Omicron vs. delta: What we know about COVID-19 disease now


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For the most up-to-date news and information about the coronavirus pandemic, visit the WHO and CDC websites.

Omicron is now the dominant coronavirus variant in the US, accounting for roughly three-quarters of COVID-19 cases the week that ended Saturday, according to data from the US Centers for Disease Control and Prevention. This happened more suddenly than the US expected, but it’s something scientists have said is inevitable given omicron’s extreme level of contagiousness and ability to get around people’s immunity — from the vaccines and past sickness with COVID-19. (Booster doses refill much of that protection.)

Omicron is also thought to have a fast incubation period, or gap of time between someone getting exposed to the virus and then showing symptoms. As The Atlantic reported, delta already made people contagious faster, but omicron’s “shorter incubation period means there’s less time to pinpoint an infection before it becomes infectious.” That means, a negative at-home COVID-19 test on Tuesday doesn’t necessarily mean a negative test on Thursday. 

Data from hospitalization rates from other countries suggest that omicron may cause less-severe disease (or at least not more severe disease) compared to earlier variants. However, health experts are pleading for people not to dismiss omicron as mild. 

“Surely, we have learned by now that we underestimate this virus at our peril,” Tedros Adhanom, director-general of the World Health Organization, said during a Dec. 14 briefing. “Even if omicron does cause less severe disease, the sheer number of cases could once again overwhelm unprepared health systems.” As the US strengthens its campaign for COVID-19 boosters, 8.1% of people in low-income countries have had a COVID-19 vaccine, according to Our World in Data. 

As COVID-19 cases surge in the US, here’s what we know now about how the omicron variant compares to delta. 

Is omicron more mild than delta? How effective are the vaccines against omicron?

A large preliminary study from South Africa suggested omicron causes less severe disease in adults (but not in children). Another preliminary study from Hong Kong found that omicron replicates less efficiently in the lungs, which could lead to less severe disease. But, the authors note, the overall threat of omicron is “likely to be very significant” given how contagious and immune-evading omicron is. 

In an interview with NBC on Sunday, Dr. Anthony Fauci said that while information from South Africa shows there might be “less requirement for oxygen” when people are sick with COVID-19 from the omicron variant, that doesn’t mean it’s a less severe virus. Because there are many factors in play, including who’s currently getting sick (a lot of young people). 

“When you have so many, many infections, even if it is less severe, that overcomes this slight to moderate diminution in severity,” Fauci told NBC. If large amounts of people become ill, many of them will inevitably need hospital care, which will inundate health care systems required to treat not only COVID-19 patients but also people seeking care for other conditions that can turn fatal without intervention. 

Delta diminished some vaccine protection against COVID-19 infection (and protection from natural immunity) because it mutated from the original virus. Omicron has reduced the vaccines’ effectiveness further, resulting in many breakthrough cases and COVID-19 reinfections. The lowered protection is also expected to spill over and decrease the effectiveness of existing treatments, including monoclonal antibodies. However, the COVID-19 vaccines have remained effective at protecting against severe disease, hospitalization and death. Health officials are encouraging booster shots for the increased antibody response and protection another dose will bring. 

Fauci told NBC that while there’s “no doubt” there will be breakthrough COVID-19 infections, there’s a “difference between a vaccinated and boosted person who has an infection, and someone who has an infection who’s never been vaccinated — a major difference with regard to the risk of severity.”

Unvaccinated people remain particularly vulnerable to severe disease and death caused by COVID-19, whichever the variant. In October, unvaccinated people were 14 times more likely to die from COVID-19 compared with fully vaccinated people, according to CDC information.

Some mutations in omicron’s spike protein are similar to ones found in the delta variant, according to the Republic of 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 WHO.

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

“What all those changes in the aggregate are going to do for the things that matter for this virus, we don’t really know yet,” Robert Garry, a virologist at Tulane University, told CNN in late November.

Read more: Think you’re ‘fully vaccinated?’ Omicron may change the CDC definition

What is a mutation or variant? 

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, 7.3% 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 an 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: Free COVID at-home test kits: Here’s how and when you’ll get yours

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.



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