The dollar strengthened on Thursday as wide optimism surrounding coronavirus vaccines ran into concerns over increasing infection numbers and risks to the fragile global economic recovery.The US death toll from coronavirus passed 250,000 on Wednesday as New York City’s public school system, the country’s biggest, called a halt to in-classroom instruction, citing a leap in infection rates.The dollar was also caught between two opposing forces, with a safety bid supporting it while fresh speculation over monetary easing to boost the economy held it back somewhat.With fiscal stimulus plans dropping by the wayside as President Donald Trump’s refusal to concede electoral defeat consumes lawmaker’s attention, speculation is increasing that the Federal Reserve may further loosen monetary policy in December.