The clue to this extraordinary possibility comes from tens of thousands of responses to US Census Bureau questionnaires showing a dwindling percentage of COVID survivors reporting long-term COVID symptoms — the lingering, often debilitating effects of the disease that studies have historically shown at about 19% of the COVID survivors.
Last June, the Census Bureau, in conjunction with the National Center for Health Statistics, added questions about long COVID to their “Household Pulse Survey,” a largely monthly online questionnaire launched in April 2020 to measure the impact of the pandemic.
The study defines long COVID as symptoms that persist for at least three months after a coronavirus infection. About 200 symptoms have been identified, of which deep fatigue, palpitations, neurological complications and digestive disorders are the most common.
On average, 58,794 COVID survivors nationwide have responded to each of the nine lengthy COVID surveys provided by the Census Bureau to date. A Chronicle analysis revealed a remarkable trend: a consistently lower number of people in each study reporting long-term COVID symptoms.
In June, 18.9% of respondents said they were “currently” experiencing long-term COVID. By the end of the year that had dropped to 11.3% and by February it had dropped to 10.8%.
In California, the percentage of people reporting current symptoms increased slightly in July and October, but otherwise mirrored the national trend, from 16.2% in June to 10.3% in February.
“Fascinating,” said Dr. Steven Deeks, a longtime COVID researcher at UCSF who was not involved in the studies. “These trends seem consistent with the anecdotal experience we and others have had.”
Research shows that people can get long-term COVID regardless of whether their initial infection was severe or mild. Scientists point to three likely causes: bits of virus that remain hidden in the body, persistent inflammation caused by the coronavirus, and autoimmunity — when the body’s own immune system turns against itself. These, in turn, can wreak havoc in the body months or even years later.
But is the virus’s “ability to cause COVID long-term waning?” Deeks asked. “That is indeed a question that this data poses.”
It is possible that the study results reflect that some people are recovering from long-term COVID. A large Israeli study published in January found that most COVID patients with mild infections saw their persistent symptoms resolve within a year of becoming ill.
Deeks and two other long COVID experts, at Stanford University and UC Davis, said the apparent decline in long COVID may be due to vaccination and evolution in the coronavirus variants that cause COVID.
Dr. Christian Sandrock, co-founder of the Long COVID Clinic at UC Davis, said he has seen a drop in long COVID referrals to his clinic.
One reason, he said, could be omicron and its offshoots. “Just as the omicron has more cold symptoms and fewer cases of serious illness and hospitalization” than previous variants, it also appears to cause fewer long-lasting symptoms, he said.
A new study from Swiss researchers to be presented in April at the European Congress of Clinical Microbiology and Infectious Diseases found that people infected with the omicron variant were less likely to report long-lasting COVID symptoms than those infected with the original delta strain . Those findings support results from previous studies suggesting that omicron is less likely to cause long-term COVID than previous variants.
Sandrock also attributed the COVID vaccine.
For example, a recent review of research in the British Medical Journal found that COVID vaccination reduces the likelihood, severity and duration of long-term COVID.
Dr. Linda Geng, co-director of Stanford Medicine’s Long COVID Clinic, agreed that vaccines and the newer variants may be responsible for the Census study’s findings.
However, she said self-reported data like that from the census surveys has limitations because people may define long COVID differently.
Geng said she is still seeing many new patients in her long COVID clinic.
“Even if the rate is lower, the overall magnitude of the problem could still be significant if a large number of infections persist,” she said. “The only guarantee against long-term COVID is not to get COVID.”
US coronavirus cases are down 40% in the past month, according to federal data. But there are still about 170,000 people who get the disease each week.
And among those still suffering from long-term COVID, there is tremendous frustration at the lack of viable treatments.
Some evidence has emerged that Paxlovid, an oral antiviral medication used to treat COVID-19, may help reduce the risk of ending up with long-term COVID.
A November study of 56,000 people with COVID found that the more than 9,000 who took Paxlovid in the first five days of their infection had a 25% lower risk of long-term COVID symptoms. But the data from one study are inconclusive.
In a webinar last month, Dr. Ashish Jha, White House director of COVID-19 Response, told Bob Wachter, chairman of medicine at UCSF, that data showing therapies like Paxlovid can reduce the risk of long-term COVID are “highly observational.”
“I wouldn’t count on it,” he said, though he noted that “it does make sense clinically.”
Researchers at Stanford hope for more clear evidence. They are looking for candidates for a clinical trial to see if Paxlovid can treat long COVID if people already have it.
Reach Aidin Vaziri: avaziri@sfchronicle.com; Nanette Asimov: nasimov@sfchronicle.com, Twitter: @NanetteAsimov