TOKYO, March 23 (Reuters) – Japan’s Nikkei share average rose to a fresh 15-year high on Monday, tracking gains in U.S. shares, while Eisai Co jumped 21 percent after a brokerage upgraded the stock, citing the effectiveness of its drug to treat Alzheimer’s disease.

The Nikkei ended 1.0 percent higher at 19,754.36 points, its highest closing level since April 2000.

The broader Topix gained 0.7 percent to 1,592.25. But trading was thin, with only 1.86 billion shares changing hands, the lowest in two weeks.

The JPX-Nikkei Index 400 advanced 0.8 percent to 14,467.45.

(Editing by Kim Coghill)