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The truth about the ‘silent’ Covid carriers

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Even if you don’t have any symptoms, you could still have Covid-19 and be a ‘silent,’ and potentially deadly, spreader of the disease. The specter of the asymptomatic carrier—the frightening idea that nobody is safe—has become one of the key drivers of lockdown and social distancing policies. 

When the SARS-CoV-2 virus was first detected, health agencies were told up to 80 percent of infected people could be asymptomatic, although that estimate has now fallen to around a third. 

But independent researchers have discovered two big problems with the asymptomatic carrier phenomenon: even the more conservative estimate of cases is still far too high, and your chances of getting the disease from a silent spreader are much lower than from someone with obvious symptoms such as coughing and sneezing.

Researchers at the Institute for Evidence-Based Healthcare at Bond University in Australia estimate that the true level of asymptomatic cases is around 17 percent—around half the current official figures—although some of the 13 studies they analyzed had the figure as low as 4 percent.1

Researchers from Wuhan, China, where the Covid pandemic was first identified, discovered just 300 asymptomatic cases among the 10 million people who had been given the standard PCR (polymerase chain reaction) diagnostic test. They stressed that the low number was probably the result of a strict 70-day lockdown in the province and shouldn’t be applied to other countries where the virus hasn’t been controlled.2

But the Wuhan researchers discovered something that other researchers can agree on: the asymptomatic people weren’t as infectious. The Bond University researchers estimate that asymptomatic carriers are 40 percent less infectious than people who have the usual Covid symptoms. Transmission rates for asymptomatic people are anything from three to 25 times lower than in people with Covid symptoms.3

Don’t follow the science

The specter of asymptomatic carriers loomed early in the Covid-19 outbreak. In one case, a woman from Wuhan infected five other family members in different areas of China while she remained asymptomatic for the entire 21-day follow-up period.

The outbreak of Covid on the Diamond Princess cruise ship apparently included many passengers who remained asymptomatic the whole time, even though tests showed they were infected.

An early review from the Centre for Evidence-Based Medicine in Oxford, UK, confirmed every health agency’s worst fears: up to 80 percent of people with the virus could be free of symptoms and yet infectious. The center also said in the same report that asymptomatic cases could equally be as low as just 5 percent of the total, but that was quickly forgotten.

The idea of asymptomatic cases quickly became front and center of policies to contain Covid’s spread—including social distancing and lockdown—but it was a decision that was more political than scientific. Although governments have insisted that their actions have been made by ‘following the science,’ the Bond University researchers could find only 13 studies out of the 2,454 papers on asymptomatic infection they reviewed that could be categorized as truly scientific.

Their conclusion that 17 percent of cases are asymptomatic mirrors a World Health Organization (WHO) study from 2015 that looked at the asymptomatic phenomenon among people with annual seasonal flu. After looking at a range of studies that had also come up with dramatically different results, the WHO researchers concluded that the average asymptomatic rate is 16 percent.4

There’s also the possibility that asymptomatic carriers are, in fact, merely presymptomatic; in other words, symptoms hadn’t manifested when they were tested. “Asymptomatic is someone who never developed symptoms ever throughout the course of their disease, and presymptomatic is somebody who has mild symptoms before they go on to develop symptoms,” said infectious disease researcher Krutika Kuppalli of the Medical University of South Carolina.5

Many of the studies the Bond University researchers uncovered had tracked people for an average of just seven days, although doctors have discovered most symptoms appear only between seven and 13 days after infection, so there is a reasonable chance they could have started displaying symptoms after the trial had ended.

One study found that 49 percent of supposed asymptomatic people went on to develop symptoms, so many of them were either presymptomatic or subclinical, which means they had only very mild symptoms. 

The researchers, from Fudan University in China, said the asymptomatic numbers could be boosted by children, with up to a third never displaying any symptoms, although they were less likely to infect others. If this is true, the real rate of silent spreaders could actually be as low as 9 percent.6

Another reason for the phenomenon could be that asymptomatic people have lower viral loads than someone who has symptoms, which means they may have the live virus in their blood but at levels that won’t trigger symptoms.7

Getting the wrong results

Even if 17 percent of infected people are asymptomatic, they may not be the deadly carriers that health agencies fear. For one, asymptomatic people don’t cough and sneeze—the two most common ways the virus is spread. It could also be because they don’t have an active infection, even though it has been detected by one of the standard tests (PCR or lateral flow).

As Carl Heneghan at the University of Oxford has observed, a person needs to have a complete live virus before it can be transmitted to someone else, and not just have the fragments detected by a test.8

With Covid, the test is the sole criterion to determine infection, even though with other diseases it is merely the beginning of a process that usually leads to a proper clinical diagnosis.

And the tests aren’t sensitive enough for doctors to rely solely on their results. The two major tests look out for RNA, which viruses carry, but they can’t detect whether the fragments are active or the remnants of a previous infection that has now passed. Only a laboratory test called viral culture can determine whether the virus is live or not, and that’s not being used for assessing Covid-19. “As things stand, a person who tests positive with any kind of test may or may not have an active infection with live virus, and may or may not be infectious,” said Allyson Pollock at Newcastle University. 9

The PCR test relies on a chemical process called polymerase chain reaction to target segments of the SARS-CoV-2 virus. The test duplicates, or “amplifies,” samples until there is enough RNA to be detected. However, amplification also means distortion, and this process can throw up false-positives— “seeing” a virus that isn’t there. Laboratory contamination is another problem that can also produce a false result.

The other test, lateral flow, is the latest, and fastest, way to detect Covid infection, producing on-the-spot results with a swab or saliva sample. But it’s very inaccurate, with one pilot program discovering it detected just 48 percent of infections that a PCR test had identified.10

Jon Deeks, leader of the Cochrane Collaboration Covid-19 test evaluation group, believes that wholesale use of lateral flow kits—which can be used in the home—is dangerous because it is not fit for purpose. 

“Low test accuracy would be less dangerous if people being tested and the public at large received accurate information about the risks and implications of a false-negative result. Instead they are being misled,” he says. Not only does it give false hope through a false-negative result, it also causes unnecessary worry with a false-positive.11

False trails

The PCR’s false-positive rate has been estimated to be as high as 5 percent, but the consensus is that it’s closer to 0.5 percent, suggesting there’s no great cause for concern—until you do the math, like Andrew Cohen from the Center for Research on Aquatic Bioinvasions. 

Based on current testing accuracy,  about 0.5 percent of uninfected people—one in 200—will get a false-positive Covid result, while 25 percent of infected individuals—one in four—will get a false-negative result. Assuming an infection rate of  1 percent (1,000 infected for every 99,000 uninfected), that works out to 495 of every 100,000 people being told they have Covid when they don’t (false-positives), and 250 people being told they’re Covid-free when they are actually infectious (false-negatives). 

This translates to a staggering 40 percent of all positive results—the false-positives divided by the total number of positives—ultimately being incorrect.

This surprising finding stems from the fact that the uninfected population is so much larger than the infected population (99 to 1), Cohen explains. And this rate will increase even more if infection rates fall below 1 percent.12

The false-positive cases could be higher still if the results of other studies are used as benchmarks. At a 4 percent false-positive rate, 3,960 uninfected people per 99,000 would be told they have the virus. Apply that to national populations, and more than 13 million Americans and 2.4 million Britons are false ‘cases’ of Covid-19.

The takeaway, says Cohen, is simple: don’t test people who have no Covid symptoms. “For these people, positive results will be more likely to be wrong than the averages indicate… and positive PCR results may likely be false.”

If the asymptomatic carrier isn’t the silent spreader we’ve been taught to fear, then full lockdowns—with all their own problems, such as mental health concerns, vital missed treatments and economic collapse—may not be necessary.

Instead, it just needs people who know they have a Covid infection—usually because of the symptoms they’re experiencing—to act responsibly and stay home. And if we’re properly protecting the old, vulnerable and frontline healthcare staff, we should have a strategy that can counter the worst that the SARS-CoV-2 virus can throw at us. 




JAMMI, 2020; 5.42; Nat Commun 2020; 11: 59173; Lancet Infect Dis, 2020; S1473-3099(20)30837-9


Epidemiology, 2015; 26: 862–72


Nature, 2020; 587: 534–5


J Med Virol, 2020; 93: 820–30 7; BMJ, 2020; 371: m4695


Clin Infect Dis, 2020; ciaa1764


BMJ, 2020; 371: m4851


BMJ 2020; 371: m4848


BMJ blog, January 12, 2021


Icd10monitor, False Positives in PCR Tests for COVID-19, Nov 9, 2020.

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Article Topics: infection, Infectious disease
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