By Martha C. White, NBC News

The world might be waiting on a Covid-19 vaccine, but thanks to policy booster shots, the stock market ended 2020 seeming to be largely immune from the contagion that still threatens Main Street businesses.

It might be hard to recall now, but 2020 started off with an economy full of potential: The Dow Jones Industrial Average was on track to break through the 30,000 threshold and the unemployment rate fell to 3.5 percent — the lowest in more than half a century. But things were already starting to unravel as an ominous viral pneumonia worked its way around the globe.

The Dow closed at a record high of 29,551 on Feb. 12 — then the patient took a turn for the worse. On March 9, 12, 16 and 18, circuit breakers designed to halt trading if the S&P 500 dropped by more than 7 percent kicked in when markets plunged. The market hit its nadir on March 23, with the S&P closing just above 2,237 and the Dow Jones a fraction below 18,592.

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