Today's biggest winners and losers in the stock market.

On this episode of Stock Movers:

- Fannie Mae and Freddie Mac have begun placing sizable orders to purchase mortgage-backed securities, stepping into a market roiled by widening bond spreads and a surge in volatility, according to a person with direct knowledge of the matter.

- Pfizer (PFE) said its experimental Lyme disease vaccine was 73% effective against the tick-borne illness, though fewer-than-expected cases in a study made it hard to determine how well it works on a large scale.

- Shares in DraftKings (DKNG) and Flutter erase earlier losses to gain after the Wall Street Journal reported that US senators are set to introduce bipartisan legislation to ban sports bets on prediction markets.