Four years ago this week, the World Health Organization designated COVID-19 as a pandemic. And on March 15, 2020, then-Gov. Charlie Baker shut down all public schools in Massachusetts, and all in-person dining at restaurants.

Bill Hanage, an associate professor of epidemiology at Harvard’s T.H. Chan School of Public Health, said those shutdowns were crucial as the previously unknown and potentially deadly pathogen rampaged through populations.

“The situation was an absolute emergency,” said Hanage, whose expertise was often called upon during the first few years of the pandemic. ”We had ICUs full to the brim with only COVID patients; we had doctors struggling with conditions that they had never experienced before in their lives.”

In Chelsea, former City Manager Tom Ambrosino remembers preparing for a challenge, but not the hardest one he’d ever face in decades of public service.

“I didn't remember feeling at all that this was something that was extraordinary, that we couldn't handle in the normal course. It really wasn't until the end of March that my office started getting some signals that something was awry in Chelsea,” recalled Ambrosino.

The densely populated minority-majority city — with numerous essential workers and multigenerational households — quickly became one of the worst hotspots of COVID-19 infections in Massachusetts.

Ambrosino said the city received help from state government and partnered with community-based nonprofits and healthcare organizations in that time of need.

“I was hearing sort of horror stories from some of our community-based organizations that the level of illness in the city was unique and extraordinary ... people getting locked out of their apartments by roommates, people dying, people in hospitals,” he said.

Ambrosino recalled setting up food distribution programs, and trying to find places for people to stay if they couldn't remain in their own homes.

The partnerships forged in Chelsea helped completely turn things around there as far as the pandemic — the city would go on to become one of the earliest and most vaccinated municipalities in the state.

Hanage said one important lesson we’ve learned in the four years since COVID-19 arrived, is that all the interventions that have faded from memory may once again be needed and needed in a more sustained fashion, when — and not if — the next pandemic arrives.

“We have been wrestling a truly, truly difficult opponent, and we've got to remember the things that we did right because of the fact that, you know, in future we're going to be facing other things like this,” said Hanage. “And we cannot afford to forget that.”