The World Health Organization (WHO) recently issued warnings about Omicron, a newly emerged variant of SARS-CoV-2, the virus that causes COVID-19. Omicron is driving a substantial uptick in the number of COVID-19 cases in South Africa and other nations. The first U.S. case was recently detected in a fully-vaccinated person living in California, and several other states are now reporting Omicron cases.

The WHO has classified Omicron as a variant of concern. To be considered a variant, a virus must have sufficient mutations to change a portion of its genetic code. Variants of concern further exhibit a wide range of characteristics that differ from the parent strain, including changes in transmissibility, disease severity, clinical presentation, immune response, or response to treatments and vaccines.

Several SARS-CoV-2 variants have been identified, including Delta, a variant of concern that first emerged in India in December 2020 and has at least 13 mutations. Delta is currently the dominant form of the virus in the U.S. By contrast, the Omicron variant has approximately 50 mutations, roughly 30 of which are present in the spike protein, the major surface protein that SARS-CoV-2 uses to enter cells, where it can replicate.

Scientists do not yet know the consequences of these mutations, which have the potential to increase transmissibility or reduce vaccine effectiveness. Early reports from Israeli health officials suggest that Omicron is highly transmissible but elicits only mild symptoms in fully vaccinated people.

Although the WHO and governing bodies worldwide have responded quickly to the news of Omicron’s emergence, determining the full extent of the variant’s effects on human health may take several weeks. Public health containment strategies such as being vaccinated, wearing masks, washing hands, and physically distancing remain essential to reducing the spread of COVID-19.

• Link to WHO brief.