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 ≈50–100 focus-forming units per milliliter SNV (isolate SN77734, titer 2 × 10⁶/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 cannot 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.