Office space in southwestern Connecticut is at a premium and a market report released Tuesday says vacancy rates are at a five-year low. Commercial real estate analysts say supply of office space is declining, leaving fewer options for large tenants and higher rental rates. They say the market is shifting from one favoring tenants to one favoring landlords. Jim Fagan from the real estate services firm Cushman and Wakefield says vacancy rates in Fairfield County for the first quarter in 2006 stood at 16.8 percent. The second quarter dropped to the lowest point in five years, 15.6 percent. Fagan says several factors, including climbing rates in New York City and increases in hiring, are influencing the market. Fagan added that he expects vacancy rates to drop even lower in the second half of the year.