Protective Immunity of the Primary SARS-CoV-2 Infection Reduces Disease Severity Post Re-Infection with Delta Variants in Syrian Hamsters.

The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) Delta variant has evolved to become the dominant SARS-CoV-2 lineage with multiple sub-lineages and there are also reports of re-infections caused by this variant. We studied the disease characteristics induced by the Delta AY.1 variant and compared it with the Delta and B.1 variants in Syrian hamsters. We also assessed the potential of re-infection by these variants in Coronavirus disease 2019 recovered hamsters 3 months after initial infection. The variants produced disease characterized by high viral load in the respiratory tract and interstitial pneumonia. The Delta AY.1 variant produced mild disease in the hamster model and did not show any evidence of neutralization resistance due to the presence of the K417N mutation, as speculated. Re-infection with a high virus dose of the Delta and B.1 variants 3 months after B.1 variant infection resulted in reduced virus shedding, disease severity and increased neutralizing antibody levels in the re-infected hamsters. The reduction in viral load and lung disease after re-infection with the Delta AY.1 variant was not marked. Upper respiratory tract viral RNA loads remained similar after re-infection in all the groups. The present findings show that prior infection could not produce sterilizing immunity but that it can broaden the neutralizing response and reduce disease severity in case of reinfection.