The dollar was headed for its worst week in a month on Friday after plumbing a 2-½ year low, as investors bet the greenback has further to drop and that the worst of the coronavirus pandemic could be over within months.A swirl of promising vaccine news has helped fuel a rally in more volatile currencies, while actions taken by the Federal Reserve have weakened the dollar.The dollar index hit a low of 90.504 on Thursday and is on track for over 1% drop over the week. It was last widely flat on the day at 90.628.Investors have turned heavily short dollars, figuring rates will remain low for a long time in the US forcing yield-seekers to head elsewhere for greater returns.