Class A office rents increased 43 percent during the fourth quarter in the Fenway and Longwood Medical submarket following the opening of Samuels & Assoc.’ Van Ness mixed-use complex.

Luxury apartment towers and trendy restaurants put the Fenway high on the list of Boston’s transforming neighborhoods.

Now the Fenway is seeing an infusion of good jobs at good wages, as back office space is converted into modernized Class A and collaborative workplaces. Hundreds of employees are migrating from Cambridge, the Longwood Medical area and the suburbs in a flurry of office leases at Samuels & Co. properties topping 430,000 square feet.

UnitedHealth Group’s Optum subsidiary, a digital health care company, will move 400 employees from Brighton, Wakefield and Waltham offices to the Van Ness complex this year and hire another 400 over the next two years.

“It made sense for us to develop a more collaborative office space where we could bring people together in a vibrant neighborhood which has a lot of technology and health care companies around it,” said Matt Stearns, spokesman for Optum.

Harvard Pilgrim inked a 58,000-square-foot office lease at Samuels’ Landmark Center, while Children’s Hospital committed to a 145,000-square-foot expansion.

Peter Sougarides, a principal at Boston-based Samuels, said the groundwork was laid with the transformation of the surrounding neighborhood over the last 15 years. Garages, gas stations and fast-food joints were replaced by high-end apartments, a City Target and the retro-chic Verb hotel. And that has given the Fenway the lively street scene that growing companies see as helping their recruitment efforts.

“The residential and retail serves as a great base for the office folks that come in,” Sougarides said. “We’re trying to create this balance so that it’s not too much residential and not too much office. It’s a comfortable mix of uses, and you’ve got a neighborhood that’s got a lot of activity.”

Short-Term Leases For Startups

Samuels’ Landmark Center is the epicenter of the transformation. A former Sears warehouse built during the Great Depression, the Landmark was acquired by Samuels in 2011 for $530 million with a long-range redevelopment scheme in mind. The departure of anchor tenant Blue Cross Blue Shield in mid-2015 signaled the beginning of the redevelopment.

Four startups occupy over 40,000 square feet within the 100,000-square-foot Hatch innovation center that opened last summer on the eighth floor. The complex contains more than 30 workrooms and a large common area equipped with indoor swing sets and ping pong tables.

Samuels has offered short-term leases – typically ranging from one to two years – to startups such as Toast, which makes point-of-sale systems for restaurants and relocated from Cambridge.

Children’s Hospital recently leased 257,460 square feet at the Landmark Center, which represents a 145,000-square-foot expansion. Staff relocation began in November and continues through March, according to Children’s spokeswoman Kristen Dattoli, with the Landmark workforce expanding from the current 700 to 1,000.

The remainder of the mixed-use redevelopment – including demolition of the parking garage for 550 residential units and repurposing of the ground floor for a Wegmans supermarket and other retailers – is expected to begin in two to three years, Sougarides said.

Spec Space Lands Anchor

Samuels’ confidence in the Fenway office market’s future was reflected in its inclusion of 237,000 square feet of speculative office space in its Van Ness mixed-use project at 1325 Boylston St. Cambridge-based health care risk manager CRICO leased 45,000 square feet in May, while Optum committed to 126,000 square feet last month.

The entry of the Van Ness space helped boost average Class A rents by 43.3 percent in the 5.3-million-square foot Fenway/Longwood submarket during the fourth quarter to $38.68, according to Transwestern RBJ research. The submarket had 242,000 square feet of positive absorption, dropping the vacancy rate to 8.6 percent.

“Many of the companies that are in the Fenway traditionally have been there because of some connection to the Longwood Medical area, but what we’re seeing is an expansion of the different types of the companies coming into the Fenway market,” Sougarides said.