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Omicron vs. Delta: Differences in transmission rates, potential symptoms, and incubation times

January 7, 2022, 7:14 PM UTC

COVID-19 infections and hospitalization rates have continued to surge in the new year thanks to the Omicron variant, which has now replaced the Delta variant as the dominant strain in the U.S.

On Wednesday, White House officials said that Omicron’s peak is likely still ahead of us, warning that the coming weeks will be “challenging.” 

And while there are still many unknowns about the Omicron variant, here’s how experts say it’s different from previous variants.  

What are Omicron’s symptoms?

Signs of COVID-19 have been well documented since the pandemic began. While people with COVID-19 have reported a wide range of symptoms—ranging from mild to severe—some of the most common ones include a cough, runny nose, fever, headache, and fatigue, according to the Centers for Disease Control and Prevention.

Throughout Delta’s peak in September, symptoms of that variant appeared to be similar to those of the initial strain of coronavirus that hit the U.S.

Since Omicron began sweeping through the U.S. last month, anecdotal reports of the variant producing some distinct symptoms have circulated among patients and health experts. 

Fever and cough are less prevalent in Omicron cases versus previous strains, according to data from the U.K. conducted by Zoe, a health science research firm, and more common Omicron symptoms include runny nose, sneezing, headaches, and fatigue.

And recently, a doctor noted that anecdotally he was seeing a new symptom among some Omicron patients: night sweats, which are repeated episodes of extreme perspiration that can soak your clothes and sheets, according to the Mayo Clinic

Unlike patients with Delta and other previous COVID strains, those with Omicron “are less likely to lose a sense of smell,” Gregg Miller, an emergency room doctor in Edmonds, Wash., and chief medical officer at Vituity, a physician-owned health care company that specializes in acute care innovation, told Fortune.

Symptoms of Omicron overall tend to be less severe than those of Delta and previous strains, according to early research from South Africa, where the variant was first detected. This may be because “there’s evidence out there that it is less likely to invade your lung tissue,” which “leads to less pneumonia and less severe disease,” Miller noted. 

This was reinforced by Dr. Anthony Fauci, the nation’s top infectious disease expert, on Wednesday, citing recent studies of Omicron’s effect on mice and hamsters. “It was shown that the virus of Omicron proliferates very well in the upper airway and bronchi, but actually very poorly in the lungs,” Fauci said at a White House press conference

However, World Health Organization official Janet Diaz, the agency’s clinical management lead, noted this week that although the variant may be less severe, Omicron should not be considered “mild.”

If I’m exposed, when will I get sick?

While much is still unknown about Omicron, experts are beginning to understand more about the variant as it continues to infect more people. One of the key observations is that people who are exposed to the strain appear to get sick more quickly.

Delta’s incubation period was about four days, and the original variant had an incubation period of about five days, across the general population. 

“Whereas with Delta, patients maybe wouldn’t develop symptoms until four or five days after infection, with Omicron, [patients] maybe are developing symptoms within two to three days after infection,” Miller said. 

Several studies of Omicron that have come out in recent weeks suggest it takes the variant about three days to develop symptoms after exposure.

How transmissible is Omicron?

The Omicron variant has managed to break records for daily positive cases in the U.S. owing to its high transmissibility compared with previous strains, according to reports from the CDC.

This is in part because Omicron is far better at circumventing the current vaccines than the Delta variant, according to a recent study conducted by researchers at the University of Copenhagen, Statistics Denmark, and Statens Serum Institut (SSI).

According to the study, Omicron was found to be 2.7 to 3.7 times as infectious as the Delta variant among vaccinated people. 

But health experts say that despite its high rate of transmissibility in comparison with other strains, Omicron is causing fewer severe cases and hospitalizations even as case numbers continue to soar. This is in part because it is hitting a population that is better prepared to battle new strains, as a  larger percentage of Americans are vaccinated against the coronavirus.

“Part of the reason we are seeing these reductions in hospitalizations and deaths from Omicron is we’ve already managed to get a large proportion of the population vaccinated and/or boosted,” Dr. William Checkley, a pulmonary and critical care physician at the Johns Hopkins Hospital and associate professor of medicine at Johns Hopkins University, told Fortune.

“For an unvaccinated person, the Omicron variant poses a risk just like Delta or the early variants prior to Delta,” he added.

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