And though some states promised to release people early to reduce numbers, in reality the entire reduction has come from admitting fewer people in the first place, says John Pfaff, of Fordham University in New York.
Now that the virus is receding, the number of prisoners may rise again, suggests Mr Pfaff, as jury trials resume.
Yet many prison officers chose to quit or retire as covid raged.
And as wages surge elsewhere, fewer are joining to replace them.
Last summer, nearly one-third of positions in federal prisons were vacant.
In September an anonymous guard at Lee Arrendale State prison, a women’s facility in Georgia, told state representatives that “on a good day” there might be as few as six or seven officers to guard 1,200 inmates.
Hannah Riley, of the Southern Centre for Human Rights, an advocacy group, reckons 70% of positions in the state are unfilled.
(The Georgia Department of Corrections did not reply to a request for comment.)
Georgia is now under investigation from federal authorities, such is the extent of violence inside.
What does this all add up to?
Even with the recent decline, America imprisons more people than any other criminal-justice system.
Black and Hispanic people are especially likely to be locked up.
In 2018 one in 45 black men was in prison (and more still in jails).
Poor conditions are not only egregious human-rights violations.
They also make prison less effective.
A Department of Justice study from 2018 found that five out of six people released from state prisons were rearrested within nine years.
The fact that prisoners are warehoused with limited access to education or mental-health treatment, in a place where drug abuse and gangs are rife, is surely part of the reason.
Worsening conditions are likely to lead to more reoffending.
Restrictions on visits mean many prisoners have lost contact with family over the past two years, says Jobi Cates, the founder of Restore Justice, a charity in Illinois which presses for criminal-justice reform.
Visits are "everything for our people", she says, but prisons have been slow to bring them back.
It is not only family members who have been kept out, but also teachers, therapists and others who help prepare people for release.
Electronic means of keeping in contact got worse, too, because of staff shortages and worries about moving people around.
“They made it to where you can only get one phone call a day,” says NaJei Webster, who was released from a prison in Illinois in September, and who now works for Ms Cates’s charity.
Prisoners can get access to email through tablet computers, but these cost money—not only for the machine but also per email sent.
Sending money to prisoners to pay for these services comes with exorbitant fees, charged by firms such as Global Tel Link and JPay, which saw its revenues spike in 2020.
向囚犯匯款以支付這些服務需要高昂的費用，由Global Tel Link和JPay等公司收取，這些公司在2020年的收入激增。
The tragedy is that falling prison populations ought to be an opportunity to close some of the worst institutions.
And state budgets are unusually replete with cash.
Mr Ossoff says he has found that improving conditions in prisons (unlike releasing people) has bipartisan support.
With several Republicans, he is pushing for more congressional oversight of prisons.
But prison-guard unions are reluctant to accept changes that make their jobs harder, and, thanks to the staff shortages, they are more powerful than ever.
It seems more likely that things will get worse.