In a column for the Washington Post, Harvard epidemiology professor Marc Lipsitch writes that as even as the number of US cases of coronavirus continues to mount, the real number may be exponentially greater – and that the implications of that fact are both complex and counterintuitive.

Mr Lipsitch’s column focuses in large part on the US’s failure to roll out a fast and widespread testing programme, which has left the country lagging behind others such as South Korea.

“The feckless federal response created such delays in testing that most cases here are not being confirmed, even now.

“We don’t know even approximately how many people are infected, but it’s certainly more than the current count of more than 35,000 confirmed cases. Even though many places are reporting relatively small numbers of confirmed cases, this is not comforting.”

After a slow and disjointed start, the US is struggling to get ahead of the epidemic. With more than 40,000 cases now confirmed, many states have now issued blanket stay-at-home orders. New York and New Jersey in particular are seeing high infection rates.

In his column, Mr Lipsitch argues that to turn the crisis around the US must “massively expand” its testing capacity; currently, the number of tests performed per capita is one-thirtieth the number in South Korea.

However, he writes, “at this point, it’s not going to be possible to find and test all the cases that actually exist, even if we massively ramped up testing.”

Mr Lipsitch explains further: “It’s just a matter of numbers. If we only know about 1 in 10 cases, then even perfectly effective interventions on known cases can block only 10 percent of transmission. More likely in the United States, we know about an even lower proportion.”

That line, that the US may only know about a tenth or less of its cases, has been seized upon by others writing up the column’s contents. However, Mr Lipsitch soon took to Twitter to reiterate to understand the complexity of what he’s arguing – and not to mistake it for doom-mongering.

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“Paradoxically, given the level of distress we are seeing in the health care system, it is good not bad that we have many more cases. That means that the horrible outcomes we are seeing (because they are getting tested) are the tip of a bigger iceberg of milder cases.

“We can only hope the proportion of unobserved cases is large, because then we are closer to achieving herd immunity, and each bad outcome brings with it a larger number of mild outcomes that contribute to herd immunity. The math of epidemics is weird.”

But the true number of cases aside, Mr Lipsitch concludes that as the government tackles the immense difficulties in sorting out a mass testing programme, the first steps are both critically important and manifestly obvious. As he wrote in his column: