Covid mutation accounts for up to 25% of state’s cases An electron microscope image made available by the U.S. National Institutes of Health in February shows the virus that causes COVID-19. A Covid mutation is spreading in California, and one expert says it accounts for about 25% of California’s cases, health officials said at a news conference Sunday night. There’s no evidence that the mutation, known as L452R, produces more dangerous symptoms but because it affects the protein in the Covid virus, there’s a chance that the new vaccines won’t be as effective in preventing the mutation.