Army largely based in the United Kingdom (See figure as well as a map of TBE risk across Eurasia from the US CDC: www.cdc.gov/tick-borne-encephalitis/data-maps/index.html). Although similar to JEV immunisation for regional Southeast Asian deployments, the TBE vaccine is not registered in Australia and must therefore be accessed through a special access scheme of the Therapeutic Goods Administration (TGA), which requires each recipient to provide informed consent.3 Although the TBE vaccine has been successfully used for decades in many European countries and travellers to Europe, this brief review has been written to provide background for ADF medical officers who are unfamiliar with the product but may be called on to supervise soldier preparation for OP Kudu.4
Figure: ADF Operation Kudu in the United Kingdom showing unit shoulder patch. Photographer LCDR Ryan Zerbe for photograph S20233386. Available from the ADF: https://images.defence.gov.au/assets/Home/Search?Query=20231031ran8557924_0005.jpg&Type=Filename
TBE epidemiology is that of an arbovirus transmitted by Ixodes ticks, typically in European forests during midyear summer months, although infection risk exists across all of Eurasia into Siberia.5 It is an uncommon infection with a few thousand cases reported each year across Europe.6 TBE may present as a biphasic febrile illness, but severe neurological symptoms, including encephalitis, may not result until weeks after exposure to a tick. Deer are thought to be the primary reservoir host, with some contribution from rodents. Its original military importance was from Soviet Russian soldiers exposed during World War II, and there were concerns that it might have been considered as a biowarfare agent.7 There is no recognised therapy for TBE other than supportive care.
European countries have been using killed TBE vaccines since the 1980s, and their use has gradually expanded across much of Europe, with emphasis on Central and Eastern Europe. During 2000–2021, over 75 million doses of TBE vaccine have been administered, and the immunisation is generally regarded as appropriate for all adult populations with a good safety profile.8 Adverse events are primarily associated with initial doses of the TBE vaccine. A recent systematic review of TBE immunisation included 17 studies.9 Cumulated information suggested that less than a quarter of vaccines had local reactions such as pain and tenderness at the injection site. Systemic reactions, such as fever, were reported in fewer than one-third of patients.9 Most studies reported no severe adverse events after TBE immunisation and those reporting severe adverse events often did not directly associate them with causation by the vaccine. United Nations forces deployed in the 1990s to the former Yugoslavia were concerned enough about TBE to vaccinate many of their soldiers, but infection rates were either non-existent or very low.10 The US Army immunised nearly 4000 soldiers deploying into Bosnia using an accelerated three-dose regimen, which was well-tolerated with less than one per cent reporting adverse events such as nausea, headache, myalgias and fever.11 Approximately 80% of US soldiers given the vaccine on days 0, 7, 28 were known to have seroconverted. The French Army concluded that the TBE vaccine was safe but cost-ineffective for preventing a rare infectious disease in soldiers.12
Understanding of TBE risk has been evolving as more physicians test for this rare disease and as ecological changes occur due to global warming. Recently, the first evidence of TBE in the United Kingdom was documented.1 With only very few reported cases from the UK, it is difficult to estimate the infection risk, but it is likely <1:100 000 exposed to summertime outdoor activities. The decision to immunise, therefore, is based on risk tolerance among otherwise well soldiers on a peacetime mission to Europe. Analogous situations in the travel medicine community have generally recommended immunisation for any person contemplating extensive outdoor exposure during the European summer.13 Despite OP Kudu’s mission to train Ukrainian soldiers, the actual documented risk in Ukraine has been very low, with the highest infection rates usually from the Baltic region.14 There have been no reports of TBE associated with distinct military missions in the recent past.
The TBE vaccine is thought to be highly efficacious, but this is difficult to test in a rare infection; >90% seroconversion rates are generally reported among travellers.4 Extending the booster doses from three to ten years for the civilian population of Switzerland did not appear to cause any increase in infections.15 Adverse events to immunisation, especially severe ones, are infrequent and the vaccine is widely regarded as safe in otherwise well travellers.8 The USA military uses the vaccine in its soldiers and their dependents in Europe, despite there being fewer than one case per year reported in the US military population.16 As the vaccine is only available under the TGA special access scheme in Australia, relatively few Australian medical officers outside of specialised travel medicine clinics have any experience with the TBE vaccine. Soldiers deploying under OP Kudu need to be counselled about the risks of infection and the expected adverse events of immunisation before giving informed consent.
To date, there have been no reports of TBE or severe adverse events with the vaccine among Australian military members. Risk and its avoidance are social constructs that evolve over time, especially in the military where some risks are accepted as part of serving the nation. Safe vaccines given for rare diseases continue to be used, but some worries increasing the total number of vaccines may cause some to refuse in the face of social media-generated antivaccine propaganda. An individual soldier’s perception of risk may be informed by their experiences during the COVID-19 pandemic more than any appreciation of rare, vector-borne diseases outside of Australia. It is the medical officer’s task to clearly state the importance of protection against infrequent but highly consequential infections such as TBE.
Author affiliations: Australian Defence Force Infectious Disease and Malaria Institute, Gallipoli Barracks, Enoggera, Queensland, Australia
University of Queensland, School of Public Health, Brisbane, Herston, Queensland, Australia
Funding: No specific funding was provided for this work. The author is an employee of the ADF, a retired US Army officer and claims no conflicts of interest.
Disclaimer: The opinions expressed are those of the author and do not necessarily reflect those of the ADF or the US Department of Defence.
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