Apartment vacancies in the U.S. fell to their lowest level since 2001 as home seizures and a growing pool of young adults forming households boosted rental demand, according to Reis Inc.

The vacancy rate fell to 4.9 percent in the first quarter from 6.2 percent a year earlier, the New York-based property- research company said in a report today. It was only the third time since Reis began gathering the data 31 years ago that the rate was less than 5 percent.

Renters are competing for a tightening supply of units as more homeowners are displaced by foreclosures, stricter mortgage-lending standards block purchases and young people move out on their own. In the three months ended March 31, 7,342 apartments became available, the fewest number of completions since Reis began publishing such data in 1999.

When vacancies drop below 5 percent, “effective rents tend to spike as landlords perceive that tight market conditions allow for greater pricing power,” Reis said in the report.

Effective rents, which take into account such landlord concessions as a free month, climbed almost 1 percent from the previous quarter to an average $1,018, the largest increase since the last recession began, according to Reis.

While low vacancies are helping to boost the performance of apartment properties nationwide, “risks may manifest later in the year” as multifamily developers pick up the pace of construction to take advantage of rising rents, Victor Calanog, head of research and economics at Reis, said in the report.

Reis expects about 70,000 units to open for leasing this year, about double the supply growth in 2011. Next year, the firm forecasts 150,000 to 200,000 new units in the 79 primary markets it tracks.