As President Biden steps home from his first 100 days in office, the general decline in new virus cases, deaths and hospitalizations since January offers signs of hope for a weary nation.

But the average number of new cases has increased 19 percent in the past two weeks, and federal health officials say complacency with the coronavirus could spark another major wave of infections.

“There is so much we can look forward to, so much promise and potential where we are, and so much reason to hope,” said Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, made an emotional appeal to Americans this week. “But right now I’m scared.”

On the positive side, nearly a third of the people in the United States have received at least one dose of a coronavirus vaccine. By early Saturday morning, an average of nearly three million people received a shot every day, up from about two million in early March.

The rising vaccination rate has led some state officials to speed up their rollout plans. This week Connecticut Governor Ned Lamont expanded access to people aged 16 and over a few days ahead of the scheduled date. And Colorado Governor Jared Polis opened general authority about two weeks ahead of schedule.

“You no longer have to sort out whether you’re inside or outside,” said Julie Willems Van Dijk, the assistant secretary of the Department of Health in Wisconsin, where anyone 16 years or older is eligible for a vaccine Monday. “It’s time to just go forward and get everyone in the arm with one shot.”

In another promising development, federal health officials said Friday that Americans fully vaccinated against the coronavirus can travel within the US and abroad “at low risk to themselves.”

But most of the signs of hope these days are counterbalanced by danger.

For the past week, there was an average of 64,730 cases per day, up 19 percent from two weeks earlier, according to a New York Times database. The number of new deaths has decreased on average but is still 900 per day. More than 960 were reported on Friday alone.

The CDC forecast this week that the number of new Covid-19 cases per week in the US “will remain stable or show an uncertain trend” over the next four weeks and that the weekly number of cases could reach 700,000 even in the US End of April.

In many states, especially in the Midwest and Northeast, cases are already increasing significantly as variants spread and some governors relax mask mandates and other restrictions. Dr. Walensky said this week that the nation could face a potential fourth wave if states and cities continued to ease public health restrictions.

Michigan, one of the hardest-hit states, reports nearly 6,000 cases a day – up from about 1,000 a day at the end of February – even though half of those over 65 are now fully vaccinated.

And in Ohio, Governor Mike DeWine said new variations added to the state’s case numbers even as vaccinations increased.

“We have to understand that we are in a battle,” he said.

As if to underscore how fragile the nation’s recovery is, a typical American ritual – the start of baseball season – has already seen a virus-related delay.

Major League Baseball officials said Friday the league found only five positive cases in more than 14,000 tests by league staff. But because four of those people were Washington Nationals players, the team’s opening game against the New York Mets was postponed, and then the entire weekend series of three games.

“It’s one of those things that brings out that we haven’t made it yet,” Brian Snitker, executive director of Atlanta Braves, told The Associated Press. “We’re still fighting against it.”