A winter wave of COVID infections is cresting as 2025 begins, one that started to swell before Christmas cookies were left out for Santa Claus and Hanukkah menorahs and Kwanzaa kinaras were lit. Nationwide test positivity was projected to be 7.5% the week ended Dec. 21, according to Dec. 30 estimates from the Centers for Disease Control and Prevention (CDC). Infections have ticked up consistently since the agency recorded a rate of 4.1% the week ended Nov. 16.

Thanksgiving, celebrated Nov. 28 this year, is a likely culprit as it marked the start of holiday travel and gatherings. In other words, “optimal circumstances” for viral spread, Dr. William Schaffner, a professor in the division of infectious diseases at Vanderbilt University Medical Center, told Fortune just before Christmas. In addition, we’re in the midst of a “quad-demic” of COVID, flu, respiratory syncytial virus (RSV), and norovirus, and uptake of the new, 2024–25 COVID vaccine has been minimal.

“Benjamin Franklin had it right: ‘An ounce of prevention is worth a pound of cure,’” Schaffner said. “Let’s do what we can to prevent serious illness this year. We’re under-vaccinating.”

Just one in five adults (20.9%) and one in 10 children (10.1%) had received the latest COVID shot as of the week ended Dec. 14, CDC records show. Even if you’ve had the illness or gotten a previous version of the vaccine, the CDC recommends the updated immunization to everyone 6 months and older to help prevent severe infection, hospitalization, and death.

Where is COVID spreading the most?

Earlier in 2024, the U.S. saw its worst COVID summer surge yet, reaching a test positivity rate of 17.8% the week ended Aug. 10. With respiratory virus season in full swing and the extent of holiday COVID spread not yet known, it’s too early to tell whether this winter’s wave will rival that of last summer. Last winter, test positivity peaked at 13.1% the week ended Dec. 30, 2023.

COVID-related emergency department visits and deaths are also creeping up this winter. The week ended Dec. 21, ER visit and death rates were 0.88% and 1.1%, respectively, compared to 0.49% and 0.9% the week ended Nov. 16. Hospitalization rates have dipped in recent weeks, going from 2% the week ended Dec. 7 to 1.6% a fortnight later.

Though no U.S. region recorded a COVID test positivity rate above 10% the four weeks ended Dec. 21, rates were higher in the Midwest, Mountain states, and South Central U.S. The CDC’s Region 8—Colorado, the Dakotas, Montana, Utah, and Wyoming—had the highest rate of 6.8% compared to the national average of 4.9%. Region 4—Alabama, the Carolinas, Florida, Georgia, Kentucky, Mississippi, and Tennessee—had the lowest at 2.2%.

Considering only the week ended Dec. 21, however, most regions showed test positivity between 5% and 9.9%. Alaska, Idaho, Oregon, and Washington, which make up Region 10, had the highest rate of 9.4%.

For more on COVID-19:

• Public health experts are warning of a ‘quad-demic’ this winter. Here’s where flu, COVID, RSV, and norovirus are spreading
• Could you have COVID? Symptoms to watch for in 2025 include sore throat and fatigue
• New COVID vaccines are here. What to know about latest shots in wake of nation’s biggest summer surge
• Yes, you can get the COVID, flu, and RSV vaccines at once. Here are the pros and cons
• Free, at-home COVID tests are back. Here’s how to order yours