Summer is now California's COVID busy season as winter waves fade

Six years after the COVID-19 pandemic upended the world, experts in California say the disease appears to be becoming more of a summer rather than a winter phenomenon.

The shift was noted over the past couple of years and has become even clearer in 2026, prompting health officials to refocus their prevention efforts. The U.S. Centers for Disease Control and Prevention says COVID-19 is growing or likely growing in 43 states at the moment, including California.

California is also one of only four states where COVID wastewater activity levels are considered moderate or high, according to the CDC.

“Our summer peak was much higher in 2025 than it was in the winter,” Dr. Erica Pan, director of the California Department of Public Health and the state public health officer, said in a recent briefing. “Summer is really the bigger peak. So if you do have patients or loved ones who have not gotten the ’25-’26 vaccines, it is still effective and highly recommended, given this surge we’re starting to see.”

The pattern is also being seen nationally. COVID-related emergency department visits peaked last summer in late August, with COVID being responsible for 1.7% of visits. The winter peak was 1%. The trend was even more stark in California, peaking last summer at 2.1%, while there was not much of a winter peak at all, maxing out at around 0.2%.

It wouldn’t be surprising for a summer increase in COVID to begin now. Large numbers of people haven’t fallen ill with COVID for quite a while.

This trend puts COVID in relatively rare territory as far as respiratory diseases go. Typically, those ailments — think flu and respiratory syncytial virus — spike in the winter, and aren’t really seen outside of that season.

In the early years of the pandemic, COVID spiked in both the summer and winter, with the winter waves being far more deadly. In 2021, for example, weekly COVID deaths spiked at more than 15,000 nationally around Labor Day, and peaked again at more than 21,000 in late January. Factors included people staying indoors due to the temperature outside, holiday travel and gatherings.

COVID deaths have dropped every year since then, but for a couple of years followed the two-peak pattern: a summer wave followed by a worse one in the winter. That has shifted in recent years, however, with doctors noticing the summer spikes were starting to get larger than their winter counterparts.

“It started in 2024. We started seeing a very large summer increase in cases and wastewater and hospitalizations, even dwarfing the winter. And then in 2025, that pattern continued, with more summer cases than winter cases,” said Dr. Peter Chin-Hong, a UC San Francisco infectious diseases expert.

It’s not entirely clear why recent summers have been more active for COVID than winters.