1. Department of Mathematics and Statistics, Georgia State University, 30 Pryor Street, Atlanta, Georgia 30303, USA
2. Division of Viral Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road NE, Mailstop H17-6, Atlanta, Georgia 30329, USA
3. Lobachevsky State University of Nizhny Novgorod, 23 Gagarin Avenue, Nizhny Novgorod, 603950, Russia