WASHINGTON, D.C. — Americans must recommit to wearing masks and taking other COVID-19 mitigation measures to avoid a new surge of the virus in the U.S., a top health official said Monday as the White House signaled that shipments of Johnson & Johnson’s vaccine could fall just short of an early target.

Rochelle Walensky, director of the Centers for Disease Control and Prevention, said cases have begun to rise slightly again, while hospital admissions remain stable and deaths continue to decline. Variants, which are in some cases more contagious or more dangerous, continue to spread, she said.

“Taken together, these statistics should serve as a warning sign for the American people,” she said at a press briefing. “We must act now and I am worried that if we don’t take the right actions now, we will have another avoidable surge” as is being seen in Europe.

Walensky repeated a warning that states are lifting restrictions too early, and called on Americans to continue to wear masks, avoid crowds and wait to travel, even if they’ve been vaccinated. “I’m calling the American people to action, whether vaccinated or not, to recommit to doing the right thing — take the steps we know work to stop COVID-19,” she said.

The U.S. is continuing to ramp up vaccinations, recording back-to-back days this weekend of at least 3 million shots administered. Another official at the briefing, White House aide Andy Slavitt, said Johnson & Johnson would increase shipments this week of its one-shot vaccine and will come close to hitting its target of providing 20 million doses by the end of March, though stopped short of saying the goal would be met.

“We are obviously working very closely with the company. We are going to see a nice increase in Johnson & Johnson this week,” he said. “I wouldn’t signal to you that they’re going to be far away from the numbers they have projected, at all, give or take a little bit.”

The administration has said that it expects to receive nearly all — or all — of its initial 100-million-dose J&J order by the end of May, though the company has only committed to providing it by the end of June. Biden has ordered another 100 million shots due in the second half of the year.