Whisk together the mayonnaise, sour cream, chives, lemon juice, 1/2 teaspoon salt and a few grinds of pepper in a large bowl.
Add the pasta to the boiling water and cook according to the package directions. About 3 minutes before the pasta is done, add the corn to the pot. Drain and rinse under cold water to cool; add to the bowl with the dressing.
Add the tomatoes and scallions to the bowl and toss to combine. Season with salt and pepper. The pasta salad is best if served right away.
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