COVID-19: Omicron infection does not provide effective immunity against other variants without vaccination

Omicron infection in unvaccinated individuals does not appear to provide effective immunity against other SARS-CoV-2 variants such as Delta, according to a new paper published in Nature. However, vaccinated individuals who were later infected with Omicron did show immunity against other variants.

Melanie Ott and colleagues studied wild-type SARS-CoV-2 (WA1), Delta and Omicron infections in a mouse model, and report that Omicron infection was significantly milder and generated a reduced immune response compared to WA1 and Delta infection. The authors collected sera from the mice seven days after infection, and tested their neutralization efficiency against WA1, Alpha, Beta, Delta and Omicron infections. Sera from Omicron-infected mice induced neutralization only against Omicron. By contrast, sera from Delta-infected mice showed effective neutralization against WA1, Alpha, Beta and Delta, and some neutralization against Omicron, and sera from WA1-infected mice showed effective neutralization against WA1 and Alpha, and some neutralization against Beta and Delta. These results were replicated nine days after infection.

The authors confirmed that Omicron infection does not provide effective neutralization against other SARS-CoV-2 variants using sera from ten unvaccinated individuals who had recovered from Omicron infection. As observed in mice, these sera showed effective neutralization against only the Omicron variant.

However, sera from vaccinated individuals with confirmed Omicron or Delta breakthrough infection showed effective neutralization against all variants. These findings suggest that breakthrough Omicron or Delta infections after vaccination can boost existing immunity by eliciting ‘hybrid immunity’ against all variants and providing broad protection against infection.