The overall office vacancy rate in Downtown Worcester has risen 10 percent in the last five years, according to a new report from the Worcester Regional Research Bureau.

Although the drop in the vacancy rate, to 79 percent in 2011 from 81 percent last year, was statistically insignificant, the ongoing recession and underinvestment in old buildings in the city will continue to hinder efforts at revitalization – which will help perpetuate vacancies, according to the report.

The Research Bureau found Class A space to be 84 percent occupied, Class B offices to be 74 percent occupied and Class C office space 87 percent occupied. At least 21 buildings downtown have 10,000 or more square feet available for lease. According to the research bureau’s past surveys, the vacancy rates in these buildings have been continuously high and rising, even prior to the current recession, making them major contributors to the total vacancy rate of downtown in general. Combined, they constitute about 10 percent of Downtown Worcester’s total office space.

Even so, there are a number of major projects underway in downtown, indicating strong market confidence among some institutions and businesses. City Square includes the demolition of Worcester Common Outlets Mall, construction of a new 66,000-square-foot St. Vincent Hospital cancer center and construction of a new, 214,000-square-foot Unum building. Massachusetts College of Pharmacy and Health Sciences is building a $10 million optometry school. Gateway Park II includes a $30 million, 100,000-square-foot building with lab, academic and office space. The $27 million rehabilitation of the Worcester Vocational School complex will create 95 new units of low-income and market-rate housing.

*This article has been modified to reflect the correct amount of space occupied in Class A, B and C buildings.