The silent spread of COVID-19: there is mounting evidence that many infections go undetected


In Iceland, almost half of people with COVID-19 showed no symptoms when they were taken for a scientific study. In the United States, preliminary results of a new antibody survey suggest that up to one in five New Yorkers may have been exposed to the new coronavirus. And in the waters off the coast of Guam, more than 850 sailors from a Navy warship were infected during a massive epidemic – with about half reporting no symptoms when tested.

At the start of COVID, experts were skeptical of the first asymptomatic case reports, which did not appear to play an important role in the spread of the disease. But four months later, there is mounting evidence that “silent diffusers” are a major driver of the pandemic, with top scientists now speculating that they could account for between 25 and 50 percent of all cases.

Many crucial questions remain, but experts believe that silent spreaders are probably one of the embers that are keeping this pandemic on fire. And the implications are potentially huge for everything from testing strategies to how vulnerable nursing homes are protected – especially as countries like Canada are moving towards relaxing restrictions.

“I flip-flopped on this,” said Dr. Isaac Bogoch, a specialist in infectious diseases at the University of Toronto and the Toronto General Hospital. “At the beginning, I was quite skeptical about the role of asymptomatic people in its transmission and I said it very clearly. But I changed my mind to reflect the data that emerged.

” It is important. And when we fully understand who is truly asymptomatic, it will likely be one of the many important components of the epidemic. “

Bogoch said that silent spreaders should be considered in three distinct categories: “pre-symptomatic” cases (people who appear healthy when they are positive but later develop symptoms); true asymptomatic, who never suffer as much from a sniff; and “subclinical” infections, where people have mild symptoms but don’t recognize them as COVIDES.

The existence of asymptomatic cases was recognized early on by the World Health Organization, which sent experts to China in February to investigate the new coronavirus. But at the time, officials said there was not enough data yet to conclude that these cases played an important role.

“There is no evidence that we see only the tip of a large iceberg, with nine tenths made up of hidden zombies shedding the virus,” said Dr. Bruce Aylward, the WHO expert who led the mission in China. York Times in early March. “What we see is a pyramid: most of it is above ground. “

But an increasing body of scientific evidence has now emerged to suggest that, although symptomatic cases are still the most important source of contagion, a surprising proportion of COVID infections are likely to occur below the surface of detection, which raises concerns. questions about current screening strategies that narrowly target people with symptoms.

Among this emerging literature are 11 “cohort” studies from countries like Iceland, Italy and Japan, where proactive testing has revealed a hidden pool of COVID. In these studies, between 31 and 88 percent of people showed no symptoms when they tested positive for the virus, said Trish Greenhalgh, a professor at Oxford University, in a written statement shared with the journalists.

In the United States, recent cases have also made the headlines, including the aircraft carrier USS Theodore Roosevelt, where an explosive COVID outbreak infected more than 850 crew members, about half of whom said they had not no symptoms during testing. At a homeless shelter in Boston, general tests recently revealed 147 positive cases.

“The number of positives was shocking, but the fact that 100% of the positives had no symptoms was just as shocking,” Dr. Jim O’Connell, president of Boston Health Care for the Homeless Program, told a local news station. .

There is now concern that silent spreaders may reignite COVID outbreaks in countries that have already postponed their first epidemic wave.

Authorities in China have recently stepped up efforts to find and quarantine infected people who have no symptoms, and on April 1 the government began publishing statistics on asymptomatic cases. In the first 24 hours, they reported 166 new infections, 130 of which were found to be asymptomatic, according to an article in the British Medical Journal. (It is not known how many of these people may have developed symptoms later.)

In Canada, the federal public health agency says it does not know to what extent the country’s epidemic is due to the silent spread “but we do know that it occurs in people in close contact or in close physical environments, “said Anna Maddison, spokesperson. for the Public Health Agency of Canada (PHAC), in an email.

But although Canada does not actively monitor asymptomatic cases, the provinces and territories that have submitted case reports to PHAC have identified at least 220 cases classified as asymptomatic, which represents 2.8% of the 7,879 reports. where a state of symptoms was known. (More than 22,200 case reports have been submitted to date.) This is certainly a low number, since the tests have been narrowly targeted at people with symptoms and the surveillance data is incomplete .

Despite the accumulated evidence of asymptomatic infections, however, there are still important gaps to be filled before we can fully understand the importance of silent spreaders, said Shelly Bolotin, infectious disease epidemiologist at Public Health Ontario and at the Dalla Lana School of Public at the University of Toronto. Health.

“We are still plagued by this first wave of disease,” she said. “We don’t have all the pieces of the puzzle yet so we can really tell. “

A missing piece of the puzzle is the effectiveness of asymptomatic people in transmitting the virus, she said. A handful of studies have shown that people can infect others before developing COVID symptoms. Others have demonstrated comparable viral loads (the amount of virus measurable inside someone) between patients with and without symptoms – a surprising and counterintuitive finding, said Bolotin.

But proving asymptomatic transmission is particularly difficult. Studies in China showing family reunification, where an asymptomatic person appears to have infected someone else in the household, is convincing – but given the scale of the epidemic at the time, there is always a chance let these two people get infected with a third person, she said. Notes.

“I think we can theorize that this is happening, but the devil is in the details,” she said. “How infectious are they really?” Overview, I think we are still learning. ”

Bogoch disputes a large part of the breathless discourse around the “asymptomatic COVID” which fails to distinguish the presymptomatic, asymptomatic and subclinical cases.

He reports two prominent publications in the New England Journal of Medicine, which some have described as evidence of COVID “asymptomatic” – one from Iceland, where 43% of positive cases reported having no symptoms, and one letter publication New York doctors who tested all pregnant women giving birth in two hospitals and found that 29 of the 33 women who tested positive had no symptoms on admission.

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But no study has been able to follow up with each study participant to verify that they did not develop symptoms afterward, said Bogoch. It is also possible that many people who consider themselves asymptomatic simply cannot recognize their symptoms as COVID, he adds.

“In the COVID follow-ups I see, many of these people were the close contact of someone symptomatic (and) classified as asymptomatic,” said Bogoch. “But all you have to do is take a story. “So did you have any symptoms, did you feel bad?” And they’ll say, “Well, I had a few night sweats for a few nights. Well, it’s not asymptomatic. ”

One of the first studies that attempted to sort out presymptomatic cases from true asymptomatic ones was an analysis of the cruise ship Diamond Princess, where 634 people tested positive for COVID after being quarantined for two weeks on the ship. After testing people throughout the quarantine period and tracking their symptoms, the researchers used a statistical model to estimate an asymptomatic rate of 18%.

This finding has since been supported by unpublished research on American passengers aboard the ship, who have been repatriated and followed by researchers at the US Centers for Disease Control and Prevention, according to one of the study’s authors, Gerardo Chowell, an epidemiologist at Georgia State University in Atlanta.

Chowell believes 18% is an underestimate of the fraction of asymptomatic cases in the larger population, however – most cruise ship passengers were older and younger people were less likely to develop symptoms. Based on what other studies have reported, he estimates that the true asymptomatic rate is closer to 30 or 40 percent of total infections.

But when it comes to really understanding the hidden “iceberg” of COVID infections, some of the best evidence will come from antibody research or seroprevalence studies. Preparations are underway to do this work in Canada, including at Public Health Ontario, but the lab is still waiting for these tests to be validated and approved by Health Canada, said Bolotin.

However, studies on antibodies have already started to spread around the world, with studies of varying size and quality disseminated from Austria to California. But many of these early efforts have been controversial, with critics pointing to flaws in methodology or concerns about tests that may be prone to false positives.

But taken together, this new body of evidence suggests that the majority of COVID cases are currently undetected, said David Fisman, epidemiologist and professor at the Dalla Lana School of the University of Toronto, who analyzed published research. until now.

“This has enormous implications for the effectiveness of case isolation, contact tracing and quarantine,” he said. “If we miss most cases, we cannot expect these strategies to be effective. “

This means that tools like physical distance are becoming more important, he added – a message difficult to hear for many people who are emotionally or financially suffering from current restrictions.

The role of silent spreaders will need to be addressed urgently in vulnerable environments like long-term care homes, where they have likely played an inordinate role, said Dr. Nathan Stall, geriatrician and researcher at Sinai Health System. On Friday, a new study by the New England Journal of Medicine on a Washington state nursing home found that 56% of COVID-positive residents had no symptoms when they were positive; half continued to develop symptoms, although many were infectious several days earlier.

Stall suspects that asymptomatic workers played a major role in the introduction of COVID into nursing homes. And once inside, the virus tends to spread unnoticed, in part because frail and elderly people with COVID tend to have atypical symptoms like confusion, decreased appetite, or falls – many of which are easily mis-attributed to other conditions.

When jurisdictions like Ontario reopen their economies, policymakers will need to ensure that vulnerable people are protected from silent spread, he said. In high-risk settings like nursing homes, he added, this could mean a continuous COVID test for all workers and residents.

“As we relax these restrictions, and we have people who spread asymptomatically, this is going to be a huge problem,” he said.

“If you plan to release restrictions, it should be done with care, and especially with close monitoring to make sure we don’t get a rebound. (Otherwise) you’re just going to be hit again with an overwhelming second wave. ”


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