Under-ascertainment of medically attended tick-borne encephalitis virus infections by tick-borne encephalitis surveillance in selected areas of Austria, Denmark, Finland, Germany, Latvia, Sweden, and Switzerland.

OBJECTIVES: Antibodies against tick-borne encephalitis (TBE) virus (TBEV) nonstructural protein 1 (NS1) immunoglobulin G are elevated in TBEV-infected individuals, but not in TBE vaccinated individuals, enabling estimation of TBEV infection prevalence in countries where TBE vaccines are used. METHODS: We used data from NS1 seroprevalence studies to estimate the incidence of medically attended TBEV infections and then compared this incidence with the incidence of surveillance-reported TBE cases to estimate the under-ascertainment multiplier for medically attended TBEV infections identified by surveillance. RESULTS: The prevalence of NS1 antibodies ranged from 0.1% to 7.0% in the identified nine seroprevalence studies. Medically attended TBEV incidence in areas where seroprevalence specimens were collected ranged from 0.5/100,000 population per year in Denmark to 21.5/100,000 population per year in Latvia. The under-ascertainment multiplier was 0.9 in Finland, 2.2 in Latvia, 2.5 in Switzerland, 3.4 in Sweden, 3.6 in Austria(Northern), 3.8 in Germany (Bavaria), 5.0 in Austria (Western), 5.0 in Denmark, and 6.0 in Germany-Baden/Wuerttemberg. CONCLUSION: Despite assumptions necessitated by data gaps, the results of this analysis indicate that more individuals are infected with TBEV and seek medical care than are ascertained by TBE surveillance. Given the increasing public health threat posed by TBE in Europe, further efforts are needed to improve TBE awareness and increase TBE vaccination among persons at increased risk of TBEV infection.