SARS-CoV-2 is highly contagious, but a few dozen imported cases would probably not be enough to spark many major undetected outbreaks. Based on related diseases such as SARS and MERS, epidemiologists suspect that the coronaviruss spreading potential is irregular. In all likelihood, some sick people infect many others, but most infect just a handful. Alessandro Vespignani, a network scientist and public-health professor at Northeastern University, estimates that in each American city that later became a hot spot for COVID-19, perhaps 10 to 20 local transmission events occurred in January. Aside from the one or two infections that did seed major outbreaks in places such as Seattle and New York, most infections that arrived from outside the country in January would have been transmitted to at most a few people, then quickly fizzle out, Bedford told me.

Read: Why America is uniquely unsuited to dealing with the coronavirus

Establishing a more precise number of how many sick people carried SARS-CoV-2 to the U.S. early this year would require data that can be difficult or impossible to collect, especially during a major global-health crisis. For one: how many people were actually sick with COVID-19 around the world in January (or earlier). The official data out of Wuhan have been unreliable from the start. And countries that have since ramped up their coronavirus-detection efforts were not looking as carefully for cases at the beginning of the year. The World Health Organization did not declare a global-health emergency until January 30.

Researchers would also need to know where people traveled around the world in the early weeks of the pandemic. One of the big challenges of looking at actual global spread of this disease is that, from January on, travel patterns have been massively disrupted, Gardner said. When researchers dont have perfect travel data for a time and place theyre studying, she explained, they often substitute or extrapolate from data in the recent past. Sometimes you can say, Well, I dont have 2016 data, but Im using 2015 data. Thats representative. That does not apply anymore. The Chinese government shut down Wuhan on January 23; even before then, individual peoples movement patterns might have begun to shift in ways that are difficult to track.

When the living cannot be fully accounted for, one way to move forward is to tally the dead. Testing shortages mean that some COVID-19 deaths have gone undetected, but researchers can get a better handle on just how many people the virus killed during a given time period by looking at the excess mortality: how many more people died than would have been expected to under normal circumstances.

Last week, the National Center for Health Statistics published preliminary data on weekly excess deaths since January 2017, which will be updated as the pandemic wears on. Bob Anderson, the chief of the NCHSs mortality-statistics branch, told me that its the first time weve done something like this before the data were final. The hope is that researchers can use the gross numbers to estimate how many Americans died of COVID-19 over a particular period, and from there estimate how many Americans were infected. But picking out excess deaths in the first few weeks of this year will be difficult. Compared with the hundreds of thousands of deaths the country experiences in a typical month, a handful of COVID-19 deaths would hardly be a blip. Indeed, by the NCHSs count, the United States did not exceed the expected number of deaths by a significant margin until the week of March 22.

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Youll Probably Never Know If You Had the Coronavirus in January - The Atlantic