Omicron vs. Delta: How the 2 COVID-19 Variants Compare, According to Experts and Research

What to know about the symptoms, severity, and transmissibility of the two strains.

The Delta variant of COVID-19 is no longer the dominant cause of infections in the US. Now, just weeks after it was identified in South Africa—and subsequently named a "variant of concern" by the World Health Organization (WHO)—Omicron accounts for the majority of COVID-19 cases in the US.

As Omicron continues to sweep through the US (and the world), scientists are working to uncover what makes this variant different than previous variants. Information is coming together slowly—Omicron, for example, is less likely to cause severe illness, but it also spreads easier, even among the vaccinated population—but there are still many things we don't yet know, which is why it's more important than ever to maintain (or ramp up) COVID-19 safety precautions.

Here, with the help of infectious disease experts and research, we explain what is known about how the Omicron and Delta variants of COVID-19 compare—and what that means for your health.

Omicron vs. Delta: How the 2 COVID Variants Compare
Alex Sandoval

Symptoms

First: Keep in mind that whether we're talking about Delta or Omicron (or any other variants of COVID-19), it's still the same SARS-CoV-2 virus—and that means while certain symptoms may appear more prominent or noticeable in one strain versus the other, the symptoms of all COVID-19 variants will be similar, the CDC says.

To reiterate, the CDC says people with COVID-19 can show a wide range of symptoms, spanning from mild to severe illness. Though it's not an exhaustive list, the following symptoms that are the most common with COVID-19 in general include:

  • Fever or chills
  • Cough
  • Shortness of breath/difficulty breathing
  • Fatigue
  • Muscle or body aches
  • Headache
  • New loss of taste or smell
  • Sore throat
  • Congestion or runny nose
  • Nausea or vomiting
  • Diarrhea

There's no official guidance on which symptoms are more prevalent with certain strains of the virus, but there have been a handful of anecdotal reports on symptoms with both variants.

The Delta variant, for instance, seemed to bring on more headaches, sore throats, and runny noses, Tim Spector, a genetic epidemiologist at King's College London who helps lead the ZOE COVID Study, told The New York Times in June. Meanwhile, fever, cough, and loss of smell were reported less often.

Omicron, too, has seemingly brought on more cold-like symptoms: According to data collected again by the ZOE COVID Study—which requires people to report their disease symptoms in an app—the top five symptoms associated with confirmed or suspected cases of the Omicron variant include: runny nose, headache, fatigue, sneezing, and sore throat. Also like Delta, people with Omicron less commonly reported fever, cough, or loss of smell or taste. Overall, the ZOE COVID Study analysis found "no clear difference in the symptom profile of Delta and Omicron."

Severity

Though the symptom profile isn't too drastically different between the Delta and Omicron COVID-19 variants, data shows that Omicron appears to be milder than Delta. The most robust information pointing to the decreased severity of Omicron comes from a study funded by the Centers for Disease Control and Prevention (CDC) using data from the Kaiser Permanente Southern California health care system. Though the study was published on the preprint server medRxiv—and thus, hasn't been peer-reviewed—CDC director Rochelle Walensky, MD, MPH, tweeted out the findings on January 12.

The study looked at data from 69,279 patients—52,297 with the Omicron variant, 16,982 with the Delta variant—between November 30, 2021, and January 1, 2022. When comparing the two variants, the study found that Omicron cases resulted in 53% less risk of hospitalization, 74% less risk of ICU admission, and 91% less risk of death. The study also found that none of the patients with Omicron required mechanical ventilation.

Other preliminary evidence from researchers at Case Western Reserve University showed similar findings: Data from 577,938 first-time COVID-19 patients—14,054 who became ill when the Omicron variant emerged and 563,884 who became ill during the Delta variant's predominance—found that the infections during the Omicron variant were "significantly less severe" than those during the Delta variant.

"The symptoms are slightly less intense, and it's a milder disease that results in less hospitalization and death than Delta," Robert Murphy, MD, a professor of infectious diseases and executive director of the Havey Institute for Global Health at the Northwestern University Feinberg School of Medicine, tells Health.

The reduced severity may be more of a testament to an increasingly vaccinated or otherwise protected population, not the variant itself. According to the most recent data from The New York Times' COVID vaccine tracker, 61.3% of the global population has received at least one dose of a COVID-19 vaccine. And since the beginning of the pandemic, a total of 321,852,826 people have contracted and recovered from COVID-19. Ultimately, "a population that has been exposed to COVID-19 before, through vaccination or infection, has some semblance of previous immunity, which protects from severe disease," Joel Chua, MD, chief of surgical infectious disease at the University of Maryland Medical Center and assistant professor at the University of Maryland School of Medicine, tells Health.

Though many experts are looking at the decreased severity of Omicron with cautious optimism, it's still important to note that, "while Omicron does appear to be less severe compared to Delta, especially in those vaccinated, it does not mean it should be categorized as 'mild,'" WHO director-general Tedros Adhanom Ghebreyesus, PhD, said in a press briefing. "Just like previous [COVID-19] variants, Omicron is hospitalizing people, and it is killing people."

Hospitalization rates are rising in the US during the Omicron surge: According to the latest CDC data, an average of 20,637 people were admitted to the hospital for COVID-19 each day from January 5–January 11; that's up 24.5% from the week prior. According to Dr. Chua, this spike in hospitalizations is more likely due to the increased transmissibility of the virus—the more people who get the virus, the more of a chance for hospitalizations. Because of the decreased severity of the variant, fewer of those hospitalizations will result in severe illness or death, but "we are still seeing people die of Omicron, particularly if they are unvaccinated or vaccinated with [a weakened immune system or] medical condition," says Dr. Chua.

Transmissibility

To see how transmissible or contagious the Omicron variant is, it's helpful to look at the timeline of the spread of disease: The variant—then known as B.1.1.529—was reported to the World Health Organization on November 24, 2021, after being detected in Botswana and South Africa. The first confirmed case of Omicron in the US was identified on December 1, 2021. Now, just over a month later, the variant makes up 98.3% of all US cases, per the CDC. (The Delta variant is still active in the US, accounting for the other 1.7% of cases.)

Back when Delta was the dominant variant—from July 2021 through December 2021—the CDC reported that it was more than two times as contagious as other variants. Unvaccinated people remained at the greatest risk for Delta infections; they were much more likely to contract the virus and then spread it to others. Fully vaccinated people were still able to get the virus—known as a breakthrough infection—but at lower rates. The vaccine still provided ample protection against severe illness and death.

The data available on the transmissibility of Omicron right now is preliminary, but it shows that the variant spreads more quickly than Delta—which is what researchers suspected early on, based on Omicron's high number of mutations (there are about 50 altogether; 36 in the virus' spike protein alone).

A preprint study from Denmark shows this increased transmissibility by looking at the spread of Omicron and Delta variants within the same household. Researchers found that Omicron is about 2.7–3.7 times more infectious than Delta in vaccinated and boosted people. In unvaccinated people, however, there was no significant difference in the rates of infection. According to the study authors, if confirmed, these findings support the idea that Omicron's increased transmissibility could be due to the variant's ability to evade immunity—not some other characteristic that makes it inherently more contagious.

But the contagiousness doesn't appear to hinge on Omicron's ability to evade immunity alone. Other factors that may add to the increased transmissibility of the variant include its shortened incubation period of just three days, as compared to Delta's four-day incubation period. This could mean those exposed to the virus have less time to take precautions to protect others.

Another possible factor in increased contagiousness: Omicron's apparent ability to stay in the upper respiratory tract and multiply quicker there. A December study from the LKS Faculty of Medicine at the University of Hong Kong found that Omicron infects and multiplies 70 times faster in the bronchus (the airway that connects the trachea to the lung) than Delta and other COVID variants, which was more likely to affect the lungs. According to researchers, this "may explain why Omicron may transmit faster between humans than previous variants."

What this means for you

It's important to keep in mind that, while COVID-19 variants can differ in severity, transmission, and sometimes even how symptoms present, they're still all the same SARS-CoV-2 virus that has wreaked havoc on the global population for nearly two full years.

That means, even with some differences, the main preventive measures remain the same: vaccines, wearing face masks, practicing social distancing, and keeping your distance from people with the virus (and staying away from others if you have the virus).

Regarding the Omicron variant specifically, the importance of boosters is a key focus at the moment, along with using higher-quality face masks. "Get the vaccine and the booster when it's available to you, and if you're in a setting beyond your core group of people or family, wear a high-filtration mask like a KN95," says Dr. Chua.

The CDC backs that up, as well: On January 5, the agency expanded booster dose eligibility to those 12–15 years old, allowing people ages 12–17 to receive booster doses five months after their primary series of a vaccine, due to a booster's ability to "help broaden and strengthen protection against Omicron and other SARS-CoV-2 variants." The CDC also updated their website on January 14 to say that some masks (like cloth masks) provide lower levels of protection than properly fitted respirators (like N95 masks) which provide the most protection.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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