A recent study conducted with Swedish blood donors from the University of Uppsala and the Uppsala University Hospital revealed that the number of previously unrecognized cases of the Tick-borne Encephalitis Virus (TBEV) is much higher than previously thought. The findings, published in the Eurosurveillance journal associated with the European Centre for Disease Prevention and Control, indicated that a significant percentage of blood donors showed traces of prior TBEV infection than what was presumed from the reported cases.
„We were very surprised to find such a high percentage of blood donors with evidence of a previous TBEV infection. This is much more than what could have been expected based on the reported cases,“ said Bo Albinsson, a doctoral student at Uppsala University and one of the lead authors of the study.
Bo Albinsson, Doctoral Student at Uppsala University
Tick-borne encephalitis (TBE) is a severe disease that is becoming more common in many parts of Europe. In Sweden, for example, the highest number of cases, 597, of this disease were reported in 2023, according to statistics from the Swedish Public Health Agency.
However, people who develop only mild symptoms or show no symptoms at all are not captured by health services and therefore do not appear in the official statistics. Thus, the relationship between the reported cases and the proportion of infected people was previously unknown.
Conventional methods to determine prior TBEV infections are not entirely reliable as those who have been vaccinated against TBE could also test positive. Furthermore, there is no national vaccination registry against TBEV in Sweden, making it difficult to accurately estimate the number of vaccinated individuals.
In the study, blood tests from 2,700 anonymous blood donors from nine regions in Sweden were examined. The researchers used the TBE-SMIA (Suspension Multiplex Immunoassay) method developed at Zoonosis Science Center (ZSC) of Uppsala University in collaboration with the Department of Clinical Microbiology at Uppsala University Hospital. This method effectively distinguishes the antibody response after a TBEV infection from the reaction after TBE vaccination, enabling researchers to accurately determine the extent of prior infections and estimate the proportion of vaccinated population in each region.
The results showed that the proportion of blood donors with a history of TBEV infection ranged from 1 to 7% in different regions, which corresponded to more than 160,000 people aged 15 to 65 years, overall. This was significantly higher than previous estimates. The researchers also found that the proportion of blood donors vaccinated against TBEV in the different regions ranged from 8.7 to 57%, totaling over 1.6 million individuals aged 15 to 65 years in the studied regions.
„Of note is that the number of confirmed TBE cases is increasing despite the relatively good vaccination coverage. Further research is therefore necessary, such as careful mapping of the distribution of the virus in different tick populations. Our results provide an important background for future vaccination strategies, and we believe that the establishment of a national vaccination registry against TBEV is worth considering,“ said Tove Hoffman, researcher at ZSC and another lead author of the study.
The study was funded by the European Union (Horizon 2020) and SciLifeLab (Pandemic Laboratory Preparedness).
Albinsson, B., et al. (2024) Seroprevalence of tick-borne encephalitis virus and vaccination coverage of tick-borne encephalitis, Sweden, 2018 to 2019. Eurosurveillance. doi.org/10.2807/1560-7917.ES.2024.29.2.2300221.