COVID-19 symptoms and the Omicron variant: What the latest studies show
Doctors studying Omicron's spread around the world have found new clues to the pattern of symptoms caused by the highly-mutated COVID-19 variant, which a growing number of reports suggest might show up differently and faster compared to the Delta variant it is now displacing.
Early evidence suggests that in many patients, Omicron is leading to a new trend of milder symptoms that mostly affect the upper respiratory system — the nose, mouth and throat. That may help explain why it appears to pose a smaller individual risk of hospitalization or severe disease than earlier strains of the virus, which often invaded the lungs.
"What is becoming clearer … is that Omicron seems to have lesser impact on lungs than prior variants," said Dr. Ronald Whelan, head of Discovery Health's COVID-19 task team.
Whelan's employer, South Africa's largest health insurer, released an early report last month on the Omicron wave there. Their analysis found that a sore throat ranked among the most common early Omicron symptoms, as well as congestion, a dry cough and lower back pain. The incubation period — the time from infection to symptoms appearing — was as short as three days. That's several days faster than previous strains of the virus.
Dr. Angelique Coetzee, chair of the South African Medical Association, echoed Whelan's observations about Omicron patients.
"The few that progressed to pneumonia only started around day 10-12 with shortness of breath, feverish feeling, and a slight cough," said Coetzee.
Most mild Omicron cases over the course of the wave in South Africa had presented with a headache, chest pain or backache, a "slight scratchy throat or dry cough and a tiredness," Coetzee recounted.
South Africa's National Institute for Communicable Diseases warned residents late last month to watch for those kinds of upper respiratory symptoms, and noted that one distinctive symptom seen in cases of earlier COVID variants — loss of taste and smell — "seems to be uncommon" with Omicron.
Investigations of outbreaks outside of South Africa have turned up similar trends.
One analysis of data compiled from reports of positive cases in London, where Omicron cases surged last month, tallied the top five symptoms as runny nose, headache, fatigue, sneezing, and sore throat.
Among mostly vaccinated people infected at a Christmas party in Norway, cough and runny nose showed up in more than three-quarters of the 81 cases. Only 12% reported reduced smell and 23% reported reduced taste.
In an Omicron outbreak between five reinfected people in a Nebraska household, a recent report published by the Centers for Disease Control and Prevention found none had reported loss of taste or smell — compared to four during their first bout of COVID-19 in 2020. Two had a fever from their reinfection. One additional unvaccinated person, who had not been previously diagnosed with COVID-19, reported "cough, joint pain, congestion, fever, and chills."
The incubation period in both outbreaks also averaged around three days, shorter than the four to five days seen with the Delta variant.
Despite signs that Omicron leads to a lower individual risk of severe illness, it's so contagious that hospitals are struggling to handle the growing caseload. A consortium of disease forecasters warned Tuesday that the U.S. is on track to exceed the Delta variant's worst days of hospitalizations and deaths due to the sheer numbers of people who may catch the fast-spreading strain.
Scientists caution that Omicron's rapid ascent may be fueled in large part by more people spreading the virus with no symptoms at all.
One study of health care workers in South Africa, which has not yet been peer-reviewed, found the rate of people carrying the disease without symptoms had climbed from 2.6% during the Beta and Delta variant outbreaks to 16% with Omicron.
"These findings strongly suggest that Omicron has a much higher rate of asymptomatic carriage than other [variants of concern] and this high prevalence of asymptomatic infection is likely a major factor in the widespread, rapid dissemination of the variant globally," the study's authors wrote.
More clues from humans — and hamsters
While the frequency of some symptoms may have changed with Omicron, doctors say there is no evidence of significantly different or new symptoms that have not been seen in previous COVID-19 cases. Instead, scientists will need to sort out how Omicron may have shifted the odds of some symptoms compared to others.
For example, loss of smell or taste — called "anosmia" and "ageusia," respectively, by doctors — emerged early during the pandemic as a telltale sign of COVID-19. However, estimates of how many cases actually result in the symptom have ranged widely over the course of the pandemic and between different parts of the world.
In November, one study estimated that as many as 1.6 million Americans were facing long-term "chronic olfactory dysfunction" following their infection.
Night sweats, another symptom cited by a doctor at a media briefing in South Africa early in the country's Omicron wave, has been touted by tabloids as "a telltale sign which only happens at night that could mean you have been hit by the Omicron variant." However, the symptom is not new — some nine in 10 patients in one report from Wuhan, China from February 2020 listed the symptom.
An analysis of cases in London last month "found no clear difference in the symptom profile of Delta and Omicron, with only 50% of people experiencing the classic three symptoms of fever, cough, or loss of sense of smell or taste," said a post from the ZOE COVID Study, which has tracked trends in previous variants too.
More data on the topic are expected in the coming weeks.
Since June, monthly government surveys in the U.K. have steadily tracked around 30% of people with positive infections reporting loss of taste or smell, and around 40% reporting cough or fatigue. A new round of survey data for December, when the Omicron variant began to drive a record spike in cases there, is scheduled to be released later this month.
New clues could also come from ongoing studies investigating Omicron's effects on animals and tissue samples challenged with the virus in labs, which can control for factors like immunity or underlying conditions that might muddy data on the variant's effects in humans.
"NIH-funded studies that are ongoing right now in both mouse and hamsters confirm the lesser virulence in the animal model. And studies here at the Vaccine Research Center at NIH, in the nonhuman primate model, are ongoing," Dr. Anthony Fauci, the president's chief medical adviser, told reporters last week.
Fauci pointed to some preprints, which have not yet completed peer review, studying the virus in mice and hamsters that found signs that Omicron may spread poorly compared to Delta in the lungs. Similar findings from researchers in Hong Kong suggest Omicron can spread faster than Delta in tissue from the bronchus (the airway that leads to the lungs) but slower in the lungs.
"Seemingly subtle change in viral biology may have significant real-world effects on the nature of the disease that results from infection," says the University of Glasgow's Brian Willett.
Willett recently authored a study, which has not yet been peer-reviewed, of how Omicron infects the body's cells.
Delta was "fine-tuned" for infecting cells by fusion, Willett said, a mechanism by which the virus can fuse with the cell membrane. Omicron seems to prefer endocytosis, a process by which the virus is able to enter the cell to infect it. Each pathway requires different molecules that show up in different amounts depending on the cell.
"If the cells in the upper respiratory tract favor infection by endocytosis, then this is where Omicron will grow preferentially. In contrast, Delta may spread deeper into the lungs as it is able to infect the cells in this environment more efficiently," said Willett.