Per the Journal of Infectious Diseases, "All viruses mutate, but influenza remains highly unusual among infectious diseases" because it mutates very rapidly, and thus, "new vaccines are needed almost every year" to protect against it. In contrast, a molecular biology journal documented in 2020 that SARS-CoV-2 "does not mutate rapidly for an RNA virus because, unusually for this category, it has a proof-reading function" in its genetics. A torrent of research has since confirmed that people who have recovered from Covid-19 are very unlikely to be seriously harmed by it, even if they are infected again. A notable exception to this rule is people whose immune systems are severely compromised, as immunity can only be as strong as each person's immune system.