Scientists need to know how many individuals have mild or asymptomatic cases of COVID-19 to have any idea whether the current social distancing efforts are helping to slow the spread of coronavirus, and to gauge how long we should go this way. The case numbers that rise by the day are not what they seem, since their symptoms are vague and possibly non-existent. The real figures may be much higher, or just a little higher.
Understanding the mild cases can help researchers get a grip on the spread of the disease; how it’s continuing to spread and how rampant it’s already become.
As many experts have demonstrated to me, it is not enough to know the amount of R0-the average number of individuals infected by each positive event. An R0 of two could mean that 20 individuals spread the disease to two individuals each, or that no one is given the disease by 19 individuals and 40 others are given it by one individual.
Jeffrey Shaman, a Columbia University professor who studies how the environment affects infectious diseases, has gathered evidence that a lot of not-very-sick people were driving the outbreak in China. He estimates that about 86% of infections were transmitted early in that outbreak by individuals who never got sick enough to go to the doctor.
The paper he co-authored, published in Science, was pioneered as a warning against individuals that have no idea that they are infected with silent spreaders. Another study found that people in one family had symptoms ranging from undetectable to extreme in China. Random studies performed in Iceland found that 50 percent of people that tested positive have no symptoms.
But deciding who gives it to others is more important than deciding who has caught the disease. Are people spreading the virus without symptoms? The research by Shaman indicated that about 86 percent of reported infections were picked up from people with undocumented infections.
Long before this new coronavirus appeared, through researching the similarities between symptoms and flu infection and colds, Shaman learned a lot about the kinds of symptoms characteristic of Covid-19. He discovered a lot of heterogeneity by identifying 2,500 individuals safe enough to be out and about in New York City, checking them for cold and flu viruses, and asking them about symptoms, people with symptoms who had no infection, and people with the infection that had no symptoms.
He and colleagues monitored 200 individuals by mobile phone in another study and noted daily symptom reports while checking them weekly for cold and flu viruses. What they discovered was that only one in four people with influenza are seeking medical care, and only one in 25 people with colds are seeking medical care.
It has been known that they will walk around for an average of five days and up to around two weeks after people become infected before they begin to feel sick, and it is not yet clear how many of those days their bodies shed enough of the virus to infect others.
The result was that people were only half as infectious as those with them without reported symptoms, and yet they made up the majority of people who got the disease during that time and spread it. It isn’t relevant if they had no symptoms, were pre-symptomatic, or had mild symptoms. What is, he said, is the fact that a lot of people with the virus were feeling pretty good, going around society, and building new transmission chains.