Dr. Francis Collins, the director of the National Institutes of Health, said Sunday that federal health officials are keeping a close eye on Delta and other coronavirus variants, such as Lambda, but as of right now there aren't major worries about any of them rendering the available COVID-19 vaccines ineffective.

"At NIH, working with [the Food and Drug Administration] and [the Centers for Disease Control and Prevention], we have a very vigorous team that looks at every new emerging variant to see what would its effect be in terms of the vaccine, will the vaccine work against this one," Collins told ABC News' George Stephanopoulos on Sunday's edition of This Week. "So far, so good. We don't have anxieties yet about Delta or Lambda or any of the others that are sort of lurking out there."

That doesn't mean they're letting their guard down, though. Collins said "we all worry about the day" that a new variant emerges that is so different from the original strain of the coronavirus that sparked an outbreak in Wuhan, China, in late 2019 that it could prove resistant to vaccines, forcing a need for widespread booster shots. But the best way to avoid that, Collins explained, is to reduce the number of infections right now.