NEW YORK, April 15 (Reuters) - The second half of the year could see strong economic growth and the labor market should continue to improve as long as there is not a surge in coronavirus variants that are resistant to vaccines, Cleveland Federal Reserve Bank President Loretta Mester said on Thursday. Mester said she expects the U.S. economy to grow by 6% or more this year and for the unemployment rate to drop to 4.5% or lower by year end. Vaccinations, school reopenings and child-care will be important to getting people back to work, Mester said during a call with reporters. (Reporting by Jonnelle Marte Editing by Chris Reese)