Limited cross-variant immunity from SARS-CoV-2 Omicron without vaccination.

Area of Science:

• Virology
• Immunology
• Public Health

Background:

• SARS-CoV-2 variants like Delta and Omicron pose global health challenges.
• Omicron typically causes milder illness than Delta, particularly in vaccinated individuals.
• The potential for Omicron to induce broad cross-variant immunity is a critical question.

Purpose of the Study:

• To investigate the humoral immune response and cross-variant neutralization following Omicron infection.
• To compare the pathogenicity and immune activation of Omicron versus Delta and WA1 variants.
• To assess the impact of Omicron infection on pre-existing immunity in unvaccinated and vaccinated individuals.

Main Methods:

• Neutralization assays using sera from infected mice and humans against various SARS-CoV-2 variants.
• Analysis of viral replication in lung and brain tissues of infected mice.
• Measurement of pro-inflammatory cytokine expression and T cell activation in lung tissues.
• Comparison of immune responses in unvaccinated individuals with Omicron infection versus Omicron breakthrough infections.

Main Results:

• Omicron infection in unvaccinated mice and humans induced limited neutralizing antibodies, primarily targeting Omicron itself.
• Unlike Delta and WA1, Omicron showed reduced replication in mouse lungs and brains, causing milder disease.
• Omicron breakthrough infections in vaccinated individuals significantly increased neutralization titers against multiple SARS-CoV-2 variants.
• Omicron infection alone did not confer broad protection against non-Omicron variants in unvaccinated subjects.

Conclusions:

• Omicron infection alone provides narrow immunity, mainly against itself, in unvaccinated individuals.
• Omicron infection can enhance existing immunity from vaccination, leading to broader protection.
• Widespread Omicron infections may not be sufficient to establish broad population-level protection against other SARS-CoV-2 variants without vaccination.