Methodological differences in serum collection timing and SARS-CoV-2 strains may explain conflicting cross-immunity results.

“First and foremost, a major difference between the two studies is when the sera was collected. In the study where there was indication of cross immunity, samples were collected 3.5 years after infection. In the study where there was no cross immunity, samples were collected within 22 days, post symptom onset, which is fairly early in the maturation of the humoral immune response when antibody titers are still increasing. It is possible that some cross immunity may have been detected if the samples were collected at a later time point. Another difference between the two studies is that they use different strains of the SARS-CoV-2 virus. One was the Hong Kong strain versus the Germany strain.”