The delta variant is fueling a sharp rise in new COVID-19 cases in some of the Massachusetts cities that were among the hardest hit in the pandemic’s earlier surges.

After seeing a precipitous drop in cases from April to June, cases have begun picking up rapidly in Boston, Fall River, New Bedford, Worcester and other Massachusetts cities.

In the two weeks from July 25 to August 7, the number of cases in Boston doubled from the prior two weeks.

During a two-week span near the end of June, there were 20 cases in New Bedford. That's now increased nearly 20 times, to 388 in the most recent two weeks.

The city case numbers are released in two-week intervals by the Department of Public Health and were compiled by Alan Geller of the Harvard School of Public Health.

“I’m discouraged,” Geller told GBH News. “So much progress was being made.”

Geller said the majority of the new infections likely come from unvaccinated people.

“There’s probably a few breakthrough infections, but the CDC data seems to indicate that well more than 90% of the cases that we’re seeing across the country are in unvaccinated people,” Geller said. “And there’s no reason to believe that that’s not the case here.”

As of this week, 67% of Boston residents had received at least one dose. But just 53% of the city’s Black residents and 54% of Hispanic residents had gotten a shot. In Worcester, 62% of all residents have gotten one dose, and in Fall River, just 54% have gotten a shot.

“I think the numbers reflect the fact that even in our most high-risk cities where the vaccination rates are not ideal, modest rates of vaccination have protected us from COVID,” Geller said of the initial drop in case numbers. “Then, most recently in July, the delta variant has kicked in. It’s extraordinarily aggressive, it’s very transmissible and it’s obviously attacking the unvaccinated population. And only by increasing our vaccination rate from half of people in the cities being vaccinated to about three quarters of people in the city being vaccinated, are we going to be able to thwart it.”

Boston Medical Center is seeing an increase in COVID-19 patients, said hospital epidemiologist Dr. Cassandra Pierre. Ninety percent of those patients, she said, are unvaccinated.

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After an early summer low, COVID cases in Boston are spiking again.
Data visualization by Lisa Williams Data courtesy of Department of Public Health and compiled by Alan Geller of the Harvard School of Public Health.

However, the hospital stays are shorter now than they were in past surges, she said, because the new patients tend to be younger than those seen earlier in the pandemic.

“This kind of pattern in the areas that have been hardest hit is exactly what we expected, but not when we expected it,” Pierre said. “I expected to see this in the fall. And we knew that these harder-hit areas that are structurally vulnerable and have a higher proportion of Black and Latinx individuals who have had lower access, as well as higher concerns and lower confidence in the vaccines were under-vaccinated relative to the rest of the state. So it’s not surprising that we would see the increases in those areas.”

She said she didn’t think the numbers would really pick up until schools opened up again in the fall and more people return to workplaces.

“But the delta variant threw off the equation,” Pierre said.