Infectious and Asymptomatic: What Makes Omicron Different

Even as it mutates into subvariants, Omicron continues to be the dominant COVID-19 strain. The current wave of Omicron poses a high risk of reinfection, raising questions about the effectiveness of COVID-19 vaccination an “natural immunity” (prior infection) against current and emerging variants.

One study, published in Nature, evaluated the neutralizing capabilities of individuals who were fully vaccinated and/or recovering from mild COVID-19 disease.

The study was conducted by a team from the Center for Emerging Viral Diseases of the University of Geneva (UNIGE) and the Geneva University Hospital (HUG). They took blood samples from 120 study participants who had been previously infected with COVID-19, and were either unvaccinated, infected before vaccination, or infected after vaccination.

“With a mean age between 28 and 52 years, without major comorbidities and a mild to moderate form of COVID-19, this cohort represents the majority of cases in the community,” said Isabella Eckerle, MD, a lead author of the study.

The investigators sought to determine how well the antibodies generated during a prior infection were able to neutralize different COVID-19 variants. The patients were infected with either the ancestral COVID-19 strain, or Alpha, Beta, Gamma, Delta, Zeta, or Omicron BA.1.

Omicron was the only variant tested that could overcome the antibodies generated by all prior variants. Not only could Omicron evade preexisting natural immunity, but vaccine-generated immunity as well, albeit to a lesser extent.

The neutralization capacity of vaccinated persons who contract Omicron is severely decreased, though not eliminated, potentially explaining why vaccination remains superior to natural immunity alone. “Reassuringly, a combination of vaccine/infection-derived immunity leads to broader antibody responses, which could enable the transition to an endemic situation,” the study authors wrote.

“Thus, Omicron can evade existing immunity and cause an infection, but hospitalization and death due to COVID-19, even with Omicron, is still reduced after vaccination,” said Eckerle. “Vigilance is still required, especially as the epidemiological curves have been rising sharply since the appearance of BA.5, the most recent Omicron subvariant.”