[1] Swedish Institute for Infectious Disease Control,
[2] 171 82 Stockholm,undefined
[3] Sweden e-mail: akelun@mbox.ki.se Tel.: +46-8-4572641 Fax: +46-8-314744,undefined
[4] Institute of Experimental and Clinical Medicine,undefined
[5] 11619 Tallinn,undefined
[6] Estonia,undefined
[7] Tartu University,undefined
[8] Hospital of Infectious Diseases,undefined
[9] 51004 Tartu,undefined
[10] Estonia,undefined
[11] Microbiology and Tumourbiology Center,undefined
[12] Karolinska Institute,undefined
[13] 171 77 Stockholm,undefined
[14] Sweden,undefined
[15] Haartman Institute,undefined
[16] Department of Virology,undefined
[17] POB 21,undefined
[18] 00014 University of Helsinki,undefined
[19] Helsinki,undefined
[20] Finland,undefined