we studied 20 serum samples from survivors of confirmed hantavirus infection , 11 from chilean patients and 9 from patients in the southwestern united states . samples were collected from 8 months to 11 years after the patient was hospitalized with hcps . the neutralizing titer was measured for each sample against snv and andes virus by a focus - reduction neutralization assay in vero e6 cells , as described previously ( 7 ) . in brief , serial 2-fold dilutions of heat - inactivated patient plasma samples were made , from 1:100 to 1:1,600 , and were mixed with equal volume of 50100 focus - forming units per milliliter snv ( isolate sn77734 , titer 2  10/ml ) or andes virus ( chilean strain of human origin , isolate chi-7913 ) and incubated at 37 for 1 hour ( 15 ) . the mixture was then used to infect a confluent monolayer of vero e6 cells ( atcc crl 1586 ) in duplicate wells of a 48-well dish , with a 1.2% methylcellulose overlay in the medium to confine the virus to the foci . after incubation for 1 week , viral foci were detected with polyclonal rabbit anti - n antibody followed by peroxidase - conjugated goat anti - rabbit immunoglobulin g. foci were enumerated under an inverted light microscope . nab titers were defined as the reciprocal of the highest serum dilution that resulted in an 80% reduction in the number of foci compared to virus controls in duplicate assays . the endpoint plasma nab titers against andes virus and snv from chilean and north american survivors of hantavirus infection are shown in the table . all chilean patients had detectable plasma nab against andes virus , with titers > 1:400 in all but 1 patient . in contrast , 9 of the 11 samples failed to show nab titers > 1:100 against snv , while the other 2 neutralized snv only at low titers . similarly , all north american patients had plasma nab against snv at titers > 400 , and only 1 showed some neutralization against andes virus , at low titer . no relationship was seen between the endpoint nab titers against the homotypic virus and time elapsed from acute disease in either chilean or north american patients , nor did a particularly high homotypic titer predict that neutralizing activity would be present against the heterologous virus . in survivors of hantavirus disease who reside in chile or the united states , we found high titers of plasma nab against the type of hantavirus that is prevalent in the patient 's own region , while substantial titers against the heterologous agent of hcps were absent . in this small group of participants , nab titers did not show any readily detectable decline with time elapsed after infection ; titers as high as 1:1,600 could be detected 11 years after illness . these results suggest that plasma from patients who survive hantavirus infection is a potential source of nab and could be used as a therapeutic alternative for patients with acute disease or as a prophylactic intervention for persons who may have been exposed to the virus . the absence of in vitro cross - neutralization makes the alternative of clinically effective cross - protection less likely and discourages the use of convalescent - phase sera to treat patients whose geographic origin is different from that of the plasma donor . our results suggest that a monovalent vaccine would not elicit protection against different types of hantavirus , even when the viruses are phylogenetically as similar as snv and andes virus . the positive results of cross - protection studies in hamster models should be interpreted cautiously , since experimental infection in those studies would tend to favor unusually brisk immune responses that go well beyond eliciting nab and likely include potent cell - mediated or innate immune responses that can not be mimicked with passive immunization ( 12 ) . similarly , some component of the cross- protective efficacy observed with genetic immunizations with hantavirus envelope genes may ultimately be related to t - cell immunity ( 13 ) . from this perspective , either multivalent or region - specific vaccines may have to be developed to protect persons at high risk from this new , relatively infrequent , but still highly lethal disease .