Omicron has swiftly become the dominant coronavirus variant in the United States, leading to a growing number of new infections every day. Here are the most common symptoms to expect from the omicron variant—and how they compare with delta.

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Omicron displaces delta in the U.S., but it may be milder

Omicron's increased transmissibility has allowed it to quickly overtake delta and become the dominant coronavirus variant in the United States. According to CDC data, the omicron variant made up 99.5% of all new coronavirus infections in the United States the week ending Jan. 15—a significant jump from the month before, when omicron made up only 38.2% of new cases.

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However, despite its increased transmissibility, early research suggests omicron may result in milder illness than other coronavirus variants.

For example, a preprint study from Kaiser Permanente Southern California, which was conducted in collaboration with CDC, found that omicron infections were tied to significantly lower risks of hospitalization, ICU admission, and death than delta infections.

Researchers analyzed data from 52,297 omicron cases and 16,982 delta cases. Overall, they estimated that, compared with delta infections, omicron infections were associated with a 53% reduction in symptomatic hospitalizations, a 74% reduction in ICU admissions, and a 91% reduction in deaths.

The study also found that omicron patients had shorter hospital stays than delta patients. "The duration of hospital stays was approximately 70% shorter, with the median of stays being 1.5 days for omicron, compared to about 5 days for delta," said CDC director Rochelle Walensky.

How omicron's symptoms compare with delta's

Not only may omicron lead to milder disease, but it may also result in different symptoms than infections from the delta variant, according to a new study released by the U.K. Health Security Agency.

For the study, researchers from Oxford University and the Office for National Statistics used data from a December 2021 U.K. Covid-19 infection survey to analyze 182,133 omicron cases and 87,920 delta cases. Individuals were asked if they had common Covid-19 symptoms, including sore throat, fever, cough, shortness of breath, loss of smell or taste, and fatigue.

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Overall, the researchers found that while some symptoms, including muscle pain, nausea, and fatigue, were reported equally in both omicron and delta cases, other symptoms were more commonly reported for one variant than the other.

For example, a sore throat, fever, and cough were reported more often in omicron cases than delta cases. On the other hand, loss of smell or taste, red or irritated eyes, sneezing, runny nose, and shortness of breath were more commonly reported among delta patients than omicron patients.

Although it is not yet clear why omicron results in different symptoms than other coronavirus variants, some early studies suggest that omicron may replicate differently in the body. For example, a recent preprint study found that omicron's viral load peaked in saliva swabs one to two days before it peaked in nasal swabs, meaning the variant seemingly replicates more rapidly in the throat. This could be why sore throats are more common among omicron cases.

In addition, health experts have noted that an individual's symptom severity is affected by their vaccination status, even with a supposedly "milder" variant like omicron.

"People that are unvaccinated go through a little bit of a longer and tougher course," said Jorge Moreno, an assistant professor of medicine at Yale School of Medicine. "People that are vaccinated have a middle-of-the-way course. The boosted people, in many cases it's almost like an old cold: the sinus symptoms, the sore throat." (Bendix/Gal, Insider, 1/14; Scribner, Deseret News, 1/17; NBC New York, 1/14; Miller, FOX 32 Chicago, 1/17; U.K. Health Security Agency technical briefing, 1/14; contains public sector information licensed under the Open Government Licence v3.0)