WASHINGTON (NewsNation Now) — A top U.S. public health official says young people are driving the latest uptick in COVID-19 cases, as the increasing rate of vaccination in older Americans is preventing the most serious cases among seniors.

Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said during a press briefing Monday that “cases are increasing nationally, and we are seeing this occur predominately in younger adults.”

She cites the increasing spread of variants and a rise in youth sports and extracurricular activities as contributing to the steady increase in cases over the last four weeks.

Walensky specifically pointed to seeing peaks in the 18-24 age group as well. Other health officials had previously expressed concerns about the increase in travel around the Easter holiday and school spring breaks.

But Walensky pointed to positive developments among the most vulnerable age group, saying senior citizens’ virus deaths have reached their lowest levels since the early fall. Greater than 75% of those aged 65 or older nationally have received at least one dose of a COVID-19 vaccine and nearly 55% are fully vaccinated.

“What we’re seeing is both a decrease in emergency department visits as well as hospitalizations associated with that demographic,” Walensky said.

While flu viruses tend to be seasonal, with cases falling as the weather warms, health officials said they have not seen similar trends with coronavirus, pointing to a wave in cases in some regions last summer.

“I don’t think we should even think about relying on the weather to bail us out of anything we’re in right now,” Dr. Anthony Fauci, the nation’s top infectious disease expert, said Monday.

This comes days after the CDC said Friday people who are fully vaccinated can safely travel at “low risk” after the agency had held off for weeks on revising guidance that discouraged all non-essential trips.

The CDC said grandparents that have been fully vaccinated can fly to visit grandkids without getting a COVID-19 test or self-quarantining as long as they follow CDC advice for traveling safely.

But the administration is not lifting restrictions that bar most-non U.S. citizens from the United States who have recently been in China, Brazil, South Africa and most of Europe. It is also keeping requirements that nearly all international U.S. air visitors getting a negative COVID-19 test before traveling to the United States.

The CDC did not revise guidance for non-vaccinated people.

About 61.4 million people, or 18.5% of the U.S. population, have been fully inoculated with COVID-19 vaccines made by Pfizer Inc/ BioNTech SE, Moderna Inc and Johnson & Johnson, according to CDC data. 32% of the U.S. population, or 100.6 million adults had received at least one dose of a COVID-19 vaccine.

The Associated Press and Reuters contributed to this report.