Level 4 means the virus has mutated until it moves between some people who have been only in brief contact, as a cold does.

And it is not clear that the vaccine would still be effective if the virus mutated.

By the time a vaccine is distributed, the virus may have already mutated.

The virus could theoretically mutate to become more dangerous or less lethal.

The virus mutated into something far more deadly, and the whole population was infected.

That could tell how the virus has spread and mutated over the years.

"And right about here seems to be the point at which the virus then mutates into a retrovirus."

The virus mutated and uses the report as a medium.

They discovered that the virus has mutated and now uses the report as a medium.

"This virus has mutated around what we've thrown at it."