The politics of this question, “How many asymptomatic carriers are out there?”, are tricky. On the one hand, evidence that there’s a large number of infected people walking around bolsters the argument that maaaaaybe the epidemic won’t be as bad as we think. Maybe the Oxford model is right and there are actually a gigantic number of people in the population who’ve had the virus but didn’t know it. We’re closer to herd immunity than we think!
On the other hand, having a huge asymptomatic population would undercut the argument that we should reopen for business soon because official case counts are low or declining in some places. If there’s a giant number of undetected infected people without symptoms then the true number of cases around us is much higher than the confirmed number. We’re all at greater risk of catching the disease than we realize even in small social gatherings once we’re no longer isolating. Actual cases could explode quickly — unless you think that the numbers of asymptomatic carriers is so massively huge that we’re already close to herd immunity. Which, as far as I’m aware, isn’t borne out by any available data here or abroad.
Plus, the more asymptomatic people there are, the more urgent it becomes to ramp up testing and contact tracing near-term. If, say, 90 percent of all patients go on to eventually develop symptoms, we could take a lot of those people out of public circulation early by encouraging people to self-isolate for two weeks at the first sign of any symptom — fever, cough, what have you. (Unfortunately, many of those patients will have already infected others before self-isolating. According to studies, coronavirus victims are most contagious *before* symptoms begin to show.) If instead only 60 percent of all patients eventually develop symptoms then we’re destined to miss a gigantic number of infected people who are out and about spreading the plague unbeknownst to everyone. The only solution: Test as many as possible, including those without symptoms.
A new study of Vo, a town in Italy that saw the country’s first COVID-19 fatality, found an average of 43 percent of residents who tested positive were asymptomatic across two different samples.
On the 21st of February 2020 a resident of the municipality of Vo, a small town near Padua, died of pneumonia due to SARS-CoV-2 infection. This was the first COVID-19 death detected in Italy since the emergence of SARS-CoV-2 in the Chinese city of Wuhan, Hubei province. In response, the regional authorities imposed the lockdown of the whole municipality for 14 days. We collected information on the demography, clinical presentation, hospitalization, contact network and presence of SARS-CoV-2 infection in nasopharyngeal swabs for 85.9% and 71.5% of the population of Vo at two consecutive time points. On the first survey, which was conducted around the time the town lockdown started, we found a prevalence of infection of 2.6% (95% confidence interval (CI) 2.1-3.3%). On the second survey, which was conducted at the end of the lockdown, we found a prevalence of 1.2% (95% CI 0.8-1.8%). Notably, 43.2% (95% CI 32.2-54.7%) of the confirmed SARS-CoV-2 infections detected across the two surveys were asymptomatic. The mean serial interval was 6.9 days (95% CI 2.6-13.4). We found no statistically significant difference in the viral load (as measured by genome equivalents inferred from cycle threshold data) of symptomatic versus asymptomatic infections (p-values 0.6 and 0.2 for E and RdRp genes, respectively, Exact Wilcoxon-Mann-Whitney test). Contact tracing of the newly infected cases and transmission chain reconstruction revealed that most new infections in the second survey were infected in the community before the lockdown or from asymptomatic infections living in the same household.
The fact that the viral load in those with symptoms wasn’t different from the load in those without is a confounding footnote to the results. Why do some infected people get sick and some don’t? If it were a question of some getting a higher dose of the virus, e.g., crossing some “threshold” where fever and cough inevitably begin to develop, it’d be easy to understand. As it is, scientists are clueless.
Vo isn’t the only discrete population with a massive number of asymptomatic infected people. I mentioned this a few days ago but it’s worth flagging it again:
The Navy is the military branch hardest hit by the coronavirus, with over 1,000 cases among military personnel. And, the majority of those cases, 660, are aboard the USS Theodore Roosevelt…
On NBC’s “Today Show” Thursday, Secretary of Defense Mark Esper revealed that more than half of the infected sailors never had any symptoms.
“What we’ve found of the 600 or so that have been infected, what’s disconcerting is a majority of those, 350 plus, are asymptomatic,” he said, adding, “So it has revealed a new dynamic of this virus that it can be carried by normal, healthy people who have no idea whatsoever that they are carrying it.”
Nearly 60 percent of sailors on the Roosevelt who tested positive were asymptomatic. That’s reminiscent of another famous ship with lots of sick people aboard: When passengers on the Diamond Princess cruise liner were tested in March, 46.5 percent who had the disease were asymptomatic. If that’s not enough for you (and if you’re willing to trust Russian data, which is risky), the head of Russia’s national public health institute claims that their own testing reveals roughly 45 percent of the infected have no symptoms.
In any population that’s been ravaged by COVID-19, then, it seems 40 percent or better of those with the virus get off scot-free. That says nothing about prevalence, though, which is what the Oxford model is ultimately concerned with. In Vo, less than three percent of the population was actively infected with the virus when the first round of testing were conducted. What’s more, I haven’t heard of a study yet that’s tracked people over the course of several weeks to see if those without symptoms at the time of testing went on the develop symptoms eventually. Surely some number in each study just so happened to be tested shortly after they were infected, at a moment before the disease was about to hit them hard. We need a way to separate the true asymptomatics from the pre-symptomatics.
But for the moment, it’s enough to say that there’s a non-negligible number of infected people without symptoms walking around, spreading the virus. Which leads to an inescapable conclusion: We’re testing the wrong people.
If the goal is to restart the American economy, the United States isn’t performing anywhere near enough tests. Worse still, we are testing the wrong people. To safely reopen closed businesses and revive American social life, we need to perform many more tests—and focus them on the people most likely to spread COVID-19, not sick patients…
If we want to control the spread of COVID-19, the United States must adopt a new testing policy that prioritizes people who, although asymptomatic, may have the virus and infect many others.
We should target four groups. First, all health-care workers and other first responders who directly interact with many people. Second, workers who maintain our supply chains and crucial infrastructure, including grocery-store workers, police officers, public-transit workers, and sanitation personnel. The next group would be potential “super-spreaders”—asymptomatic individuals who could come into contact with many people. This third group would include people in large families and those who must interact with many vulnerable people, such as employees of long-term-care facilities. The fourth group would include all those who are planning to return to the workplace. These are precisely the individuals without symptoms whom the CDC recommends against testing.
What’s the point of testing patients who have symptoms at this stage? Everyone with a fever, cough, chills, aches, and especially shallow breathing or some combination thereof logically assumes they have the disease and acts accordingly. Most are told to ride out their illness at home anyway. Maybe you test them if they need to go to the hospital and there’s some doubt about whether it’s COVID or the flu, and for some reason a chest CT scan can’t resolve that doubt. But otherwise we’re better off using scarce tests to sniff out the asymptomatic.
