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If that statement was true, this would be a massive sea change in how we deal with the virus. Essentially, rather than taking steps as a community to prevent the spread, we simply would have to focus on those who show symptoms. Thermometers and symptom checks would open the doors to any activity: cruise ships, sports arenas, dance floors, anything.
Unfortunately, the truth is that that headlines likely don’t reflect reality. And it’s worth figuring out what the real story is with asymptomatic transmission of the virus.
First, this wasn’t an official statement from the World Health Organization, it was one doctor’s response to a journalist in a question and answer session. That doctor was Dr. Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit — certainly qualified to be discussing such matters. The official transcript of her remarks is here.
In answering the question about asymptomatic transmission, Van Kerkhove first sought to separate truly asymptomatic people from pre-symptomatic people.
There are some people who get the coronavirus who never feel sick at all. But as she pointed out to begin her response, many people who are labeled as asymptomatic at one juncture “are in their pre-symptomatic phase, which means it’s a few days before they actually develop severe symptoms.”
How many people are truly asymptomatic? Studies vary wildly. Most look at a specific population, whether that be people on a cruise ship, a jail, a homeless shelter, or living in Iceland, and find what percentage of people who tested positive reported symptoms at the specific time of the test. When you do that, you find huge percentages of asymptomatic people, from 30% to 90% depending on the group.
But when you look at only studies in which people are monitored for 14 days, you find a far smaller percentage of people who remain asymptomatic. A review of studies found that only 15% of cases stayed asymptomatic the whole time. Many or most of the people who test positive while asymptomatic get symptoms later on.
There’s more. Van Kerkhove said that “[What] we’re finding is that when we go back and say, how many of them were truly asymptomatic, we find out that many have really mild disease.” In other words, many people who report no symptoms for one reason or another actually do have some symptoms: a sore throat, an undetected fever, a rare cough that doesn’t seem out of the norm, etc.
Here are the symptoms reported by people who tested positive in Salt Lake County.
Think about how many of these symptoms you could explain away for reasons other than COVID-19. People cough all of the time. Muscle aches, headaches, and sore throats are common for all sorts of reasons. How many people proudly report diarrhea symptoms? Reporting being asymptomatic is one thing, but being truly asymptomatic is a higher bar.
Yes, Van Kerkhove said that, among those people who never get even mild symptoms, “from the data we have, it still seems to be rare that an asymptomatic actually transmits onward to a secondary individual.” That is good news: if a doctor rules out symptoms over close watch, we know that person is really unlikely to transmit that disease to another person.
For example, take the case of a truly asymptomatic 22-year-old woman in China who saw many folks in the time around her positive test. She was sent to the hospital for other reasons, and because her test came in early January before we understood coronavirus at all, she had tons of contacts: 35 patients, 224 hospital staff members, and 196 relatives — imagine their family parties! They tested all 455 people who came in contact with her. None of them tested positive.
But from the standpoint of everyday life, what we really want to know are the transmission details of both the truly asymptomatic people and the pre-symptomatic people. At any given point, we can’t tell without medical examination if someone is an asymptomatic carrier, a pre-symptomatic carrier, or doesn’t have the disease at all. Our policies need to be shaped with that majority of the population in mind.
And there, we have varying evidence. One study looked at the first 157 people who got the disease in Singapore and figured that pre-symptomatic transmission was most likely for 10 of those people, a rate of 6.4%. Another study from China said 12.6% of cases were likely caused by pre-symptomatic transmission. Another study of a Chinese hospital from Nature found 44% of cases that were from pre-symptomatic transmission.
Several study authors point out another problem: because our testing likely misses most asymptomatic cases and many mildly symptomatic cases, we may miss a big chunk of those people in our data. Models that attempt to address this have found anywhere from 33% to 79% of cases are caused by asymptomatic or pre-symptomatic spread.
We do know that people have significant amounts of the virus stuck in their throats and lungs before they develop symptoms, ready to be spewed. The Nature study referenced above found that viral load in patients typically started 2.3 days before they feel symptoms, and was at its peak 0.7 days before symptom onset.
Of course, when someone is symptomatic, they’re more likely to share the virus. Coughing releases a ton of virus-laden droplets, as does sneezing. Virus can get all over people’s hands from runny noses, and those caring for sick people can have closer contact.
But asymptomatic people can create those droplets too. We now know that singing releases a huge quantity of droplets, but so too does normal speaking — the louder the volume, the more droplets released. Again, people cough or sneeze for all sorts of non-coronavirus reasons, releasing the potentially high viral load they carry.
That’s probably why the WHO clarified the day after Van Kerkhove’s comments, saying “scientists have not determined yet how frequently people with asymptomatic cases of COVID-19 pass the disease on to others.”
Van Kerkhove’s response was taken out of context. Still, a greater degree of care needs to be taken by those who answer questions at the WHO.
The other issue at play is that pre-work or pre-activity symptom checks aren’t always effective, especially when only thermometers are used. First, look again at the symptom list from Salt Lake County. Only 36% of positive coronavirus cases had a measured fever when tested. That’s despite 41% of them feeling feverish!
If self-reported symptom checks are done, employers need to make sure that their employees aren’t incentivized to lie about or hide their symptoms. The Smithfield meat processing plant that offered their employees a $500 “responsibility bonus” to come to work every day from April 1 to May 1 was doing the exact opposite. (Smithfield has since reversed the terms of the “responsibility bonus,” instead giving it to people who don’t come to work after testing positive.)
I’m also reminded of the fate of the Antarctica cruise ship that sailed in mid-March after the pandemic was declared by WHO. They figured they were safe, because they didn’t let anyone board from high-risk countries. The ship had two physicians who screened the 223 passengers and crew before boarding for symptoms, a temperature check was also performed. Everyone was examined “regularly” on board by the doctors.
And yet, on Day 8, someone had a fever, and everyone quarantined. By the time the ship had tests delivered 12 days later, 128 people on board (59%) had been infected.
In short, minimizing the risk of asymptomatic transmission doesn’t do us any favors. Too many will confuse asymptomatic people for pre-symptomatic people, who do seem to create a large share of cases. Looking to stop symptomatic transmission alone doesn’t prevent putting people in dangerous situations.
It’s unfortunate, but true: beating this disease will take the efforts of everyone, not just those who are sick.