DES MOINES — Following a top-secret briefing Tuesday morning, Sen. Chuck Grassley, R-Iowa, expressed confidence the United States is “on top of” efforts to prevent the spread of the coronavirus.That doesn’t mean the virus, which already has claimed about 2,700 lives, is contained, Grassley said during his weekly conference call with Iowa reporters.“I don’t think that I could give Americans any certainty that it couldn’t crop up in the United States like it did in Italy over last weekend or 10 days ago in South Korea, where all of a sudden, you get an explosion of it,” Grassley said after joining more than a dozen senators at the briefing.