Another just-reported study examined a statistically representative sample of 3,330 people in early April in Santa Clara, California, and looked for evidence of an immune response to SARS-CoV-2—a measure of how many had been exposed to the virus (perhaps without even knowing it) and had recovered. The result: between 2 percent and 3 percent of the overall population, a figure that’s “50-85-fold more than the number of confirmed cases,” according to the authors.

In other words, at least according to this estimate, far more people are exposed to the virus than are getting diagnosed. In part, this could simply reflect the lack of adequate testing—many of these patients might have been demonstrably sick. Or it could suggest that the percentage of asymptomatic cases is much higher than we currently believe. Or, despite the best efforts of the researchers, the sample may have been unknowingly skewed in such a way that makes extrapolation from this one study not especially solid.

But wait, there’s more.

As if the problem of asymptomatic patients wasn’t problematic enough, consider fresh data suggesting that among COVID-19 patients who do show symptoms, the most contagious time is likely one to two days prior to symptom onset. Which, if true, would mean that stopping the spread through measures such as temperature checks would be difficult.