Did a Record Flu Season ‘Hold Back’ COVID-19 This Winter?

Winter 2024 – 2025 produced a surprising split-screen in U.S. respiratory surveillance. COVID-19 indicators stayed lower than many people expected for a winter stretch. Meanwhile, the flu season surged across clinics and hospitals nationwide. CDC classified the 2024–25 influenza season as high severity across age groups. CDC’s surveillance also recorded a record-high pediatric death total for seasonal influenza. Those facts made the season stand out, even before anyone compared it with COVID-19.

In a CDC MMWR report, 280 pediatric influenza deaths were reported for the 2024–25 season. The CDC season summary reported 279 laboratory-confirmed pediatric deaths through August 30, 2025. Either way, the count was historically high. That contrast raised a fair question about winter virus dynamics. Could widespread flu infection have delayed a COVID-19 wave? Viral interference is one theory, and researchers take it seriously. Still, it is not proven as the main driver for this winter. Population immunity, behavior, treatments, and surveillance limits can also shift the curves. The best framing is multi-cause, with viral interference as one candidate piece.

The flu season really was severe

CDC tracked multiple signals that point to a hard-hit flu season. Influenza activity rose in mid-November 2024 and peaked in early February 2025. Hospitalization surveillance showed clear pressure on inpatient care. In FluSurv-NET, CDC reported 39,319 laboratory-confirmed influenza hospitalizations from October 1, 2024, through April 30, 2025. The peak weekly hospitalization rate reached 13.7 per 100,000 people. CDC noted that the peak tied for the highest since 2010 – 2011. Mortality surveillance also climbed in midwinter. The CDC reported that influenza deaths peaked at 2.8% of all U.S. deaths in mid-February 2025. It also publishes modeled burden ranges to capture untested illnesses. It is estimated that 43 million – 73 million symptomatic illnesses occurred for that season. Furthermore, it is estimated that 560,000 – 1,100,000 hospitalizations occurred over the same span. CDC reported that adults 65 and older accounted for 57% of hospitalizations.

The pediatric toll was the headline. The CDC reported 279 laboratory-confirmed pediatric influenza deaths. It wrote, “This is the highest number of deaths reported during a seasonal influenza epidemic.” CDC reported a mean age at death of 7 years, with a wide age range. It described where deaths occurred, including after hospital admission and in emergency departments. The CDC reported that 147 of 260 children with a known history had at least 1 higher-risk condition. They reported that 185 of 207 vaccine-eligible children with known status were not fully vaccinated. It also warned that additional deaths may be reported as they are identified. Those details suggest many children lacked the best available protection. They also show how fast seasonal flu can still overwhelm families.

What “low COVID” means in modern surveillance

COVID-19 did not disappear in winter 2024–25. It still caused sickness and some hospital care. However, counting infections is harder now than early in the pandemic. Many people use home tests and never report results. Others never test at all. For that reason, agencies rely on layered signals. CDC’s respiratory virus activity page explains that ARI “captures a broad range of diagnoses” from emergency department visits. They also highlight wastewater, since it can detect viral traces even when testing drops.

Furthermore, CDC notes that wastewater can reflect virus circulation, even without symptoms. It also cautions that wastewater trends can differ from hospital trends. It links that gap to COVID-19, causing severe disease less frequently than earlier. So, a lower hospitalization curve does not guarantee lower transmission. It can also mean fewer infections become severe. The CDC classifies ARI-related emergency visits into 5 activity levels for regions and states. It also reports the percentage of tests positive for COVID-19, influenza, and RSV as another trend signal.

Independent research supports the idea that SARS-CoV-2 activity stayed lower in some winter settings. Marie-Céline Zanella and colleagues studied Geneva, Switzerland, using primary care, hospitals, and wastewater. Their window ran from September 2024 through April 2025, which overlaps the winter season. In their abstract, they wrote, “SARS-CoV-2 activity remained consistently lower than previous seasons across all surveillance systems.” During the same window, they observed strong influenza activity. They reported a clear winter peak for influenza in their combined indicators. Their approach matters because it combines signals, not just tests. Integrated surveillance still cannot prove why one virus stays lower. It can show timing and relative burden across streams. Even so, combined indicators reduce the chance that missed testing explains everything. It also reduces the chance that a single data stream drives the conclusion.

Viral interference explained 

Viral interference is a real concept in respiratory virology. One infection can make the airway less welcoming for another virus for a period. Timing drives the effect. If a second virus arrives soon after the first, it meets an immune system on alert. Interferons are a key part of that alert. Infected cells release interferons, and nearby cells switch on antiviral defenses. This can slow replication of a second virus, at least early on. Some researchers also discuss simple competition for susceptible cells in the airway. Others focus on innate immune cells that arrive after the first infection begins. However, interference is not guaranteed, and it can vary by virus pair. It can also fade as the first infection resolves. Therefore, timing between exposures can matter as much as which virus circulates.

Benjamin Davido, an infectious diseases physician at Université Paris-Saclay and AP-HP, explored this idea for the 2024 – 2025 winter. He linked a heavy influenza season with a smaller COVID-19 winter wave in the United States. In his published abstract, he hypothesized, “Influenza’s robust interferon response might have contributed to suppression of SARS-CoV-2 replication.” That sentence captures the promise and the limits. It points to a plausible biological pathway, yet it does not quantify the effect. Davido framed interference as one possible contributor among broader forces. He also emphasized that surveillance describes correlations, not lab-confirmed causation. Interference can also work both ways, depending on order and immunity. Real people carry immune memory from past infections and vaccines. That memory can change how strongly interferons rise after exposure. Interference is a candidate explanation, not a singular explanatory reason on its own.

What laboratory studies can show, and what they cannot

Laboratory studies can test interference with clean timing. Researchers infect airway cells with influenza, then add SARS-CoV-2, and measure viral growth. That design avoids many community confounders. It removes differences in access to testing and differences in care seeking. It also standardizes exposure dose and infection order. Those strengths matter when you want to test a mechanism. However, the price of control is realism. A lab model cannot recreate a city with layered immunity and uneven mixing. It also cannot mimic weather, travel surges, and school holidays. In real life, many infections happen without a clear “first virus” moment. Some people get exposed to both viruses within the same week. So, lab results should guide questions, not settle them. Airway epithelium models capture key cells, yet they miss full-body immune responses. They also cannot capture reinfections across months, which shape winter immunity.

Multiplex panels can detect co-infection in real patients. Coverage varies by hospital system, so datasets remain uneven across states and seasons. Even so, recent experiments support influenza-to-COVID interference. In 2024, Stéphanie Gilbert-Girard, Lucie Piret, and Guy Boivin published a PLOS Pathogens study using human airway epithelium. They tested sequential infections with influenza A and SARS-CoV-2 variants. In their paper, they stated, “Thus, influenza A causes a viral interference towards SARS-CoV-2 most likely through an IFN response.” Then they reported reduced SARS-CoV-2 replication after prior influenza infection in their model. They also reported that SARS-CoV-2 induced marginal interferon production in their system. That finding can help explain directionality in some coinfections. Yet their work also shows that viral interactions depend on timing. A longer gap between infections can reduce the interferon window.

Immunity and vaccines can lower severe COVID-19 outcomes

By winter 2024 – 2025, most people had some SARS-CoV-2 immune memory. That memory comes from vaccination, infection, or both. It does not block all infections, especially as variants evolve. Yet it often reduces severe disease risk. Therefore, hospital curves can stay lower even when infections continue. This can make a winter look calmer, even when people still get sick. It can also reduce the share of infections that reach emergency care. Another factor is the timing of recent exposure. If many people were infected months earlier, protection against severe disease can persist longer. That can blunt a winter hospitalization rise. So, a lower COVID-19 winter can reflect immune history more than viral competition.

CDC published interim estimates for 2024 – 2025 COVID-19 vaccine effectiveness in adults. The analysis was led by Ruth Link-Gelles and colleagues at CDC. It used data from September 2024 through January 2025 in established networks. The report states, “Interim effectiveness of 2024–2025 COVID-19 vaccines was estimated” for emergency visits and for hospitalizations. Those estimates vary by group and outcome, yet they support ongoing benefit. On the influenza side, CDC continues to press vaccination before and during flu season. The CDC writes, “Everyone 6 months and older should get a flu vaccine every season with rare exceptions.” CDC estimated that flu vaccination prevented 170,000 – 360,000 hospitalizations in 2024 – 2025 if vaccination and prior infection lowered severe COVID-19 risk, which alone could keep winter indicators down.

Behavior still drives exposure, even without mandates

Viruses spread through contact and shared air, so human behavior sets the stage. Winter brings more indoor time, so exposure chances rise. Schools amplify the spread because children mix closely and share air for hours. Workplaces and public transport add daily contact, especially in dense cities. Travel moves viruses between regions, then local gatherings do the rest. These forces can lift influenza quickly, since it often spreads fast among children. CDC’s season summary shows how rapidly influenza indicators rose in late 2024. They can also fight COVID-19, yet the timing can differ by variant and immunity. Some COVID-19 waves peak outside winter, which changes seasonal expectations. So, a winter with heavy influenza does not automatically mean COVID-19 must spike too. Ventilation, crowding, and local travel can shift exposure risk week by week.

Behavior also reacts to what people see around them. A heavy flu season can raise caution, even without formal rules. Parents keep sick children home more often, and that reduces mixing. Some households skip gatherings when close relatives are ill. Clinics may advise people to stay away unless symptoms worsen. These choices can reduce exposure opportunities for several respiratory viruses at once. They can also shift who seeks tests and who seeks care. That makes winter comparisons tricky, because “low” can reflect fewer visits, not fewer infections. CDC notes that ARI includes common cold diagnoses, so it captures behavior and healthcare use. WHO encourages people to stay updated, since guidance can change with conditions.

Testing and reporting can bend the curves we compare

COVID-19 case numbers now miss many infections. Home testing is common, and reporting is often optional. Some people only test when work requires documentation. Others rely on symptoms and never test. Therefore, “case counts” are a weaker yardstick than they once were. Hospitalizations and emergency visits are steadier signals, yet they also carry biases. People choose the emergency department based on cost, worry, and access. During holidays, care seeking can drop, even if infections rise. CDC flags holiday-related reporting issues and urges cautious interpretation. Those lags can make one virus look like it peaks later than another. WHO also warns that reported statistics may not reflect the actual number of cases. It notes that some countries have stopped reporting or changed reporting frequency.

Influenza surveillance has limits, too, and the CDC states them plainly. In its season summary, CDC writes, “not everyone who gets sick with influenza will seek medical care or be tested.” CDC adds that reporting is not required in most areas. Therefore, CDC models the burden each season and labels early numbers as preliminary. The same report flags timing, stating, “Data presented are current as of September 9, 2025.” It also warns that additional pediatric deaths can be reported later. These caveats do not erase the flu season’s severity. They do warn against over-reading small differences between virus curves. CDC even shades preliminary respiratory test data, signaling that recent values can shift. Backfilled laboratory reports can also raise past weeks after initial posting. That is common in respiratory surveillance. So, early comparisons across viruses should stay provisional. If COVID-19 looks lower, part of that may reflect measurement choices.

Read More: New Blood Test Could Finally Diagnose Chronic Fatigue and Long COVID

So, did the flu season “hold back” COVID-19?

Viral interference remains a reasonable hypothesis for winter 2024 – 2025. It has credible biology, and lab work supports it. Influenza can trigger interferon responses that limit later viral replication in airway tissue. If influenza spread widely first, some people may have gained short-lived antiviral protection. That could delay a COVID-19 rise, or flatten it across weeks. However, no national dataset can prove this mechanism drove the winter outcome. Real-world spread mixes biology with immunity history and contact behavior. Different regions also peak at different times, even in the same season. So, the most honest answer keeps uncertainty in view for now. A balanced explanation uses more than one driver. The 2024 – 2025 flu season was unusually severe, including record pediatric deaths. At the same time, COVID-19 severe-outcome indicators appeared lower in several surveillance streams. 

That likely reflects immune memory across the population, plus vaccination and treatment for higher-risk people. Behavior shifts during a heavy flu season can also reduce exposure opportunities. Surveillance limits add another layer, since some signals track trends, not totals. The CDC explains this directly when it models epidemic trends from emergency visits. CDC writes, “it does not tell us the actual number of current infections” with SARS-CoV-2, influenza, or RSV. Therefore, the best way to watch the next winter is multi-signal tracking. Combine emergency visit trends with hospital admissions, test positivity, and wastewater. Then interpret them with caution, even when one virus dominates headlines. To test interference in communities, researchers need multiplex testing and clear timing data. CDC notes it is “almost impossible to know exactly when transmission occurred” in the data. Therefore, better winter answers will come from focused studies.

A.I. Disclaimer: This article was created with AI assistance and edited by a human for accuracy and clarity.

Read More: New H3N2 K-Variant Flu Is Spreading Rapidly, Prompting Severe Flu Season Warnings