Providence, R.I. — The federal government has told states to prepare for a coronavirus vaccine to be ready to distribute by Nov. 1.

The timeline raised concern among public health experts about an “October surprise” — a vaccine approval driven by political considerations ahead of a presidential election, rather than science.

In a letter to governors dated Aug. 27, Robert Redfield, director of the U.S. Centers for Disease Control and Prevention, said states “in the near future” will receive permit applications from McKesson Corp., which has contracted with CDC to distribute vaccines to places including state and local health departments and hospitals.