Long COVID may now be less common than previously thought
Immunity from vaccination appears to lower risk over time, according to new data
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Long COVID can be a severely debilitating condition for those who live with it, but the growing list of symptoms and conflicting estimates on how often it occurs make it incredibly difficult to measure exactly how many people it affects.
Post-COVID-19 condition, as it's called by the World Health Organization (WHO), is also not an inevitability for most people who get infected, and it now appears significantly less common than earlier research suggested — thanks in part to vaccination.
Based on data from the early in the pandemic, the WHO estimates placed the condition at a rate of between 10 to 20 per cent of COVID-19 patients, while the Public Health Agency of Canada (PHAC) states it can occur in between 30 to 40 per cent of those not hospitalized.
Canada's Chief Public Health Officer Dr. Theresa Tam went as far as to say back in May that long COVID can affect up to 50 per cent of all patients, adding that the symptoms can be "quite broad and non-specific."
But with estimates that more than half of Canadians have been infected with COVID since December after the emergence of Omicron and its highly contagious subvariants, there is a lack of evidence to suggest there are currently millions of COVID long haulers in Canada.
Newer research suggests long COVID is occurring at a much lower rate than estimates from early in the pandemic, before widespread vaccination. PHAC is now working to better understand the true number of cases — while acknowledging their data is outdated.
"Long COVID is real. There are a lot of people suffering from it," said Bill Hanage, an epidemiologist at Harvard University's T.H. Chan School of Public Health in Boston.
"But you don't serve those people by pretending that 40 per cent of the population is in that boat. In my view, it's actually a bit disrespectful to the people who are genuinely suffering from long COVID to pretend that that is the case."
Estimates based on outdated research
Many of the estimates cited by health organizations are based on early data that largely looked at patients in 2020, long before COVID-19 vaccines and Omicron dramatically changed the immunity landscape in Canada and around the world.
One study published in The Lancet in July 2021, cited by PHAC as one of its main sources for its estimate that 30 to 40 per cent of non-hospitalized patients develop long COVID, looked at fewer than 1,000 patients between April 2020 and December 2020.
"I assume that due to vaccination and the Omicron variant, fewer people will now be affected by long COVID," Clara Lehmann, a lead author of the study and professor at the department of Internal Medicine at the University of Cologne in Germany, said in a recent email.
PHAC also cites two systematic reviews as evidence for its high estimates of long COVID — a preprint study authored by its researchers from late 2021 that has not yet been peer reviewed, and a study in The Journal of Infectious Diseases from April.
Many of the papers analyzed in the studies are from before the emergence of Omicron and COVID-19 vaccines, while a significant proportion also had no control groups from the general population to compare against. The lead author of The Lancet study PHAC cited also said she expected the rate to be much lower.
"I believe that the proportion [of long COVID] has gone down," said Bhramar Mukherjee, lead author of the Lancet study and biostatistics and epidemiology professor at the University of Michigan.
"There are many more studies now with a vaccinated population, and initially it was not really clear what the prevalence is, but it seems like there is a considerable effect."
A U.K. study published this week in Nature identified up to 62 symptoms associated with long COVID, including hair loss and erectile dysfunction, and found 5.4 per cent of non-hospitalized patients reported at least one symptom three months after an infection.
In comparison, 4.4 per cent of people with no recorded evidence of COVID-19 infection reported at least one symptom — a difference of just one per cent. People in this group were not specifically tested for the study and found to be negative.
That's in line with a recent survey from the U.K.'s Office for National Statistics that found the rate of long COVID was just over four per cent with Omicron BA.1 or BA.2 breakthrough infections in triple vaccinated adults, which was lower than with Delta at five per cent.
Akiko Iwasaki, a professor of immunobiology at Yale School of Medicine in New Haven, Conn., said it's not entirely clear yet how much vaccination helps in preventing long COVID. Some studies have shown it can reduce the risk by half and others showed significantly less benefit, but emerging research suggests they lower the rate significantly.
"That could likely be related to the fact that we have immunity to some extent from vaccination and potentially prior infections," she said. "Also there may be some intrinsic difference between the variants of concern."
In a statement to CBC News, a spokesperson for PHAC clarified that "there is currently insufficient pan-Canadian data to estimate the number of long COVID patients in Canada" and the rates of 30 to 40 per cent on their website "predate the arrival of Omicron."
"The estimates should not be used to extrapolate how many Canadians may have [long COVID] in 2022 since the arrival of the Omicron variant and sub-variants," the statement read, adding they are currently in the process of updating their ongoing systematic review.
"The evidence reviewed by PHAC suggests, based on a small number of studies, that COVID-19 vaccination prior to COVID-19 infection may help to reduce the risk of developing [long COVID]."
Confusion over long COVID symptoms
The confusion lies with the different definitions of what long COVID actually is, coupled with the fact that the level of immunity in the population from prior infection and vaccination has vastly changed the risk of developing it.
And while some symptoms can be life-altering, others can be much less severe or hard to attribute to COVID-19 altogether — making it incredibly difficult to study accurately.
"It's fuzzy, the criteria are not sufficiently settled to permit statements that are as strong as some people make," said Hanage from Harvard. "You need to decide exactly what you mean by long COVID and recognize that there are a lot of different sorts of long COVID."
The WHO lists dozens of long COVID symptoms that aren't explained by another diagnosis — from fatigue, shortness of breath and cognitive dysfunction, to anxiety, depression, sleep disorders and loss of taste or smell — that can last at least two months after an infection.
The U.S. Centers for Disease Control and Prevention classifies long COVID as at least 19 symptoms that range widely from general tiredness to respiratory and heart conditions, neurological symptoms and digestive issues that can occur after one or even three months.
PHAC states there can be more than 100 symptoms of long COVID weeks or months after infection but narrowed its list of common ones to nine — including general pain and discomfort, difficulty thinking or concentrating and posttraumatic stress disorder (PTSD).
"How frequently it occurs kind of depends on the definition of long COVID, and there is no universal definition currently," said Iwasaki. "As with everything else, the statistics are changing at different stages of the pandemic."
She said the fact that there are currently more than 200 long COVID symptoms across various health organizations that range in severity and duration in different populations throughout the pandemic only adds to the confusion.
"The estimates are all over the place," said Dr. Angela Cheung, a senior scientist-clinician at the University Health Network in Toronto who researches long COVID.
"Some will count any one symptom, like if you have one lingering symptom you have long COVID, and that symptom may be very mild and doesn't really affect your daily life. Whereas some people have multiple symptoms and are totally debilitated and can't work."
Canada updating estimates on long COVID
Canada may soon have a better handle on the true rate of long COVID occurring in the population with the release of a survey from PHAC and Statistics Canada to determine the prevalence, risk factors, symptoms and impacts on daily life of the condition.
The first leg of the survey was launched in April 2022, with results expected early next year. PHAC said in a statement it also plans to conduct followup studies to examine changes in long COVID over time and longer-term outcomes in those who are affected.
"We need to get a better understanding of the degree as well," said Cheung, who is working with PHAC and Statistics Canada on the survey.
"Because while people may be more willing to put up with one or two symptoms, that doesn't really affect their activities of daily living or work, whereas people are less accepting of something that really disrupts their life."
Iwasaki said that while the rate of long COVID may be changing over time, the condition severely affects a significant proportion of the population who need ongoing support.
"People who've gotten long COVID in the original wave are still suffering," she said. "Some of them haven't recovered."
Hanage said the situation for severe long COVID can be improved by ensuring people have prior protection from vaccination, improving research into the condition and finding therapies to help those who need it most.
"Even if the actual risk of serious long COVID symptoms is pretty low, and I actually think it is, that's not much comfort to the millions of people who are going to end up suffering severe long COVID," he said.
"It's just that you individually being infected are more likely than not to make a full recovery."
- A previous version of this story was updated to clarify the results of a study published in Nature that found 4.4 per cent of people with no recorded evidence of COVID-19 infection also reported at least one symptom of long COVID.Jul 30, 2022 10:36 AM ET