Secretary of Homeland Security Kirstjen Nielsen said Friday that next week’s midterm elections will be the “most secure” the U.S. has ever held, but cautioned that U.S. election systems are under “constant attack.”

“This is going to be the most secure election we’ve ever had,” Nielsen said at a Council on Foreign Relations event. “But as we all know, this is a dynamic threat. It changes by the minute.”

So far, there is no indication of a “sustained effort to hack voting infrastructure,” the DHS chief said, adding that the department will alert state and local officials “the moment that changes.”

“What we’ve seen is continued attempts to scan,” Nielsen told the audience. “In some very limited cases, we have seen access, but they’ve been quickly prevented or mitigated. But not that we can attribute to a foreign country.”

“The midterm is . . . just the warm-up, or the exhibition game,” said senior DHS official Christopher Krebs. “The big game, we think, for the adversaries is probably 2020.”

Foreign influence campaigns on social media are “much more difficult to combat” than threats to the integrity of the vote itself, Nielsen said, before adding that her “biggest concern” is that a foreign entity will “sow discord on social media, suggesting something did not work as it should” in the election.