Americans' mental health is the worst it's been in two decades, according to a new Gallup poll.

Between the lines: It's fairly obvious why, given that the once-in-a-generation pandemic was only the start of 2020's stressors. But once the pandemic ends, this unfortunate side effect could linger.

By the numbers: 34% of poll respondents said their mental health is excellent — a drop from 43% a year ago.

• A significantly higher number — 76% — said their mental health is good or excellent, but that's still a nine-point drop from 2019.
• 18% said their mental health is only fair, and 5% said it was poor.
• Although the vast majority of demographic groups saw declines, some started off with better reported mental health than others.

The other side: The portion of Americans who said their physical health is good or excellent was 79%, which is roughly the same as it has been for the last two decades.

The big picture: This portrait of the country's mental health is self-reported. But it's safe to assume that some portion of people whose mental health has taken a turn for the worse will need help, and won't just bounce back to normal once the pandemic ends.

• For example, substance use disorders and addictions won't simply go away with a coronavirus vaccine.
• The U.S. had a poor record of providing mental health care before the pandemic. It will take a concerted effort to adequately deal with its mental health aftermath.

Go deeper: The overwhelming aftershocks of the pandemic