Omicron’s ‘fat tail:’ Why aren’t COVID infections falling faster?

The omicron wave has crested, but health experts fear infections aren’t falling as fast as they had hoped.

The omicron variant fed Florida’s highest COVID-19 growth rate since the pandemic started nearly two years ago. In the last weeks of December, the state infection rate doubled every 4-5 days. When it peaked in mid-January, omicron was infecting an average of more than 65,000 Floridians a day.

Health experts then predicted that the highly contagious variant would rapidly burn through the state’s population and depart as quickly as it had arrived. But the omicron wave has stuck around longer than predicted, exhibiting what experts call a “fat tail” — meaning infections aren’t going down as fast as they went up.

This matters because as long as infection numbers remain at such heights, COVID-19 will continue to sicken more people and strain healthcare resources for longer than expected. As of last week, COVID-19 was infecting an average of nearly 18,000 Floridians and sending more than 1,000 to the hospital each day, according to federal health data.

The secret to omicron’s success is an abundance of mutations that made the variant about 2.7 to 3.7 times more infectious than the delta variant in vaccinated households.

The mutations allowed it to evade many of the immune defenses that individuals had accumulated through vaccination or exposure, and made two of the most popular monoclonal antibody treatments ineffective. It also means that an omicron infection might not provide immunity from future variants.

Experts say the omicron variant appears to cause less severe symptoms than did previous strains, while still threatening the vulnerable and unvaccinated. But omicron has posed a different kind of problem through its sheer volume of cases: a record 1.9 million infections in just two months.

The silver lining that experts hoped for was that the variant would recede at the same pace that it arrived. Omicron isn’t doing that.

“Theoretically, epidemic waves are pretty symmetrical,” said University of Florida epidemiologist Thomas Hladish. That means cases generally go up and then down at the same rate.

To illustrate this idea, think of taking the daily case rate three weeks after cases reached a peak. If the wave is symmetrical, the case rate three weeks after the peak should equal those three weeks before cases peaked.

The delta wave that rocked the Sunshine State last summer fits this pattern. The wave peaked at 21,984 cases on Aug. 13, and after plateauing for more than a week, soon receded at roughly the same rate that it grew. Three weeks after the spike, delta was generating 16,500 cases a day — just 22 percent higher than expected based on the variant’s growth rate.

Had the omicron variant followed this pattern, case rates would have plummeted dramatically once the wave hit its apex. When the omicron wave peaked 32 days ago on Jan. 8 at 65,306 cases, it was generating the highest average daily case rate at any point in the pandemic.

If this wave was symmetrical, as previous waves have been, then by now cases should have fallen to fewer than 5,000 cases a day. Instead, Florida’s daily average was nearly 18,000 cases on Sunday, according to CDC data — nearly 270 percent higher than expected had the wave been perfectly symmetrical.

Similar patterns have emerged in the U.K. and, to a lesser extent, in South Africa, where the omicron variant first was detected. In both countries, elevated case counts linger more than a month after infection rates hit their peak, according to data collected by The New York Times.

Hladish offered some theories as to why omicron has lingered longer than experts predicted. One is that the new BA.2 subvariant — dubbed “stealth” omicron — could be spreading far more widely than believed.

The new subvariant is about 34 percent more infectious than the original omicron strain, according to a recent Danish study. The virus can reach more people even faster than the original omicron, so the subvariant could already be replacing the variant as it fades.

Florida so far has recorded five cases of the BA.2 omicron variant, all from patient samples taken in mid-January. Given how quickly the new subvariant has spread elsewhere, Hladish said, it already could account for a substantial number of cases in Florida at this point.

Another possible explanation is that Floridians dropped their guard against omicron too soon. Scientists say omicron can produce milder symptoms than delta, and residents who heard that message may have stopped masking and social distancing sooner than they should have — or have stopped practicing pandemic precautions altogether.

”People will become concerned when the numbers are bad and take additional precautions,” Hladish said. “And then the peak has passed and things are improving, and they may actually cause the wave to draw out.”

They may also be following the cues of Gov. Ron DeSantis and Florida lawmakers, who do not support mask or vaccine mandates and have stymied local government officials, school districts and private employers from imposing their own pandemic rules.

Hladish said the virus may have finished going through the population of people who weren’t taking any precautions, who never stopped going to bars and restaurants and eating and drinking indoors. Now omicron could be making its way through whoever is left, those who wear masks, socially isolate and try to stay safe.

University of South Florida epidemiologist Jason Salemi is concerned about omicron’s “fat tail” and slow decline. But he said it’s too soon to tell which, if any, of Hladish’s theories will bear out.

He raised another possible factor: Florida’s recent chilly weather forced residents indoors after omicron’s peak, and that could have made it easier for people to continue to infect each other.

But both Hladish and Salemi agree that omicron’s fat tail means Floridians will have to wait longer than expected for infections to return to safer levels. Last week the state’s positivity rate was 18 percent.

There are no blanket recommendations, Salemi said. But health professionals and the CDC stress that precautions like wearing masks and getting vaccinated and boosted remain essential to preventing infection and curbing viral spread, especially now that the ever more contagious BA.2 subvariant is out there.

“It’s a bad idea to let (precautions) go back to normal,” said Hladish, “it’s not the first time we’ve had to learn this lesson.”

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How to get tested

Tampa Bay: The Times can help you find the free, public COVID-19 testing sites in Citrus, Hernando, Hillsborough, Manatee, Pasco, Pinellas, Polk and Sarasota counties.

Florida: The Department of Health has a website that lists testing sites in the state. Some information may be out of date.

The U.S.: The Department of Health and Human Services has a website that can help you find a testing site.

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How to get vaccinated

The COVID-19 vaccine for ages 5 and up and booster shots for eligible recipients are being administered at doctors’ offices, clinics, pharmacies, grocery stores and public vaccination sites. Many allow appointments to be booked online. Here’s how to find a site near you:

Find a site: Visit vaccines.gov to find vaccination sites in your ZIP code.

More help: Call the National COVID-19 Vaccination Assistance Hotline.

Phone: 800-232-0233. Help is available in English, Spanish and other languages.

TTY: 888-720-7489

Disability Information and Access Line: Call 888-677-1199 or email DIAL@n4a.org.

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OMICRON VARIANT: Omicron changed what we know about COVID. Here’s the latest on how the infectious COVID-19 variant affects masks, vaccines, boosters and quarantining.

KIDS AND VACCINES: Got questions about vaccinating your kid? Here are some answers.

BOOSTER SHOTS: Confused about which COVID booster to get? This guide will help.

BOOSTER QUESTIONS: Are there side effects? Why do I need it? Here’s the answers to your questions.

PROTECTING SENIORS: Here’s how seniors can stay safe from the virus.

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