A pharmacy technician holds a dose of the Johnson & Johnson Covid-19 vaccine before it’s used for a clinical trial on December 15, 2020, in Aurora, Colorado. | Michael Ciaglo/Getty Images

The vaccine is very effective — and most importantly, it only requires one shot.

One big reason to be excited about the new Johnson & Johnson vaccine for Covid-19, which was authorized by the Food and Drug Administration over the weekend for emergency use in the US: Unlike the Moderna and Pfizer vaccines already in use, it requires only one shot for full protection.

That’s a big deal. From a practical standpoint, it means that the new vaccine could really speed up America’s vaccination campaign — certainly more than another two-dose vaccine would. It also fixes a problem that’s long bedeviled medical treatments that require multiple doses: A lot of