As rabbit polyclonal anti-SARS S1 antibodies did not show significant neutralization activity against SARS-CoV-2 S pseudovirions, we then asked whether sera from recovered SARS and COVID-19 patients could neutralize SARS-CoV S and SARS-CoV-2 S pseudovirions. We obtained sera from one recovered SARS patient and five recovered COVID-19 patients and tested their neutralization activities against transduction on 293/hACE2 cells by SARS-CoV and SARS-CoV-2 S pseudovirions. Serum from recovered SARS patient showed strong inhibition on transduction by SARS-CoV S pseudovirions, and it had modest neutralization activity against SARS-CoV-2 S pseudovirons (Fig. 6a and Table 1). In contrast, sera from all five COVID-19 patients neutralized SARS-CoV-2 S pseudovirions effectively, and none of them had an effect on transduction by SARS-CoV S pseudovirions (Fig. 6b and Table 1).
Fig. 6 Limited cross-neutralization of SARS and COVID-19 sera.
All sera were incubated on 56 °C for 30 min to eliminate complement. SARS-CoV S and SARS-CoV-2 S pseudovirons were pre-incubated with serially diluted SARS patient serum (a) or COVID-19 patient sera (b) for 1 h on ice and then added on 293/hACE2 cells. Pseudoviral transduction was measured 40 h later. Experiments were done in triplicates and repeated twice, and one representative is shown. Error bars indicate SEM of technical triplicates. Source data are provided as a Source Data file.
Table 1 Neutralization activities of antisera from SARS-CoV and COVID-19 patients.
Patient serum SARS-CoV S pseudovirion SARS-CoV-2 S pseudovirion
SARS-CoV patient >80 40
COVID-19 patient #1 Not detected >80
COVID-19 patient #2 Not detected 40
COVID-19 patient #3 Not detected >80
COVID-19 patient #4 Not detected >80
COVID-19 patient #5 Not detected >80
SARS-CoV and SARS-CoV-2 S pseudovirions were pre-incubated with serially diluted patient sera for 1 h on ice, then virus-antibody mixture was added on 293/hACE2 cells. Cells were lysed 40 h later and pseudovirus transduction was measured. The highest dilution of the serum sample that decreased transduction by 50% or more was considered to be positive. Source data are provided as a Source Data file.