As Covid-19 Surges, the Big Unknown Is Where People Are Getting Infected

Wall Street Journal:

Western nations face a big challenge in fighting the Covid-19 pandemic: Ten months into the health crisis, they still know little about where people are catching the virus.

The problem is becoming more acute as new cases are breaking records in the U.S. and Europe and pressure grows on authorities to impose targeted restrictions on places that are spreading the virus, rather than broad confinement measures that are wreaking havoc on the economy.

In Germany, authorities say they don’t know where 75% of people who currently test positive for the coronavirus got it. In Austria, the figure stands at 77%. In Spain, the health ministry said that it was able to identify the origin of only 7% of infections registered in the last week of October. In France and Italy, only some 20% of new cases have been linked to people who previously tested positive.

Jay Varma, senior adviser for public health in the New York City mayor’s office, said 10% of the city’s infections are due to travel, 5% from gatherings, and another 5% from institutional settings such as nursing homes.

“The vast majority of the remainder—somewhere probably around 50% or more—we don’t have a way to directly attribute their source of infection,” Mr. Varma said. “And that’s a concern.”

One reason for this deficit is that most contact-tracing systems set up to investigate infections haven’t been identifying enough contacts to map how the virus spreads. And whatever data they generate isn’t always being mined to inform how to craft more discriminating lockdowns.

Asian nations that have used contact tracing successfully to control the disease interview 10 or more contacts for each case. In the U.S., France, the U.K. and Spain, tracers are identifying fewer than four contacts for each case, according to government data.

Even when data can be gleaned from such limited and partial contact-tracing records, it is likely to be skewed by statistical distortions.

To illustrate this, Marc Lipsitch, an epidemiologist from Harvard T.H. Chan School of Public Health, points to government data in several countries showing most people are becoming infected at home. The home, while undoubtedly a driver of infections, probably tops the list, he said, because of how hard it is to trace infections that originate elsewhere.

“We may be putting too much emphasis on what we can trace…and not acting on the types of establishments that are very likely contributing but for which data isn’t as compelling,” he said.

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