What are the symptoms of a COVID-19 omicron variant infection?

With cases of omicron rising throughout the United States, Americans are scrambling to distinguish the symptoms of this new variant from those of other coronavirus variants, including delta.

Most PCR and rapid antigen tests can detect omicron — the Food and Drug Administration has noted there are only a few tests that don't — but results do not indicate to the user which variant they are infected with, leaving people to guess.

Some symptom differences have emerged from preliminary data, but experts are not certain they are meaningful. Data released last week from South Africa's largest private health insurer, for instance, suggest that South Africans with omicron often develop a scratchy or sore throat along with nasal congestion, a dry cough and muscle pain, especially low back pain.

But these are all symptoms of delta and of the original coronavirus, too, said Ashley Z. Ritter, a nurse practitioner at the University of Pennsylvania. Given that omicron has been circulating for only about three weeks, she added, "it's still too early to say that there's any difference in symptoms between the omicron variant and previous versions."

It's likely that the symptoms of omicron will resemble delta's more than they differ.

"There's probably a huge amount of overlap between omicron and the prior variants, because they are essentially doing the same thing," said Dr. Otto O. Yang, an infectious disease physician at the University of California, Los Angeles, David Geffen School of Medicine. "If there are differences, they're probably fairly subtle."

One possible difference is that omicron may be less likely than earlier variants to cause a loss of taste and smell. Research suggests that 48 percent of patients with the original SARS-CoV-2 strain reported loss of smell and 41 percent reported loss of taste, but an analysis of a small omicron outbreak among vaccinated people in the Netherlands found that only 23 percent of patients reported loss of taste, and only 12 percent reported loss of smell. It's unclear, though, whether these differences are because of omicron or some other factor, like vaccination status.

Indeed, many COVID-19 symptoms vary depending on a person's vaccine status. Maya N. Clark-Cutaia, an assistant professor at the New York University Meyers College of Nursing who has been following up with COVID-19 patients throughout the pandemic, said that vaccinated patients with delta or the original coronavirus tend to present with headache, congestion, sinus pressure and sinus pain, while unvaccinated patients are more likely to have shortness of breath and cough, along with flulike symptoms.

With omicron, Dr. Clark-Cutaia said patients she's talked to in Pennsylvania are presenting with similar symptoms to delta. Vaccinated omicron patients complain of headaches, body aches and fever — "like a really bad cold," she said. Unvaccinated people have the same shortness of breath, cough and flulike symptoms she saw among unvaccinated people with delta and the original coronavirus.

One other difference between omicron and other variants is that omicron seems to have a shorter incubation time — after a person is exposed, it takes as few as three days for them to develop symptoms, become contagious and test positive compared with four to six days with delta and the original coronavirus, said Dr. Waleed Javaid, the director of infection prevention and control at Mount Sinai Downtown in New York City. That could be because the variant's mutations help it attach to and go inside cells, he added.

What about the claim that omicron infections are milder than delta infections?

According to last week's data from South Africa, after controlling for vaccination status, the risk of hospitalization for adults diagnosed with omicron was 29 percent lower there than in the first wave of the pandemic, and hospitalized South African COVID-19 patients have been less likely to be admitted to intensive care.

But South Africa's observations may not apply to the United States and other countries. Most South Africans have already been infected with COVID-19, and the median age in South Africa is 27 — both of which might cause the variant to have milder effects there than in the U.S., where the median age is 38, Dr. Yang said. The data also showed that, although children tended to have mild symptoms, they were 20 percent more likely to be hospitalized during the omicron wave compared with the first wave.

"There are people who get severe illness from omicron," Dr. Javaid said. Sometimes, early mild symptoms can develop into serious symptoms later, so it's crucial that people with cold or flulike symptoms be tested and stay home. "It is still a coronavirus. We're still in a pandemic," he said.