New research shows how and when COVID-19 first spread to Massachusetts

Respiratory Therapist Angelika MacClellan (left) talked with nurse Natasha Cacciatore through a patients door in the Special Pathogens Unit ICU at Brigham and Women's Hospital in Boston on April 27, 2020. The hospital converted the cardiovascular center into a COVID-19 intensive care unit.Craig F. Walker/Globe Staff/The Boston Globe

Massachusetts had the third-highest rate of COVID-19 infections in the United States by July of last year, according to a Northeastern University study that modeled how the coronavirus emerged on the nation’s coasts but spread most rapidly in the Northeast in the first wave.

By July 4 last year, 13 percent of Massachusetts residents had been infected with the virus, behind only New Jersey, with 15.2 percent, and New York with 13.3 percent, according to the paper published online Monday in “Nature.” In contrast, Oregon had the lowest so-called attack rate at that point, with only 0.7 percent of residents infected.

The first US case had been detected in Washington state on Jan. 21 last year, a few weeks after the announcement of a puzzling cluster of unusual pneumonia cases in Wuhan, China. The first infection in Boston was diagnosed on Feb. 1 in a student at the University of Massachusetts Boston who had traveled from Wuhan. Still, COVID seemed a remote threat to many in Massachusetts until the Biogen conference in late February ignited a super-spreader event.

But the Northeastern researchers said it’s impossible to rule out transmission of the virus in parts of the United States and Europe as early as December 2019. International air travel was the key driver at first, with the earliest cases likely introduced on the West and East coasts in cities with major airports, they said. The Trump administration imposed restrictions on travel from China that went into effect on Feb. 2 last year.

After that, the contribution from mainland China became “barely visible,” according to the study, and interstate travel became the main transmission source. By April 30, 2020, domestic travel accounted for 69 percent of introductions in Massachusetts.

Once the virus arrived in the United States and Europe, the researchers said, public health surveillance systems were slow to detect its spread within communities.

“We really didn’t understand what was happening around us in early 2020 because we didn’t have the testing capacity, at least at that point,” said Jessica T. Davis, a Northeastern PhD candidate and one of the lead authors of the study.

By early March 2020, health officials were detecting only 1 to 3 in 100 COVID infections in parts of the United States and Europe, the researchers said.

“Our results suggest that many regions in the US and Europe experienced an onset of local transmission in January and February 2020, during the time when testing capacity was limited,” the study read. “If testing had been more widespread and not restricted to individuals with a travel history from China, there would have been more opportunities for earlier detection and interventions.”

The study did not say what proportion of infections are being detected today.

Bronwyn MacInnis, director of pathogen surveillance at the Broad Institute of MIT and Harvard and co-lead author of a recent study of an outbreak of breakthrough cases in Provincetown, said it wasn’t surprising that the virus spread undetected early in the pandemic. But the Nature study, she said, underscores “just how much we might have missed.”

Alessandro Vespignani, director of Northeastern’s Network Science Institute, oversaw the study, a follow-up to another paper his team published in April last year.