S&P 500 exchange-traded funds on Thursday were signaling that stock-market circuit breakers, which halt the market briefly in order to mitigate volatility, will likely be triggered for a second time this week as the market reels from uncertainty around the spread of COVID-19, the infectious disease that was first identified in Wuhan, China and that has spread world-wide. The SPDR S&P 500 ETF Trust SPY, -1.15% , which tracks the S&P 500 index SPX, -1.11% , was down 7.8% in premarket trade on Thursday. If S&P 500 falls by at least 7% in regular trade a circuit breaker will kick in and result in the pause of trading for 15 minutes. If the benchmark index were to fall 13% on the day once trading resumes, it would trigger another 15-minute halt. Trading wouldn't halt if the decline occurred after 3:25 p.m. Eastern. A 20% drop in the S&P 500 would trigger what's known as a level three cirucit breaker, which would stop trading for the remainder of the day. Circuit breakers were triggered on Monday after crude-oil prices were roiled following Saudi Arabia's announcement that it would ramp up crude output, cratering crude prices CLJ20. On Thursday, markets are downbeat after apparent disappointment with a European travel ban announced by President Donald Trump during a late-Wednesday speech to the nation. Stock-index futures triggered limit-down rules which don't allow trading to move more than 5% in any direction. The SPDR Dow Jones Industrial Average ETF Trust DIA, -0.85% was down 8.9% in premarket while the Nasdaq-100 ETF, the Invesco QQQ Trust Series I DIA, -0.85% , was down 6.2% before the bell, according to FactSet data.