Results of the Screening of Tick-Borne Encephalitis Virus Antibodies in Human Sera from Eight Districts Collected Two Decades Apart

The sera from healthy individuals aged 10-59 years randomly selected from the general population during repeated cross-sectional surveys were stored at -20 degrees C at the serum bank of the National Institute of Public Health in Prague. The sera included in the present study were collected in the 1980s and in 2001 in eight districts of the Czech Republic. The proportional representation of the study localities was similar in both periods. The sera were uniformly distributed in 5-year age groups for 10- to 19-year-olds and in 10-year age groups for 20- to 59-year-olds. Males and females were nearly equally represented in both periods. Altogether 704 sera, 434 from the period 1978-1989 and 270 from 2001, were screened for antibodies against tick-borne encephalitis virus (TBEV) by the virus neutralization test. The seroprevalence rates were 11.5% in the 1980s and 26.3% in 2001. From the logistic regression model, it follows that the chance of detecting anti-TBEV antibodies was more than twice higher in 2001 than in 1978-1989 (odds ratio [OR]=2.22). The differences between males and females were not statistically significant, nor was sex-period interaction. The time trends in the seropositivity rates were similar in all age groups, with the exception of the 10- to 14-year-olds (p=0.914). The rate of seropositives in the age group 15-59 years increased 1.9 times, whereas that in the age group 10-14 years rose 5.1 times. In areas comparable to those where the study sera were collected, the average incidence rates of tick-borne encephalitis (TBE) per 100,000 population aged 10-59 increased significantly from 3.35 in 1978-1989 to 8.96 in 2001 (p<0.001). The age-specific antibody trends in adult age groups in both periods suggest that clinically manifest or inapparent TBE cases do not induce lifelong immunity, but they are likely to reflect the previous epidemiological situation.