More COVID-19 tests are being done, but . (Public Health – Seattle & King County via Twitter)
Billionaire Elon Musk knows his stuff when it comes to rockets or electric cars. But does that translate into epidemiological expertise? Not completely, according to Washington state’s coronavirus trackers.
Musk weighed in today in a Twitter response to a GeekWire story that focused on Washington’s rising number of COVID-19 cases as well as Washington Gov. Jay Inslee’s appearance on CBS’ “Face the Nation.”
“The situation is critical in many places across the United States, and all the happy talk and wishful thinking in the world is not going to wash that away,” Inslee said. “So we are taking very strong measures to get people to mask up. We know that’s the solution from a health standpoint, and the way to reopen our economy.”
Musk, in contrast, has voiced skepticism about some of the restrictions that were put in place in California to quell the spread of coronavirus. During an earnings call with financial analysts in April, the Tesla CEO said the state’s stay-at-home order was “fascist” in nature. “Give people back their goddamn freedom,” he said.
He was more measured in today’s tweets, which called attention to the fact that the number of COVID-19 cases was rising dramatically while the number of coronavirus-related deaths was not:
There are a ridiculous number of false positive C19 tests, in some cases ~50%. False positives scale linearly with # of tests. This is a big part of why C19 positive tests are going up while hospitalizations & mortality are declining. Anyone who tests positive should retest.
— Elon Musk (@elonmusk) June 29, 2020
Musk didn’t respond to our tweet asking about his sources, and he doesn’t have a great track record when it comes to handicapping the pandemic. For example, in March he said the number of new cases would be “probably close to zero” by the end of April. (The seven-day moving average is actually around 40,000 per day and rising in the U.S.) But we think we see where he’s coming from: It has to do with the nuts and bolts of testing, including concepts such as pre-test probability, test sensitivity and test specificity.
Let’s take the example of Washington state: Nearly 550,000 COVID-19 tests have been conducted in the state, and roughly 6% of the people tested (nearly 33,000) have gotten back positive results. Suppose the specificity of a given test is 97%. That means that a test could come back positive for 3% of the people who don’t have the virus that causes COVID-19, typically because the test is actually being triggered by a different kind of virus.
In that scenario, there might be as many “false positives” (16,500) as there are “true positives” (16,500). You can see how it works using this interactive from The BMJ, a British medical journal. Just plug in 3% for pre-test probability, 97% for test specificity, and any number above 85% for test sensitivity.
That could explain how you get to a 50-50 mix of true positives and false positives. In reality, though, PCR tests tend to have higher than 97% specificity, which means it’s rarer that they would mistake a different sort of virus for the virus that causes COVID-19.
Charissa Fotinos is deputy chief medical officer for the Washington State Health Care Authority. (WSHCA Photo)
The interplay of variables also makes a difference. “If you’re in a hotspot like Houston, or Yakima, right now the prevalence [of the virus] is higher,” said Charissa Fotinos, who oversees the state’s testing program as deputy chief medical officer for the Washington State Health Care Authority. Relatively speaking, false positives should account for less of the total in a virus hotspot.
So if your coronavirus test comes back positive, should you take Musk’s prescription and insist on another test? That depends, Fotinos said in a statement that includes a heaping helping of “ifs.”
“If someone who is asymptomatic and had no contacts with anyone who had symptoms, or who was known to have COVID, tests positive, you could question if that was a true positive — if there’s a low number of infections in the community,” she said. “But if you have symptoms, or if you were told that you were in contact with someone who has COVID, you should heed that.”
Fotinos is more concerned about false negatives. That’s because not isolating yourself if you have COVID-19 has more serious consequences for the wider community than isolating yourself if it turns out you don’t have the disease. (At the same time, Fotinos acknowledged that it’s a “bummer” for the person involved.)
As more tests are conducted, public health experts want to see a decline in the proportion of positive tests. “Generally we’d like that to be below 2%, or even 1%,” Keith Jerome, a virologist at Seattle’s Fred Hutchinson Cancer Research Center and UW Medicine, said in an email. “That means you’re doing enough tests to find the people who actually have the virus, and get those people away from others so that the virus doesn’t spread further.”
Unfortunately, that’s not the case for Washington state’s Yakima County or other hotspots. “What you’re seeing now in too many parts of the country is lots of tests being run, with a much higher percentage of those tests being positive,” Jerome said. “In some places 10% or more of tests are positive, and that means the virus is not at all under control.”
Fred Hutch biologist Trevor Bedford said in an email that the sheer numbers argue against false positives being the explanation for the reported rise in cases.
“The rapid rise in percent positive of tests in Arizona, Florida and Texas strongly indicates that the rise in cases is real,” Bedford wrote. “In Arizona, daily tests since June 1 have a bit more than tripled from ~4000 to ~13,000, while daily confirmed cases have increased tenfold from ~300 to ~3000. There are large increases in hospitalizations as well, which won’t result from increased testing.”
Bedford has explored the dynamics of coronavirus testing in multiple Twitter threads. The way he sees it, the statistics support the view that the focus of the pandemic has shifted from older to younger age ranges. Researchers also have noted that COVID-19 tends to hit older people harder than younger people (though it still hits hard). That could partly explain why the number of deaths hasn’t risen as steeply as the number of cases.
But Jerome said the jury is still out. “Unfortunately, hospitalization and fatalities usually trail the first positive test by a week or two, so those measures are likely to get worse in the coming days as well,” he told GeekWire.