Tick-borne encephalitis vaccination: schedule and seasonality

Tick-borne encephalitis is a viral neuroinfection from the flavivirus family, transmitted primarily by the bite of an infected tick. There are no specific antiviral drugs for treatment, so the primary prevention strategy is vaccination and bite protection. In recent years, an expansion of the area and foci of infection has been reported, including in Central and Northern Europe, increasing the importance of prevention for residents and travelers. The vaccine is inactivated, so it does not contain live virus and cannot cause disease. [1]

In the United States, the first vaccine against tick-borne encephalitis was licensed in 2021 under the trade name Ticovac, while in Europe, vaccines based on the European strain of the virus have long been used. Their effectiveness has been confirmed by long-term observations and immunization programs in endemic countries. [2]

Who is eligible for vaccination and when?

Vaccination is recommended for people living in endemic areas or planning to travel to such areas during tick season if they plan to spend a lot of time outdoors. High-risk groups include hikers, campers, mushroom pickers, fishermen, cyclists, hunters, forestry and agricultural workers, military personnel, and field researchers. The vaccine is also recommended for laboratory workers with potential exposure to the virus. The decision is based on a combination of factors: the likelihood of exposure to ticks, the duration and nature of the activity, the season, age, underlying medical conditions, and personal risk tolerance. [3]

European and British sources are emphasizing the same: the vaccine is considered for anyone living in or frequently visiting outbreak areas, especially those planning outdoor activities. Travelers are advised to consider local risk maps and seasonal peaks. [4]

What vaccines are used and what do they consist of?

Two European inactivated whole-virus vaccines—FSME-IMMUN and Encepur—are produced using chicken embryo cell cultures, adsorbed on aluminum hydroxide, and stabilized with albumin. They are registered under national names in various countries, including Ticovac. They may contain traces of substances from the manufacturing process in minimal quantities. This technology results in the presence of trace amounts of chicken-derived proteins and antibiotics, which is important for assessing allergic risks. [5]

For Ticovac, regulatory documents detail the doses, excipients, and process impurities, including trace amounts of chicken-derived proteins, neomycin, gentamicin, and formaldehyde within acceptable safety limits. This is standard for inactivated cell culture-based vaccines. [6]

Table 1. Inactivated vaccines against tick-borne encephalitis: main parameters

Parameter	FSME-IMMUN Ticovac	Encepur
Type	Inactivated whole virus preparation	Inactivated whole virus preparation
Cellular substrate system	Chicken embryo fibroblast cells	Chicken embryo fibroblast cells
Age forms	Children's 0.25 ml, adults 0.5 ml	Children's 0.25 ml, adults 0.5 ml
Adjuvant	Aluminum hydroxide	Aluminum hydroxide
Features of the composition	Traces of chicken protein, antibiotics, albumin	Traces of chicken protein, antibiotics
Registration	Europe, USA as Ticovac	Europe, national registrations

Sources: registry documents and summary reviews. [7]

Vaccination schedules: standard, accelerated, booster

The basic course consists of 3 doses. For children aged 1 to 15 years, the dose is 0.25 ml, and for people aged 16 years and older, 0.5 ml. The first and second doses are administered at intervals of 1 to 3 months for children and 14 days to 3 months for adults. The third dose is administered 5 to 12 months after the second. Vaccination should be completed no later than 1 week before expected exposure to ticks. [8]

Accelerated schedules are available for faster protection. For Encepur, a so-called "express schedule" is available: doses on days 0, 7, and 21, followed by a booster dose 12 months later. For FSME-IMMUN, an accelerated schedule is used: days 0, 14, followed by a third dose 5-12 months later. The choice of schedule depends on age, time before the season, and clinical circumstances. [9]

A booster dose is indicated 3 years after completion of the basic course if the risk of exposure remains. Subsequently, the booster interval in most European recommendations is 5 years for individuals younger than the threshold age and 3 years for older individuals, as their serological responses decline more rapidly. The specific age threshold and intervals depend on the instructions for the specific vaccine. [10]

Table 2. Administration schemes by age and purpose

Group	Basic 3-dose regimen	Accelerated options	First booster	Subsequent boosters
Children 1-15 years old	Day 0, after 1-3 months, after 5-12 months	For Encepur, completion in 21 days is possible according to indications	In 3 years	Usually every 5 years, see instructions
16 years and older	Day 0, after 14 days-3 months, after 5-12 months	FSME-IMMUN option with a dose on day 14 in case of time constraints	In 3 years	5 years for younger children, 3 years for older children, see instructions

Sources: full text of Ticovac instructions and materials on Encepur. [11]

Efficiency and population effect

Immunization demonstrates high levels of seroprotection after completion of the course and a significant reduction in disease incidence nationwide with widespread coverage. An analysis of the Austrian program showed a sixfold reduction in disease incidence as vaccination coverage increased. This is a classic example of a population effect in an endemic country. [12]

Current estimates in the Baltic countries for 2025 show efficacy of at least 97% with at least three doses as prescribed. For Austria, an analysis of hospitalizations from 2000 to 2024 shows the prevention of thousands of hospitalizations and hundreds of severe cases with high adherence to boosters. These data confirm that vaccination works both at the individual level and within the healthcare system. [13]

It is known that the immune response may be weaker in older age groups, so adherence to booster intervals is especially important, and more frequent revaccinations are sometimes discussed. This is consistent with immunological observations and practical recommendations. [14]

Table 3. Efficiency and impact on population health

Study area	Design and period	Final indicator	Conclusion
Austria, national program	Observational data, many years	Approximately six-fold reduction in incidence with increased coverage	Mass vaccination dramatically reduces the incidence of disease
Baltic States	Performance Assessment 2025	Efficiency is not less than 97% with a course of 3 doses	High individual protection when following the diagram
Austria, 2000-2024	Analysis of hospitalizations	Hundreds of severe cases prevented, boosters make a significant contribution	The importance of booster vaccination

Sources: summary reports and publications. [15]

Safety and tolerability

Inactivated tick-borne encephalitis vaccines have a good safety profile. The most common adverse reactions are local pain and sensitivity at the injection site, and some vaccinated individuals experience short-term malaise or headache. In pediatric studies, the incidence of fever after the first dose is higher in young children and decreases after subsequent doses. Serious adverse events are extremely rare and are generally not causally related to vaccination. [16]

The combined international experience spans tens of millions of administered doses, with a favorable benefit-risk ratio. Accumulated data from regulators and expert bodies confirm good tolerability, especially after booster doses, given an already established immune response. [17]

Table 4. Frequent reactions and their estimated frequency according to the regulator data

Reaction	Children 1-15 years old	Adults 16-65 years old	Comment
Sensitivity at the injection site	About 18%	About 30%	It goes away on its own
Pain at the injection site	About 11%	About 13%	Mild to moderate
Fever	About 10% after the first dose, lower after the following	Rarely	Higher in children, transiently
Headache, fatigue	Up to 11% and up to 7%	Up to 6% each	The malaise is short-term

Source: Ticovac complete instructions. [18]

Contraindications and precautions

An absolute contraindication is a severe allergic reaction to any component of the vaccine, including trace substances from the manufacturing process. Since the vaccine is grown in chicken embryo cells, vaccination is contraindicated in individuals with severe hypersensitivity to egg or chicken protein. The vaccine also contains albumin and trace amounts of antibiotics, requiring caution in patients with relevant allergies. [19]

Pregnancy is not a strict contraindication, as the vaccine is inactivated. Several European guidelines permit vaccination for clear indications when the risk of infection is high and the benefits outweigh the theoretical risks. Breastfeeding is also not considered a contraindication for the administration of inactivated vaccines, according to best practices. [20]

In people with immunodeficiencies, the immune response may be reduced, so more careful timing and prioritizing completion of the baseline series before the season may be necessary. For older people, booster discipline is important due to a more rapid decline in titers. [21]

Table 5. Contraindications and precautions

Category	What to consider	Practical tactics
Severe allergy to components	True anaphylaxis to egg, chicken protein, albumin, and the listed antibiotics	Do not vaccinate, consider alternatives for bite prevention
Acute illness with fever	Postpone vaccination	To be administered after recovery
Pregnancy, breastfeeding	Inactivated vaccine, possible in high risk situations	Individual assessment of benefits and risks
Immunodeficiency, immunosuppression	Reduced response	Strict adherence to schemes, control of boosters

Sources: registry documents and recommendations. [22]

Compatibility with other vaccinations

Experience with concomitant administration with other vaccines is limited, but available studies show no clinically significant interactions when administered concurrently with other vaccines using different syringes and different anatomical sites. General guidelines for inactivated vaccines permit concomitant administration if necessary. [23]

Post-exposure prophylaxis: what to do after a tick bite

Vaccination specifically "post-bite" in previously unvaccinated individuals is not recommended, as the immune response will not have time to develop in time. Specific immunoglobulin against tick-borne encephalitis virus is no longer used in European countries due to concerns about its effectiveness and potential adverse effects on the course of the disease. The approach involves monitoring the patient's well-being, and if symptoms develop, early diagnosis and supportive therapy. [24]

It is important to remember that in addition to tick bites, infection is possible through alimentary means through unpasteurized dairy products from infected animals, so when on holiday in endemic regions, you should avoid unboiled milk and homemade cheeses. [25]

Table 6. Actions after a tick bite in an unvaccinated person

Step	Content
Immediate action	Carefully remove the tick, treat the skin, record the date and region
Observation	Monitor your health for 28 days; if you experience fever or neurological symptoms, consult a doctor.
Vaccine	Do not use as a first aid after a bite in non-immune individuals.
Immunoglobulin	Not recommended in European countries
Prevention further	Schedule a vaccination course before next season

Sources: position papers and clinical reviews. [26]

Practical tips for course planning

If there are 2-3 months left before the start of the active season, the standard regimen allows for the first two doses to be completed comfortably before peak tick activity. If time is less available, the accelerated Encepur regimen, with completion within 21 days and a booster after 12 months, or the accelerated start of FSME-IMMUN with a dose on day 14, can be considered. The third dose is always scheduled for 5-12 months. The decision is based on age, availability of the specific vaccine, and timing. [27]

If there is prolonged or recurring risk, remember to take a booster after 3 years and further boosters according to age and instructions. For the elderly and those with compromised immune systems, it is especially important to follow the intervals. [28]