Dan Kitwood/Getty Images

The United Kingdom is experiencing an uptick in coronavirus cases. It's "tiny" compared to previous spikes, David Leonhardt writes in The New York Times, but he adds that the country will still be "important to watch" in the next few weeks. So, considering the U.K. has had a mostly successful vaccine rollout, why is the increase happening?

The primary culprit is likely the more transmissible Delta variant, first identified in India, that's been spreading in the U.K. The good news is that it appears vaccines hold up pretty well against the mutatated virus, but less so for people who have received only one vaccine dose, which is more common in the U.K. than it is in the United States. The U.K. chose to prioritize first shots early in the rollout, which allowed a wider swath of people to get at least some protection. That was "not necessarily" a mistake, despite the Delta variant changing the game a bit, Leonhardt writes, but it does mean more people — especially younger people who are still in the middle of the vaccination process — have a higher risk of getting infected. Fortunately, about 90 percent of U.K. residents older than 65 have received both doses, and the country "is accelerating second shots for vulnerable people" now, Leonhardt writes.

Dr. Monica Gandhi, who specializes in infectious diseases at the University of California, San Francisco, also pointed out that the U.K. "does a lot of asymptomatic testing" even among the vaccinated population, which may play a role in the increase in cases and help explain why, for now at least, they aren't accompanied by rises in hospitalizations and deaths.