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 be the longest reported infection,” said Dr. Luke Blagdon Snell, an infectious disease expert at the Guy’s & St. Thomas’ NHS Foundation Trust.

Snell’s team plans to present several “persistent” COVID-19 cases at an infectious diseases meeting in Portugal this weekend.

Their study investigated which mutations arise — and whether variants evolve — in people with super long infections. It involved nine patients who tested positive for the virus for at least eight weeks. All had weakened immune systems from organ transplants, HIV, cancer or treatment for other illnesses. None were identified for privacy reasons.