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.

After it was named a "variant of concern" in November 2021 by the World Health Organization (WHO), the COVID-19 Omicron variant replaced the Delta variant as the dominant cause of infections around the world.

As Omicron continues to be the variant of concern in the U.S. (and the world), according to the Centers for Disease Control and Prevention (CDC), scientists are working to uncover what makes this variant different from previous variants.

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 still important to continue to practice 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 of Delta or Omicron

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, says the CDC.

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.

Omicron 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
  • Sore throat

Also like Delta, people with Omicron less commonly reported fever, cough, or loss of smell or taste. According to a May 2022 study published in the journal Otolaryngology — Head and Neck Surgery, compared to rates of smell and taste loss during the early phase of the pandemic in 2020 before variants were identified, chances of smell and taste loss were just 17% for omicron, 44% for delta and 50% for the alpha variant.

Overall, the ZOE COVID Study analysis found "no clear difference in the symptom profile of Delta and Omicron."

Severity of Delta or Omicron

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.

Per the CDC, Omicron and its subvariants cause milder disease than previous variants like Delta. An August 2022 study published in JAMA Network Open found that over 56% of people who were likely infected with Omicron didn't even know they had the virus.

One study published in January 2022 on the preprint server medRxiv—and thus, had not been peer-reviewed—indicated decreased severity of Omicron. CDC director Rochelle Walensky, MD, MPH, tweeted out the findings in January 2022.

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 and published on medRxiv in December 2021 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, told 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 September 2022 data from The New York Times' COVID vaccine tracker, 69.7% of the global population had received at least one dose of a COVID-19 vaccine.

According to the World Health Organization (WHO), there have been a total of 610,866,075 cases of 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, told Health.

Though the CDC says Omicron results in less severe illness and death in general, it also notes that a surge in cases may lead to increases in hospitalizations and deaths.

"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 in January 2022. "Just like previous [COVID-19] variants, Omicron is hospitalizing people, and it is killing people."

Fortunately, hospitalization rates are declining in the U.S. According to the latest CDC data, the 7-day daily average for COVID-19 daily hospitalizations from September 14–20, 2022, was 3,971. This is a 9.9% decrease from the prior 7-day average.

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," said 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 the 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. Just over a month later, the variant made up 98.3% of all U.S. cases, per the CDC, comprising 100% of U.S. cases by September 2022.

And while the Omicron may be somewhat milder, it also spreads more easily than earlier variants, including the Delta variant. Researchers suspected this early on, based on Omicron's high number of mutations (there are about 50 altogether; 36 in the virus' spike protein alone).

According to the CDC, anyone with Omicron infection, regardless of vaccination status or whether or not they have symptoms, can spread the virus to others. In addition, Omicron can cause reinfection, even in people who have recovered from COVID-19.

The March 4, 2022 Morbidity and Mortality Weekly Report (MMWR) reported on a study of household transmission in four U.S. jurisdictions. The study found that Omicron infection resulted in high transmission among household contacts. This was especially true among those who lived with unvaccinated people or people who did not take measures to reduce the risk of transmission in the household.

And a Danish study published in Nature Communications in September 2022 showed the increased transmissibility of Omicron by looking at the spread of Omicron and Delta variants within the same household. Researchers found that Omicron was generally 2.4–3.2 times more transmissible than the Delta among vaccinated household contacts. In unvaccinated people, however, there was no significant difference in the rates of infection.

According to the study authors, 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 2021 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 since early 2020.

That means, even with some differences, the main preventive measures as per the CDC remain the same: staying up to date with vaccines, getting tested for COVID-19 if you have symptoms, following recommendations if you have been exposed, keeping your distance from people with the virus, and seeking medical treatment and staying away from others if you have the virus.

The CDC recommends wearing face masks and practicing social distancing in certain circumstances or at medium or high COVID-19 community levels.

Practicing these measures to keep Omicron at bay will help keep you, and others, safe from COVID-19.

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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