In-Depth: Why is omicron less severe?

Posted at 7:50 PM, Dec 30, 2021
and last updated 2021-12-31 00:31:41-05

SAN DIEGO (KGTV) – The nation’s top health officials say the omicron variant appears to cause milder symptoms on average than delta, and new research spearheaded by San Diego scientists is offering one of the leading theories as to why.

Dr. Anthony Fauci, the president’s chief medical adviser, said Wednesday that all indications suggest omicron causes a lower hospitalization-to-case ratio than delta. Over the last two weeks, cases rose126 percent while hospitalizations increased only 11 percent, he said.

“The spike in cases is out of proportion to the increase in hospitalization,” said Fauci.

Dr. Fauci cited studies on hospital trends in South Africa and U.K., where fewer infected individuals have been admitted to hospitals compared to previous waves. In the U.K., the risk of hospitalization was 62 percent lower for individuals with omicron than delta in late November and early December.

Fauci said pre-existing immunity, changes in omicron, or some combination of the two might explain omicron’s lessened severity.

New research from scientists at La Jolla Institute for Immunology shows omicron can evade many of the antibodies produced by previous infection or vaccination, but not the specially trained white blood cells called T cells. Between 83 and 85 percent of the T cell response was maintained, said LJI professor Dr. Alessandro Sette.

“We predicted that this was going to be a likely scenario, that the T cells would still hold, and it has indeed been reassuring that this is the case,” he said.

The LJI team found the Moderna, Pfizer and Johnson and Johnson vaccines produced detectable T cells against omicron for at least six months after vaccination.

“We found similar behavior, regardless of what vaccine that different people had taken,” Sette said.

T cells are the body’s second line of adaptive immune defense. They won’t prevent an infection from taking hold in the first place like antibodies can, but they can rapidly swoop in and keep mild symptoms from getting worse, Sette said.

A booster shot restored neutralizing antibody levels against omicron, the study found.

Other studies suggest changes in the omicron variant itself might be playing a role in its reduced severity. A study in Hong Kong on different types of human cells found omicron replicated faster than delta in certain parts of the upper airways, but ten times slower than delta in the lungs.

That suggests the virus may cause less damage inside the organ where people are most susceptible to breathing problems and life-threatening disease.

“It still grows enough to be transmitted, but because it doesn’t induce as much inflammation, the inflammatory response that makes people really sick isn’t getting them into the hospital,” said Dr. Robert Schooley, an infectious disease expert at UC San Diego.

"I think all the data we have suggests it is is less severe," he added. "Having said that, it is still severe for people who are unvaccinated."

U.S. cases are at record highs. Health officials say omicron poses the highest risk of any variant for breakthroughs and reinfections.

“I feel like I’ve just gotten back-to-back variants,” said San Diegan Patrick McCoig.

The fully vaccinated standup comedian was finishing his final day of COVID quarantine Thursday morning. It was his second stint this year.

McCoig had a breakthrough case in August that kept him out of work for nearly a month. “Aches, fever, chills, extreme exhaustion. I could barely get out of bed,” he said.

Then around Christmas, he tested positive again. This time his symptoms were far milder, although he missed another week of work in quarantine. He’s started a GoFundMe to help with expenses.

"That day I got COVID, I was signing up to get my booster. So I was a little too late on that one," he said.