When it was discovered, omicron alarmed scientists.

The variant looked wildly different from earlier versions of the coronavirus — and it quickly became clear that these mutations gave omicron an uncanny ability to sidestep our vaccines and spread very rapidly. But it has taken longer to untangle what, if anything, sets an omicron illness apart from that of its predecessors. And most of all, does this variant cause less severe disease than the variants that have come before it?

With infections at all-time highs in the U.S., the clinical picture is now coming together and starting to confirm what other countries have found — a typical case of omicron not only presents slightly differently but also likely carries a lower chance of getting seriously ill.

Scientists at Case Western Reserve University have preliminary evidence that the risk of being admitted to the hospital or the intensive care unit during the omicron surge in the U.S. is about half of the risk observed during the delta surge. And this reflects what doctors across the country are now seeing firsthand with their patients.

"This is a pretty different surge," says Dr. Brendan Carr, chair of emergency medicine for the Mount Sinai Health System — where the emergency rooms are busier than ever but many of the COVID-19 patients are not sick enough to be admitted.

But as with any variant of SARS-CoV-2, your absolute risk depends on many factors, including whether you're vaccinated and boosted, your age, your overall health and your economic situation.

"In the older age group, it's still a nasty disease, even if it's less [nasty] than the delta variant," says Dr. Pamela Davis, who's a pulmonologist at Case Western Reserve University and a senior author on the new study. "You don't get off scot-free just because you happen to be infected in the time of omicron."

"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,' " said the World Health Organization's director-general, Tedros Adhanom Ghebreyesus, on Thursday. "Just like previous variants, omicron is hospitalizing people and it is killing people."

Indeed, hospitalizations across the U.S. now stand at more than 126,000, and more than 1 in every 4 ICU beds is filled with a COVID-19 patient, according to the latest data from the Department of Health and Human Services.

Omicron Symptoms

When You Get Sick with Omicron, What Are the Symptoms?

What those hospital numbers don't tell us is what a typical case looks like. As with previous variants, the vast majority of people infected with omicron have a mix of symptoms that resolve relatively quickly and don't require hospital care. And doctors are finding many of these cases tend to look like an ordinary upper respiratory infection. In other words, what you think of as the common cold.

"It's mostly that runny nose, sore throat and nasal congestion," says Dr. John Vanchiere, the associate director of the Center for Emerging Viral Threats at LSU Health Shreveport. "The cough is milder [than previous variants], if there's any cough at all, and fever seems to be a little less common."

This fits with early data from the U.K. showing that fever and cough are not as prevalent with omicron cases there and that the five top symptoms are runny nose, headache, fatigue, sneezing and sore throat.

With omicron, the symptoms also come on more quickly once you're infected. Several studies have found that the incubation period — the time it takes to develop symptoms after being exposed — is about three days. In contrast, delta took about four days, and the original variant took more than five.

Another difference doctors are noticing: Loss of smell and taste — considered a telltale sign of COVID-19 — is not nearly as common with omicron infections. And fewer patients have symptoms related to lower respiratory problems, such as shortness of breath, says Vanchiere, including older patients.

At the same time, it appears — anecdotally at least — that certain symptoms show up more with omicron than they did with delta. Three that have gained attention are nausea, night sweats and lower back pain.

But it's very possible that doctors and patients are simply paying more attention to these symptoms than they did with earlier variants, says Dr. Scott Roberts, an assistant professor of infectious diseases at the Yale School of Medicine.

"A lot of this is probably magnifying these symptoms under a microscope instead of clear changes," he says. "Omicron versus delta are really more similar than they are different." And just like earlier variants, omicron can't be defined as causing only a narrow group of symptoms. As at earlier stages in the pandemic, many patients are still having some combination of fever, gastrointestinal problems, aches and pains, brain fog, weakness and, less often, trouble breathing, says Mount Sinai's Carr. "Omicron can present in a myriad of different ways," he says.

"It is clear that if you're vaccinated, particularly if you’ve had a booster, omicron tends to produce milder infections," said Dr. William Schaffner, an infectious disease expert at the Vanderbilt University Medical Center in Nashville, Tennessee.

"What we haven't seen yet is a substantial body of information about what omicron will do in unvaccinated people," he added.

Indeed, at least one person who was not vaccinated is reported to have died of omicron. Officials in Houston announced in December that the unvaccinated man in his 50s succumbed to the virus.

For people who have been vaccinated, but have not had a booster, typical symptoms include more coughing, more fever and more fatigue than those who have received an extra dose, said Dr. Craig Spencer, director of global health in emergency medicine at New York-Presbyterian/Columbia University Medical Center.

So, If I Do Get Omicron, What's My Risk of Getting Very Sick?

With SARS-CoV-2, the big danger is that a mild illness will turn into a life-threatening one. Although that could definitely still happen with omicron, the risk appears to be lower than it was with delta. A study published online on Jan. 2 provides some of the first compelling evidence from the U.S. that the chance of ending up in the hospital is lower with omicron compared with the delta variant.

Scientists at Case Western Reserve University analyzed health records from more than a half-million people infected with SARS-CoV-2 across the country, including 14,000 people possibly infected with omicron from Dec. 15 to 24, after the variant became dominant.

"In this period, we still have delta circulating in the community. But you're pushing more and more and more toward the omicron variant," says Davis, who contributed to the study.

Then the researchers looked to see if there was a difference between people infected during the end of the delta wave and those infected during the early stage of the omicron wave. "The difference was huge," says data scientist Rong Xu, who led the study and is also at Case Western Reserve University. "We didn't need to do any complicated statistics to see the difference."

Xu and her colleagues found that the risk of needing to go to the ER dropped from about 15% during the delta surge to 5% during the early omicron surge (about a 70% decrease) and the risk of being hospitalized dropped from 4% to 2% (or by 50%).

If a person did end up in the hospital, the person's risk of being admitted to the ICU or being put on a ventilator also decreased substantially at the end of December compared with during the delta surge. Specifically, the risk of being admitted to the ICU fell from 0.8% to 0.4% (or by 50%) and the chance of being put on a ventilator fell from 0.4% to 0.1%.

This lower risk with omicron is also consistent with what scientists have observed in South Africa and the United Kingdom.

Xu and her team estimate that, in their study, about 60% of the people were vaccinated. So some of this lower risk could be because of vaccination, but the data altogether suggests that there is a reduced risk for hospitalization with the omicron variant compared with the delta variant.

In particular, Xu and her team observed a similar reduction in risk across all age groups, including children under age 5, who are not eligible for vaccination, and children ages 5 to 15, who may have been vaccinated but haven't been boosted. That consistency, Xu says, suggests the reduction in severity is due, in part, to something inherent with omicron itself and not simply because of changes in vaccination status.

"So this is really something that's different between omicron and delta," Xu says. That all said, this reduction in risk doesn't mean omicron will be mild for everyone. For people who are at high risk for severe disease, such as older people or those with underlying health issues, the chance of being hospitalized is still quite significant. For example, if you're over age 65, your risk of being hospitalized with COVID-19 is still 5% with the omicron variant, which means 1 in 20 people infected in this age group will end up in the hospital. (By contrast, with the original version of the virus, the rate was 1 in 10.)

"The risk is not zero," says Xu's colleague Davis, speaking of omicron. "Many people are still going to be admitted to the ICU, and some people are still going to need to have mechanical ventilation." That's why, she says, everyone should be vaccinated and boosted. As with previous variants, being vaccinated greatly protects you from severe disease with omicron. A study from the U.K. government, published last week, found that three doses of vaccine cuts the risk of hospitalization due to omicron by about 80% compared with a person who's not vaccinated at all.

If You Do End Up at The Hospital, How Bad Is It?

Even though early data shows that omicron is milder than delta, many hospitals are packed because the sheer number of people getting infected is enormous. And doctors are finding a key difference among their patients who are ending up in the ER or being admitted: Many are neither struggling to breathe nor dealing with perilously low oxygen levels.

Those two conditions were "a hallmark of the first disease and of delta and not nearly as prominent in omicron," says Mount Sinai's Carr. In the past, it was basically a given that a severe case of COVID-19 would wreak havoc on the lungs, at times leading to pneumonia and uncontrolled inflammation. But this apparent change in the disease — that a severe infection in the lungs doesn't seem as common — means fewer people need supplemental oxygen or intubation.

"They're not short of breath, and really the lungs are OK," says Roberts, of Yale. These observations also line up with lab research that shows omicron does not replicate in lung tissue as well as delta.

Many of the patients who are being hospitalized often have some underlying health condition, or they're older and more vulnerable to a viral infection. "What we're seeing is something really tips these patients over the edge," says Roberts. For example, an omicron infection may lead to complications of an existing condition such as diabetes or heart failure.

But Roberts says it's still quite rare for people who are vaccinated and boosted to get seriously ill from omicron. About 80% of the patients at Yale New Haven Hospital are unvaccinated. And among those who are vaccinated, almost all have not received a booster shot.

While it's welcome news that omicron is easier on the lungs, ProHEALTH's Griffin says it's not that way for some of his patients. And among unvaccinated people, he says, an omicron infection can feel like the same unforgiving disease to him.

"If we have a patient who's younger, if we have a patient who's vaccinated, if we have a patient who recently recovered from delta, we're tending to see very mild disease with omicron," says Griffin. "But people who are fresh, with no preexisting immunity, it's hard to see that the virus is milder."

Omicron Vaccine

Omicron is spreading rapidly across the globe, and researchers are racing to understand how vaccines will hold up against this new variant of the coronavirus. Several preliminary studies have assessed the effectiveness of current COVID-19 vaccines in use against the Omicron variant.

So far, one- or two-dose vaccines provide far less protection than those paired with a booster, but they still do appear to protect against severe disease.

Studies conducted in the lab and the real-world show that full vaccination plus a booster shot provide stronger protection against infection with Omicron.

It is important to remember most of these are lab studies and may not reflect the vaccines’ real-world performance.

Here is what we know so far.

Omicron vs. Delta: Vaccines seem less effective

Early estimates suggest that vaccine effectiveness against symptomatic infection with Omicron is significantly lower compared with the Delta variant.

A report by Imperial College London indicated that the risk of reinfection with Omicron was 5.4 times greater than the Delta variant. Previously having COVID-19 also afforded little protection against reinfection with Omicron.

“This is such a contagious virus that it can spread not only among the unvaccinated, where I think it still has a very substantial risk of causing serious disease that might require hospitalization, but it can also spread among vaccinated persons, although the illness it produces among the vaccinated, particularly if you’ve had a boost, is generally mild and even without symptoms,” said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center in Tennessee.

However, the Imperial College London study suggests there is “no evidence of Omicron having different severity from Delta,” despite hospitalization remaining relatively low for the time being.

“[Even if] Omicron may not lead to more severe illness than Delta, a rapid and massive surge in infections could still overwhelm hospitals with sick patients. People who are unvaccinated remain at the highest risk, but also those who have not received a third dose of an mRNA vaccine,” said Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York City.

2-dose Pfizer vaccine vs. Omicron

Few real-world studies so far have managed to estimate effectiveness for the Pfizer-BioNTech vaccine, and a number of lab studies show that a booster increases antibody production significantly.

“The studies are largely laboratory studies that would indicate that levels of antibody induced by boosting will have a noteworthy impact on preventing serious disease,” said Schaffner. A preprint study conducted by Oxford University reported that two doses of the AstraZeneca or Pfizer-BioNTech vaccines offered little protection against infection with the Omicron variant.

However, a real-life study from South Africa found that two doses of the Pfizer-BioNTech vaccine still protected people from severe disease. Researchers found that two doses provided 70 percent protection against hospitalization and 33 percent protection against infection. This was a drop from about 93 percent and 80 percent, respectively, for the Delta variant.

A study published in the New England Journal of Medicine echoed these figures, finding that a two-dose regimen of the Pfizer-BioNTech vaccine was 70 percent effective against hospitalization with Omicron.

Dr. Monica Gandhi, an infectious diseases specialist with the University of California, San Francisco, told Healthline that although these lab studies show that antibodies, the body’s main line of defense against the coronavirus, can wane over time or be affected by mutations along the virus’s spike protein, vaccines were able to generate other types of immune cells to fight off infection.

“We know now that T cells from the vaccines still work against Omicron, and B cells generated by the vaccines adapt the new antibodies they produce to work against variants. So, I think the clinical scenarios we are seeing in those with prior immunity, vaccinated, and even boosted individuals [show that] the vaccines are likely still protecting Omicron-infected individuals from severe disease,” she said.

Pfizer booster vs. Omicron

Early analysis from the United Kingdom has suggested that boosters have moderate to high vaccine effectiveness against symptomatic infection, offering 70 to 75 percent protection in the initial weeks after the booster.

Researchers found a slight difference depending on whether the initial doses were from the AstraZeneca vaccine or Pfizer-BioNTech vaccine. Both groups had Pfizer-BioNTech boosters.

Two weeks after a Pfizer-BioNTech booster dose, vaccine effectiveness for people who received the AstraZeneca vaccine increased to around 71 percent, and to about 76 percent for those who initially got the Pfizer-BioNTech vaccine.

As for severe disease, modeling by Imperial College London found that the Pfizer-BioNTech vaccine with a booster was around 85.9 percent effective against Omicron, compared with about 97 percent against Delta.

Early data from Pfizer had indicated that a booster dose significantly increased neutralizing antibodies, bringing the vaccine’s protection close to what two doses provided against the original variant of the coronavirus. The data comes from a series of lab experiments testing the neutralizing ability of blood samples from people who had two doses of the vaccine and those who had received a booster.

The neutralizing ability of the antibodies collected from those who did not have a booster had a more than 25-fold decline against Omicron than the original variant.

Similarly, a study in Israel comparing blood samples from two groups of healthcare workers who had or had not received Pfizer-BioNTech boosters found that the third dose increased antibody levels 100 times compared with two doses alone. The most recent report by the UK Health Security Agency (UKSHA) showed that a third dose offers significantly higher protection.

Early results showed that vaccine effectiveness dropped to 52 percent against Omicron 6 months after the second dose, while an mRNA booster brought this up to 88 percent against hospitalization.

Moderna vaccine vs. Omicron

No studies offer definitive estimates for the Moderna vaccine’s effectiveness against Omicron, though scientists believe it may be similar to Pfizer-BioNTech’s vaccine. A preliminary study of blood samples from 30 people who had received two doses of the Moderna vaccine showed that their antibodies were at least about 50 times less effective at neutralizing Omicron.

With a 50-microgram booster, however, antibodies increased 37-fold. On the other hand, a 100-microgram booster, the amount given to immunocompromised people as a third primary shot, increased antibody levels more than 80 times. A UKHSA analysis of nearly 200,000 cases showed that vaccine effectiveness of three doses offered 88 percent protection against hospitalization with the Moderna or Pfizer-BioNTech vaccines.

Johnson & Johnson vaccine vs. Omicron

A Centers for Disease Control and Prevention (CDC) panel in December 2020 recommended that people get the Pfizer-BioNTech or Moderna vaccine over the Johnson & Johnson vaccine due to the rare risk of blood clots. Furthermore, data from South Africa showed that the J&J vaccine produced virtually no antibodies at all against Omicron in lab experiments.

“Those who received the J&J vaccine (in the past few months) and are worried are past the period of concern. But if they’ve just received one dose of J&J, they should be urged to get a booster,” said Schaffner, commenting on the latest data. Although it is not yet a formal recommendation, mixing vaccines has been shown to increase immunogenicity, which is why physicians are advising that booster doses for J&J be Moderna or Pfizer-BioNTech.

“They (those who received J&J vaccines) will get a much higher antibody level [with mRNA boosters],” said Schaffner. Gandhi, meanwhile, said that although she generally agreed with the CDC panel recommendation, there were still groups of people who may benefit from this vaccine.

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