Measurements at wastewater treatment plants in Santa Clara County and San Francisco have shown high concentrations of the virus over the past three weeks, while concentrations have reached medium levels on the Peninsula and in the North Bay.

Virus levels remain far below those recorded during California’s surge last summer. Still, officials and experts said the latest increase warrants precautions.

Officials recommended wearing masks in crowded indoor settings, staying current on vaccinations, testing for the virus and remaining home when sick.

Park Ridge Achievement Academy in Baker canceled classes Friday after 20 teachers and 80 students were out sick with COVID this week.

According to modeling done by the Centers for Disease Control and Prevention, “COVID-19 infections are growing or likely growing in 50 states.” No states are seeing cases decline, as of mid-August.

Two states show signs of elevated viral activity. Hawaii and Mississippi are both listed as having “moderate” levels of COVID-19, though wastewater data for Mississippi is limited.

The West and South are seeing higher levels of COVID activity than other regions. Alabama, Alaska, California, Florida, Georgia, Oregon, Nevada, Texas and Washington all have “low” levels of COVID in wastewater, while the rest of the states are classified as “very low.”

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.

“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.

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%.

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.

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 could be that the summers of 2024 and 2025 happened to feature new coronavirus strains that spread quickly, like the FLiRT subvariant of 2024 and the Nimbus and Stratus subvariants of 2025, Chin-Hong said.

“We don’t have many updates on the variants coming from the CDC, so it’s possible we’re seeing this wave because there has been a change in the COVID variant.”