Check out the companies making headlines in midday trading. 

Tilray — Shares of Tilray surged more than 20% after the cannabis company and peer Aphria reached an agreement to combine businesses, creating a new giant in the growing industry. The all-stock deal will result in a combined company valued at about $3.9 billion.

Moderna — Moderna shares fell more than 6% after Morgan Stanley downgraded the drugmaker to equal-weight from overweight. The bank cited its elevated valuation after the stock's 600% rally this year amid high expectations around its coronavirus vaccine.

Dish Network — Shares of Dish Network fell more than 7% after announcing the company would raise $2 billion by way of a convertible note offering. Dish said the funds will be used for general corporate purposes and 5G network costs.

United Airlines — An analyst at JPMorgan downgraded United Airlines to underweight from overweight, pushing the stock down 3.5%. The analyst cited concerns around the stock's valuation, noting: "The recent ascent in airline equities has significantly diminished the implied potential upside to several of our Dec 21 price targets, with some having already passed through said targets."

Twitter — The social media stock gained 3.9% after JPMorgan upgraded it to overweight from neutral and named it a top pick. The firm said it is bullish on the online advertising market in 2021 and that Twitter should be one of the biggest beneficiaries of that sector rebounding.

Teladoc — Shares of the remote health care company shed more than 5% after Business Insider reported that Amazon was working on a business that would provide medical care to workers at major companies. The effort would include providing primary care online, according to the report.

Chipotle – Shares of the restaurant chain jumped more than 4% after Stifel upgraded Chipotle to a buy rating. "Chipotle has demonstrated impressive resilience during the pandemic, and we believe the brand should benefit in 2021 from increased consumer mobility and several sales-building initiatives," the firm said in a note to clients. Stifel also raised its 12-month price target to $1,500 from $1,400.

Southwest Airlines — Southwest shares dipped 2.3% after the airline lowered its guidance for December operating revenue. The company expects revenue to fall between 65% and 75% on a year-over-year basis. Southwest had previously forecast a decline ranging between 60% and 65%.

Spotify — Shares of the streaming platform lost more than 2% after Credit Suisse downgraded the stock to neutral from overweight. The Wall Street firm said Spotify's valuation has expanded and most of the positive factors are priced in the stock.

GoDaddy – Shares of the website builder advanced more than 5% after GoDaddy said it is acquiring payment company Poynt. Under the terms of the agreement, GoDaddy will pay $320 million in cash and $45 million in deferred cash payments subject to certain factors over three years. The deal is expected to close in the first quarter of 2021.

— CNBC's Maggie Fitzgerald, Pippa Stevens, Jesse Pound and Fred Imbert contributed reporting.

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