Omicron has been shown to be highly transmissible and have extensive evasion of neutralizing antibody immunity elicited by vaccination and previous SARS-CoV-2 infection. Omicron infections are rapidly expanding worldwide often in the face of high levels of Delta infections. Here we characterized developing immunity to Omicron and investigated whether neutralizing immunity elicited by Omicron also enhances neutralizing immunity of the Delta variant. We enrolled both previously vaccinated and unvaccinated individuals who were infected with SARS-CoV-2 in the Omicron infection wave in South Africa soon after symptom onset. We then measured their ability to neutralize both Omicron and Delta virus at enrollment versus a median of 14 days after enrollment. Neutralization of Omicron increased 14-fold over this time, showing a developing antibody response to the variant. Importantly, there was an enhancement of Delta virus neutralization, which increased 4.4-fold. The increase in Delta variant neutralization in individuals infected with Omicron may result in decreased ability of Delta to re-infect those individuals. Along with emerging data indicating that Omicron, at this time in the pandemic, is less pathogenic than Delta, such an outcome may have positive implications in terms of decreasing the Covid-19 burden of severe disease.
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