COVID-19 Shifts to a Summer Phenomenon as Cases Rise Across the US
核心洞察
COVID-19 (搜索) infections are growing or likely growing in all 50 states, with active outbreaks in at least 15 states and territories as of mid-August.
Experts report that summer peaks have surpassed winter peaks since 2024, with California's 2025 summer peak "much higher" than its winter peak.
Nationwide test positivity reached nearly 4% last week, still below the more than 9% recorded during the same week a year ago.
COVID-19 (搜索), a virus many Americans still associate with winter, is now flourishing in the summer months, and the latest data suggest the bug is gaining ground nationwide. Infections are growing or likely growing in all 50 states, according to the Centers for Disease Control and Prevention (搜索) (CDC), drawing renewed attention from health officials after last summer's surge.
"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, told the Los Angeles Times. "Summer is really the bigger peak."
A Seasonal Shift in COVID-19 Transmission
The shift toward summer transmission has become increasingly clear over the past couple of years, prompting health officials to refocus their prevention efforts. The CDC reports that COVID-19 (搜索) is growing or likely growing in 43 states, including California, which is one of only four states where COVID wastewater activity levels are considered moderate or high.
The pattern is evident nationally. COVID-related emergency department visits peaked last summer in late August, with COVID responsible for 1.7% of visits, compared with a winter peak of 1%. The trend was even more pronounced in California, where visits peaked last summer at 2.1%, while the winter peak maxed out at around 0.2%.
"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.
This trend places COVID in relatively rare territory among respiratory diseases, which typically spike in winter. In the early years of the pandemic, COVID spiked in both summer and winter, with winter waves proving far more deadly. In 2021, weekly COVID deaths spiked at more than 15,000 nationally around Labor Day and peaked again at more than 21,000 in late January.
Current Wave and Projected Peak
For now, this summer's wave is not as severe as last year's. Nationwide wastewater viral activity levels were considered "very low" as of Aug. 14, as were emergency department visits related to the respiratory disease. Last week, nearly 4% of COVID tests came back positive, compared with more than 9% during the same week a year ago, according to the CDC.
However, infections are climbing nationwide, with rates increasing sharply over the last month, particularly in the South and West. As of Aug. 17, at least 15 states and territories were experiencing active COVID outbreaks, according to the Pandemic Mitigation Collaborative's data tracker.
"That is troubling, especially if you're in one of those areas," Michael Hoerger, an assistant professor at Tulane University School of Medicine who leads the PMC, told Today. "We're expecting transmission to peak about a week into September."
The reasons for the summer shift remain unclear. It could be that the summers of 2024 and 2025 happened to feature new coronavirus strains that spread quickly, such as the FLiRT subvariant of 2024 and the Nimbus and Stratus subvariants of 2025, Chin-Hong said.
"This past fall and winter marked a change in how the COVID waves have been coming, with very little activity. The reasons for this are likely myriad, including that there was not a marked shift in the viral variant, plus the cumulative effect of people having both vaccine and natural immunity against COVID," said Dr. Elizabeth Hudson, regional physician chief of infectious diseases at Kaiser Permanente Southern California (搜索). "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."
Vaccination Guidance and the Fall Season
The surge comes as Americans face another fall vaccine season, with new guidance on who should get a COVID shot. Until last year, COVID-19 (搜索) vaccines were recommended for everyone 6 months and older. But the Trump administration's CDC has moved away from a universal recommendation, instead adopting a shared clinical decision-making model for people in that age group.
Still, other major medical groups continue to recommend COVID vaccines for the wider population, and NYC Health says they "remain our best protection" against the virus.
"The message is that COVID vaccine and flu vaccine will be available, they're covered by insurers, they're recommended by medical societies. People should get them and get them for their kids," Claire Hannan, executive director of the Association of Immunization Managers, told NPR.
An updated COVID-19 (搜索) vaccine is expected to become available in early fall, with the new shot targeting the XFG variant. The California Department of Public Health recommends the 2025-26 COVID-19 vaccine for seniors age 65 and up, those age 6 months to 23 months, pregnant women, anyone with risk factors and those in contact with them, children and teens never vaccinated against COVID, and anyone else who chooses to be vaccinated.
The California Department of Public Health also advises that people age 65 and up, and those who are immunocompromised, should get a second COVID-19 (搜索) vaccination six months after their first dose.
"Anyone who hasn't received a 2025-26 dose, or is due for the second dose, is encouraged to get vaccinated now, before the next COVID-19 (搜索) surge," according to a slide in Pan's presentation to health professionals.
Particularly for those who are older and immunocompromised, "go and get the shot now," Chin-Hong said, "especially when we're seeing that summer is the new winter in the last two years in California. Don't wait for the fall."
Ongoing Disease Burden
While COVID-19 (搜索) is no longer the threat it was during the early years of the pandemic, infection can still become serious or even deadly. There is also the lingering risk of long COVID (搜索), and people who get COVID have been found to have an increased risk of stroke, heart attack and blood clots.
For the 2025 calendar year, 20,685 deaths were attributed across the nation to COVID-19 (搜索) on death certificates, according to the CDC. There were 47,541 deaths in 2024; 76,055 in 2023; 247,168 in 2022; 463,267 in 2021; and 385,676 in 2020. In California, there were 1,813 deaths involving COVID-19 in 2025.
Those most vulnerable to severe illness include older adults, people who are immunocompromised, pregnant women and children younger than 5.
COVID hospitalizations in California are rising but remain lower than they were for the past two summers. California most recently reported a rate of 0.84 hospitalizations for every 100,000 residents; a year ago, the rate was 1.92; and the year before that, it was 8.12.
The big question is how many Americans will actually roll up their sleeves. Fewer than 20% of US adults received the most recent COVID shot. Severe illness, hospitalization and death from COVID "is preventable" with vaccination, Chin-Hong said. State health officials forecast a moderate increase in COVID hospital admissions later this summer and in early fall.