[1] Inst Nacl Enfermedades Resp, Mexico City 14080, DF, Mexico
[2] Pontificia Univ Catolica Chile, Santiago, Chile
[3] Univ Republica, Montevideo, Uruguay
[4] Cent Univ Venezuela, Caracas, Venezuela
[5] Univ Fed Sao Paulo, Sao Paulo, Brazil
[6] Univ Fed Pelotas, Pelotas, Brazil