Why even a small increase in Covid-19 cases is so scary

Over the past few days, Covid-19 cases have taken an upward turn — a trend that led Centers for Disease Control and Prevention Director Rochelle Walensky to say she has a sense of “impending doom.”

The increase might seem small; the US is still better off than it was in January. And news about America’s Covid-19 vaccine rollout keeps getting better and better. But there’s a reason Walensky and other public health officials and experts are still so worried about the uptick in cases: exponential spread.

With Covid-19, as well as other infectious pathogens, the start of new waves of disease comes slowly. But as more people get infected with the virus, the surge starts to pick up. Pretty soon, daily new cases can start doubling in a matter of days or weeks — and by then, any reaction from the public or policymakers is doomed to be too little, too late. It’s made even worse by the possibility of coronavirus variants: As the virus spreads and replicates exponentially, it gets more chances to mutate, potentially leading to another new, possibly more infectious variant.