Over a holiday period marked by canceled flights, out-of-stock at-home COVID-19 tests and unease about new quarantine guidelines from the US Centers for Disease Control and Prevention, the omicron variant joined delta in a stampede across the US. But while COVID-19 cases have hit unprecedented numbers in many parts of the country, hospitalization and death rates have stayed "comparatively low," CDC Director Dr. Rochelle Walensky said last week during a White House COVID-19 response briefing.
Hospitalization data lags behind the COVID-19 case count by a couple of weeks, and many of the people getting infected in the US or abroad have some protection from the COVID-19 vaccines or a prior infection, but the hospitalization gap adds to evidence showing that while incredibly contagious, omicron may lead to less severe disease than earlier variants, including delta.
"All indications point to a lesser severity of omicron versus delta," Dr. Anthony Fauci, President Joe Biden's chief medical adviser, said last week at the White House briefing. But we shouldn't become complacent, Fauci said, as our hospital system "could still be stressed" in parts of the country while large numbers of people get sick at once. (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.)
Omicron is the dominant variant, but delta is still responsible for a large number of cases (and hospitalizations) in the US, according to the latest data from the CDC. The agency had previously overreported the number of COVID-19 cases caused by omicron, adjusting it from roughly two-thirds for the week ending Dec. 18 to over 58% for the week ending Dec. 25. However, as the US follows the path of other countries that experienced a sharp rise in COVID-19 cases caused by the omicron variant, scientists learn more each day. Here's what we know now about how omicron compares to delta.
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Is omicron milder than delta? How effective are the vaccines against omicron?
A preliminary study from Hong Kong found that while omicron replicates much faster in the bronchus (which carries air from the windpipe to the lungs) -- likely explaining how fast it spreads between people -- omicron isn't as good at replicating in the lungs as delta and other variants, which indicates lower disease severity.
Another study, on hospital data
from South Africa, found that the omicron-driven wave resulted in shorter hospital stays, fewer ICU admissions and less need for supplemental oxygen compared to earlier waves of the pandemic. However, the authors of the study noted, the changing clinical presentation of COVID-19 in this study might be due to high levels of prior COVID-19 infection or vaccination.
At last week's press briefing, Fauci referenced additional information that suggests omicron causes less severe disease, on average, compared to delta, including UK information that shows the risk of hospitalization admission was 40% the risk from delta.
"So, in conclusion, the data are encouraging but still, in many respects, preliminary," Fauci said.
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.