With a coronavirus variant spreading quickly throughout Great Britain, health officials in that country have changed how they plan to vaccinate the public. Now they will stretch out the interval between the two doses of the vaccines already in use, from three to four weeks to up to three months. And if necessary, someone given one manufacturer’s vaccine could get the other one if the one they received first is unavailable. They are trying to quickly vaccinate as many people as possible, but it’s hard to know how well that one dose will work, and for how long. The scariest possibility is that this adjusted vaccine administration creates forms of the virus that are resistant to the vaccine.