For some, COVID-19 just won’t go away. They are called “persistent” coronavirus cases, and some can count the duration of their infections in months, not just days. But one seems to stand alone. A U.K. patient with a severely weakened immune system had COVID-19 for almost a year and a half, scientists reported, underscoring the importance of protecting vulnerable people from the coronavirus. There’s no way to know for sure whether it was the longest-lasting COVID-19 infection because not everyone gets tested, especially on a regular basis like this case. But at 505 days, “it certainly seems to…