Boston’s COVID-19 positivity rate has spiked to 4.2 percent, acting mayor Kim Janey said Tuesday.

The rate, which had previously plateaued at around 3.7 percent, translates into more than 170 cases per day. Speaking to reporters at City Hall, Janey called the jump “troubling.”

“The 17 percent increase in positivity rate illustrates that many people are still getting COVID. The change is largely due to a higher rate of infection among younger residents,” Janey said, adding that “more than half” of the new cases of the past two weeks have come from Bostonians 29 and younger.

The increase comes a week after Boston and the state, stepped into the final phases of re-opening, with outdoor dining launched, large sports venues opened at 12 percent capacity and public gathering caps increased. All the steps allow for more opportunities to socialize outside the home and contract the virus.

Last week, Health and Human Services Chief Marty Martinez said the city would unveil a new campaign targeted towards the young adults who account for the increased infections.

Janey, who also announced a new pot of $50 million dollars for Boston’s rental relief fund, said the pandemic’s impact extends beyond the realm of public health into financial and housing stability.

“We are working together to protect public health, to promote housing stability and to ensure an equitable recovery,” she said. “These additional funds, at amounts up to $15,000 dollars, will make a big impact.”

The money, she said, will go towards rent and ultility payment assistance, including for services like internet access.

More than 30 percent of the city has now received a dose of the COVID-19 vaccine.

“The arrival of vaccines bring new hope, but stopping the spread requires our continued vigilance,” Janey said, urging people to sign up for a vaccination when eligible.

Since the pandemic started, 63,748 Bostonians have been infected and 1,341 have died from the virus.