The Math That Explains the End of the Pandemic

...Every case of Covid-19 that is prevented cuts off transmission chains, which prevents many more cases down the line. That means the same precautions that reduce transmission enough to cause a big drop in case numbers when cases are high translate into a smaller decline when cases are low. And those changes add up over time. For example, reducing 1,000 cases by half each day would mean a reduction of 500 cases on Day 1 and 125 cases on Day 3 but only 31 cases on Day 5.

The end of the pandemic will therefore probably look like this: A steep drop in cases followed by a longer period of low numbers of cases, though cases will rise again if people ease up on precautions too soon...

...It is possible to bring Covid-19 case numbers down quickly via exponential decay even before vaccination rates reach herd immunity. We just need to keep transmission rates below the tipping point between exponential growth and exponential decay: where every person with Covid-19 infects fewer than one other person, on average. Every single thing people can do to slow transmission helps — including wearing masks, getting tested and avoiding crowded indoor spaces — especially given concerns about current and future variants, since it could be what gets us past the threshold into exponential decay...

https://www.nytimes.com/2021/04/29/opinion/covid-exponential-decay.html

Zoë McLaren (@ZoeMcLaren) is an associate professor in the School of Public Policy at the University of Maryland, Baltimore County. She studies health and economic policy to combat infectious disease epidemics, including H.I.V., tuberculosis and Covid-19.