The Atlantic Why did this coronavirus change faster than scientists expected? In the early, uncertain days of the coronavirus pandemic, scientists delivered one comforting pronouncement: The virus that caused COVID mutates rather slowly. If that remained true, the virus would not change much to become more dangerous soon, and any vaccine could provide years of durable protection.
What actually happened was that SARS-CoV-2 began mutating quickly, first to be more transmissible and then to evade our immunity, causing breakthrough infections and reinfections. Five years and an alphanumeric soup of variants later, most of us have gotten COVID at least once. The vaccine is still being updated to match new circulating variants. And the virus itself is still changing.
In truth, scientists were both right and wrong about the speed at which SARS-CoV-2 mutates. The rate of mutations as this virus jumps from person to person is indeed unimpressive. But scientists were not aware of a second, accelerated evolutionary track: When SARS-CoV-2 infects a single immunocompromised patient, it can persist for months, accumulating countless mutations in that time.
And if we are unlucky, that highly mutated virus might spread to others. This is the likely origin of Omicron, which appeared in fall 2021 with more than 50 mutations—an astounding evolutionary leap. Omicron looked like it had achieved four or five years’ worth of expected evolution in just months, Jesse Bloom, who studies viral evolution at the Fred Hutchinson Cancer Center, told me at the time. These mutations enabled Omicron to cause a massive wave of infections even among the vaccinated.