Gov. Charlie Baker at a press conference at the Pfizer manufacturing facility in Andover. (Pool photo by Stuart Cahill/Boston Herald)

GOV. CHARLIE BAKER said on Thursday that the recent runup in Massachusetts COVID-19 cases is occurring not because the state is reopening too quickly but because some individuals are becoming careless and forgetting the basics about dealing with the virus.

The governor said case clusters are popping up because of parties or other activities where participants are not complying with COVID guidance. “In many cases, people haven’t been wearing face coverings or socially distancing,” Baker said.

Case counts have risen a bit over the last several weeks, with the seven-day positive test rate (positive cases as percent of all tests) rising to 2 percent, which is still well below where it was during the surge of cases in April and May. Baker said his administration is keeping a close eye on the numbers but continues to believe the key to holding the virus in check is responsible individual behavior.

“It’s not taking the summer off and we can’t either if we want to continue to fight and  contain the virus and keep our economy going,” Baker said. “Don’t become careless or complacent.”

The governor’s press conference on Thursday was at Pfizer Inc. in Andover, where production of a vaccine for COVID-19 has already begun even though it has not received approval yet from the US Food and Drug Administration. That approval will depend on the results of a study of how the two-dose vaccine works on 30,000 patient volunteers. The study should be completed in October.

Meg Reusch, research and development site leader at Pfizer Inc. in Andover. (Pool photo by Stuart Cahill of the Boston Herald)

Meg Ruesch, the research and development site leader at Pfizer, said the company and its German partner, BioNTech, expect to have 100 million vaccine doses ready by the end of the year. “This manufacturing is being undertaken at risk,” said Ruesch.

Assuming the vaccine receives regulatory approval, Ruesch said, 1.3 billion doses could be ready by the end of 2021.

The vaccine works by providing genetic instructions to cells to make a protein that triggers an immune response by the body. “Clinical data for the vaccine has been encouraging so far,” Ruesch said.

Baker brushed aside suggestions by President Trump and others that this year’s elections could be postponed. Despite COVID-19, Baker said, Massachusetts has carried out municipal and special legislative elections with relatively few problems. He said elections have been held in the United States during World Wars and even the Civil War.

“Elections need to happen and there’s no reason why this one can’t happen,” he said.

Bruce Mohl oversees the production of content and edits reports, along with carrying out his own reporting with a particular focus on transportation, energy, and climate issues. He previously worked...