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Omicron causes less severe disease than delta, leads to fewer hospitalizations, new data shows

While new data from the US confirms what's been seen in other countries, omicron continues to cause unprecedented numbers of COVID-19 cases.

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Compared to the delta variant, omicron causes less severe disease for patients, according to data shared Wednesday by Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention, at a COVID-19 response briefing

Walensky cited a preprint study published Tuesday that looked at data from about 52,000 people infected with the omicron variant, and about 17,000 infected with delta, in southern California. Compared to patients who had the delta variant, omicron patients had a 53% reduced risk of hospitalization, a 74% reduced risk of ICU admission and a 91% reduced risk of death. The study has not yet been peer-reviewed.

Also, Walensky said, none of the people infected with omicron required mechanical ventilation. This backs up another study from researchers in Hong Kong who found that while extremely contagious, omicron isn't as good at replicating in the lungs as delta, likely leading to less severe disease. 

The new research also adds to what researchers from many other countries including South Africa have found: Though more contagious than any other variant, omicron doesn't seem to be making people quite as sick as the delta variant did. And when it does make people sick enough to require hospitalization, those hospital stays tend to be shorter, Walensky noted Wednesday. 

But shorter hospital stays doesn't mean no hospital stays, and it doesn't mean no deaths. The huge number of people getting infected with COVID-19 will strain hospital systems -- already struggling in some states -- which could affect care and the outcomes for people who do need to be hospitalized. And overwhelmed hospitals don't only affect COVID-19 patients, they affect everyone who needs care for something that could turn serious or even fatal if left untreated.

In addition to omicron's incredible degree of transmissibility or contagiousness compared to earlier variants, the new variant is also leading to many breakthrough cases in people who are fully vaccinated, and reinfections in people who've already had COVID-19. In order to lessen the burden of large numbers of people getting sick at once, health officials are encouraging everyone eligible (age 12 and up) to stay "up to date" and get a COVID-19 booster shot. 

Omicron has been causing record-breaking numbers of COVID-19 cases in the US, and this "staggering" daily case rate has led to a high number of total hospitalizations, Walensky said. Data from other countries will continue to provide understanding on what we can expect in the coming weeks when COVID-19 cases are predicted to peak in the US, Walensky said. 

"I want to underscore the importance that the up-to-date COVID-19 vaccination with boosters prevents severe disease and keeps you out of the hospital," Dr. Anthony Fauci, the chief medical advisor to the president, said Wednesday. 

Read more: US Army's 'super vaccine' could protect against all COVID variants. What do we know?

As scientists continue to learn more about omicron each day, and adjust guidance accordingly, here's what we know about the new variant and COVID-19 symptoms now. 

What is a mutation or variant? 

Some mutations in omicron's spike protein are similar to ones found in the delta variant, according to South Africa's Department of Health, as well as mutations found in the alpha, gamma and beta variants -- all classified as variants of concern by the World Health Organization.

Omicron has more mutations on its spike protein than the delta variant does, but scientists are working to understand what that means.

The coronavirus enters our cells using its "corona," or layer of protein spikes, then makes copies of itself in our bodies, where inevitably there are some errors or mutations, as explained by Yale Medicine. Sometimes those mutations in the virus are harmless, but other times -- like in the case of the delta and omicron variants -- they make it much easier for the virus to spread from person to person and infect more people. 

The more people who are unvaccinated or without immunity from COVID-19, the more opportunities there are for the coronavirus to spread and form concerning variants. 

"I think what you're seeing is just the manifestation of what we've been talking about," Fauci told NBC in November. "Why it is so important for people to get vaccinated, and for those who are fully vaccinated to get boosted."

Apart from vaccine hesitancy, many people in countries outside the US don't have access to a COVID-19 vaccine. According to Our World in Data, 9.5% of people in low-income countries have received a dose of coronavirus vaccine. 

"The emergence of the omicron variant should be a wake-up call to the world that vaccine inequality cannot be allowed to continue," South Africa's president, Cyril Ramaphosa, said in a November address

At a recent press briefing, White House COVID-19 Response Coordinator Jeff Zients said the US has shipped 300 million COVID-19 vaccine doses out for donation, a milestone, he said, in the White House pledge to donate 1.2 billion doses of COVID-19 vaccine to other countries. 

What are the symptoms of omicron vs. delta?

In a Dec. 10 report by the CDC, the details of 43 omicron cases (some of the first in the US) were revealed. When it came to symptoms, most people (89%) reported a cough, 65% were fatigued and 59% of them were congested or had a runny nose. Only 8% of the 43 people reported losing their sense of smell or taste, which has affected many people with previous COVID-19 infections, caused by other variants. Fourteen percent of people in the report had COVID-19 previously. 

Dr. Angelique Coetzee, a South African doctor who helped discover omicron, told the BBC in late November that so far patients she's seen with the omicron variant have "extremely mild cases" of COVID-19. Those symptoms included fatigue, headache and a scratchy throat, she said, not the telltale loss of smell or a cough associated with earlier COVID-19 infections. 

However, the delta variant may have slightly changed the way COVID-19 presents. Cough and loss of smell are also less common symptoms of COVID-19 caused by the delta variant compared with earlier variants, per the Baton Rouge General, a Mayo Clinic network. Cold symptoms like a headache and runny nose are now more common symptoms of COVID-19, according to the UK's ZOE COVID Study.

How do you test for omicron? 

A COVID-19 test won't tell you which variant you have. In order for scientists to determine whether it's omicron or another coronavirus variant, the CDC uses genomic sequencing. According to Walensky, the CDC director, the US is now testing 80,000 positive COVID-19 samples per week (about one in seven positive tests), up from 8,000 per week earlier this year.

Fortunately, the omicron variant is easily detected through PCR tests, according to Fauci, which can then be confirmed through labs that use genomic sequencing. 

Read more: Can you tell which COVID variant you have?

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.