Results of a blood serum examination of residents of Moscow and the Moscow region after the end of the West Nile fever outbreak in 2021

The aim of this study was to determine the level of humoral immunity to the West Nile virus (WNV) in the Moscow region population after the end of the outbreak in October 2021, as well as to confirm the specificity of antibodies to WNV by comparatively testing patient sera for antibodies to the antigenically related endemic tick-borne encephalitis orthoflavivirus (TBEV) using ELISA-IgM, ELISA-IgG, and 50% plaque reduction neutralization test. Materials and methods. We analyzed 1,594 sera from outpatients-residents of Moscow and the surrounding region–who underwent outpatient examination in the winter of 2021 at Infectious Diseases Clinical Hospital No. 1 in Moscow (IKB No. 1) and medical institutions of the Moscow Regional Research Institute named after M.F. Vladimirsky Regional Research Institute (MONIKI) conducted a study without any connection to WNV infection in the summer-autumn of 2021. Results. All samples were negative in ELISA-IgM test with WNV and TBEV antigens. In an ELISA-IgG test with the WNV antigen, antibodies were detected in 64 samples (4.0%). All samples were tested for IgG antibodies to TBEV in an ELISA and neutralizing antibodies to WNV and TBEV in a 50% plaque reduction neutralization test. Specific antibodies to WNV were detected in 44 samples (68.8%), to TBEV in eleven (17.2%), and group-specific antibodies in nine (14.0%). According to the total data from the test of 1,594 sera from residents of the Moscow region, specific antibodies to WNV were detected in 2.8% of cases, to TBEV in 0.7%, and group-specific antibodies in 0.6%. Ten of the 11 individuals with specific IgG antibodies to TBEV were undergoing outpatient examination at Moscow’s Infectious Diseases Clinical Hospital No. 1 for a history of tick-borne encephalitis or for post-vaccination immunity testing following vaccination against this infection. The detection rate of specific antibodies to WNV in similar studies conducted in the same region in 2013 was 0.2%, while in 2021 it was 2.8%. The difference between these rates is statistically significant (p 0.01). Conclusion. Based on these data, it can be concluded that sporadic undiagnosed cases of WNV infection occurred in Moscow and the surrounding region between 2013 and 2021.