In February 2025, flu rates spiked to the highest levels seen in at least 15 years, with flu outpacing COVID-19 infections and hospitalizations for the first time since the beginning of the COVID-19 pandemic. The Centers for Disease Control and Prevention has classified this flu season as having “high” severity across the U.S.

The Conversation asked epidemiologist Annette Regan to explain why this flu season is different from last year’s and what people can do to help reduce the spread.

How do flu cases and hospitalizations this year compare with previous years?

Beginning in late January and extending through February 2025, flu hospitalizations have been higher than any other week since before 2009.

Most flu cases appear to be from influenza A strains, with a split between influenza A/H3N2 and influenza A/H1N1. These are two different subtypes of the influenza A virus.

Researchers believe that historically seasons that are predominated by influenza A/H3N2 infections tend to be more severe, but infections from influenza A/H1N1 can still be very severe.

This year’s season is also peaking “late” compared with the past three flu seasons, which peaked in early or late December.

Unfortunately, there have been a number of deaths from flu too this season. Since Jan. 1, 2025, alone, over 4,000 people, including 68 children, have died from flu. While the number of deaths do not mark a record number, it shows that flu can be a serious illness, even in children.

Why are flu cases so high this year?

There are a number of factors behind any severe season, including poor community protection from low immunization rates and low natural immunity, virus characteristics, vaccine effectiveness and increased human contact via travel, office work or schools.

Unfortunately, flu vaccination rates have declined since the COVID-19 pandemic. At the end of the 2023-24 flu season, 9.2 million fewer doses were administered in pharmacies and doctors’ offices compared with an average year before the pandemic.

In addition, since 2022, fewer and fewer doses of flu vaccine have been distributed by private manufacturers. Flu vaccination rates for adults have historically been in the 30% to 60% range, much lower than the recommended 70%. Before the COVID-19 pandemic, flu vaccination rates were increasing by around 1% to 2% every year.

Flu vaccination rates began dropping after the COVID-19 pandemic, especially in higher-risk groups. Flu vaccination in children has dropped from 59% in 2019-20 to 46% in 2024-25. In adults 65 years and older, the group with the greatest risk of hospitalization and death, flu vaccination rates dropped from 52% in 2019-20 to 43% in 2024-25.

Lower vaccination rates mean a greater portion of the population is not protected by vaccines. Data shows that vaccination reduces the risk of flu hospitalization. Even if a vaccinated person gets infected, they may be less likely to experience severe illness. As a result, low vaccination rates could contribute to higher flu severity this season.

However, low vaccination rates are probably not the only reason for the high rates of flu this season. In previous severe seasons, genetic changes to the viruses have made them better at infecting people and more likely to cause severe illness.

The effectiveness of annual flu vaccines varies depending on how well the vaccine matches the circulating virus. The effectiveness of vaccines ranges from 19% to 60% in any given season. In the 2023-24 flu season, the vaccine was 42% effective.

Similarly, early 2024-25 data from the U.S. shows that the vaccine was 41% to 55% effective against flu hospitalizations in adults and 63% to 78% effective against flu hospitalizations in children.

How do seasonal flu symptoms differ from COVID-19 and other illnesses?

It’s important to remember that people often incorrectly refer to “the flu” when they have a common cold. Flu is caused only by the influenza virus, which tends to be more severe than common colds and more commonly causes a fever.

Many of the signs and symptoms for flu, COVID-19 and other respiratory viruses are the same and can range from mild coldlike symptoms to pneumonia and respiratory distress. Common flu symptoms are fever, cough and fatigue, and may also include shortness of breath, a sore throat, nasal congestion, muscle aches and headache.

Some symptoms, such as changes in or loss of taste and smell, are more common for COVID-19. For both COVID-19 and flu, the symptoms do not start until about one to four days after infection, and symptoms seem to last longer for COVID-19.

The only way to know what virus is causing an infection is to test. This can be done using a rapid test, some of which now test for flu and COVID-19 together, or by seeing a doctor and getting tested using a nasal swab. There are prescription antiviral medications available to treat flu and COVID-19, but they need to be taken near the time that symptoms start.

Some people are at high risk of severe flu and COVID-19, such as those who are immunosuppressed, have diabetes or have chronic heart or lung conditions. In these cases, it is important to seek early care and treatment from a health care professional. Some doctors will also prescribe via telehealth calls, which can help reduce the strain on doctors’ offices, urgent care centers and emergency rooms when infection rates are high.

What can people do now to help steer clear of the flu?

There are a number of ways people can reduce their risk of getting or spreading flu. Since the flu season is still underway, it’s not too late to get a flu vaccine. Even in seasons when the vaccine’s effectiveness is low, it is likely to offer better protection compared with remaining unvaccinated.

Handwashing and disinfecting high-traffic surfaces can help reduce contact with the flu virus. Taking efforts to avoid contact with sick people can also help, including wearing a mask when in health care facilities.

Finally, remember to take care of yourself. Exercising, eating healthy and getting sufficient sleep all help support a healthy immune system, which can help reduce chances of infection.

Those who have been diagnosed with flu or are experiencing flu-like symptoms should avoid contact with other people, especially in crowded spaces. Covering coughs and sneezes can help reduce the amount of virus that is spread.

Annette Regan, Adjunct Associate Professor of Epidemiology, University of California, Los Angeles

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The Sun periodically ejects huge bubbles of plasma from its surface that contain an intense magnetic field. These events are called coronal mass ejections, or CMEs. When two of these ejections collide, they can generate powerful geomagnetic storms that can lead to beautiful auroras but may disrupt satellites and GPS back on Earth.

On May 10, 2024, people across the Northern Hemisphere got to witness the impact of these solar activities on Earth’s space weather.

Two merging CMEs triggered the largest geomagnetic storm in two decades, which manifested in brightly colored auroras visible across the sky.

I’m a solar physicist. My colleagues and I aim to track and better understand colliding CMEs with the goal of improving space weather forecasts. In the modern era, where technological systems are increasingly vulnerable to space weather disruptions, understanding how CMEs interact with each other has never been more crucial.

Coronal mass ejections

CMEs are long and twisted – kind of like ropes – and how often they happen varies with an 11-year cycle. At the solar minimum, researchers observe about one a week, but near the solar maximum, they can observe, on average, two or three per day.

When two or more CMEs interact, they generate massive clouds of charged particles and magnetic fields that may compress, merge or reconnect with each other during the collision. These interactions can amplify the impact of the CMEs on Earth’s magnetic field, sometimes creating geomagnetic storms.

Why study interacting CMEs?

Nearly one-third of CMEs interact with other CMEs or the solar wind, which is a stream of charged particles released from the outer layer of the Sun.

In my research team’s study, published in May 2024, we found that CMEs that do interact or collide with each other are much more likely to cause a geomagnetic storm – two times more likely than an individual CME. The mix of strong magnetic fields and high pressure in these CME collisions is likely what causes them to generate storms.

During solar maxima, when there can be more than 10 CMEs per day, the likelihood of CMEs interacting with each other increases. But researchers aren’t sure whether they become more likely to generate a geomagnetic storm during these periods.

Scientists can study interacting CMEs as they move through space and watch them contribute to geomagnetic storms using observations from space- and ground-based observatories.

In this study, we looked at three CMEs that interacted with each other as they traveled through space using the space-based observatory STEREO. We validated these observations with three-dimensional simulations.

The CME interactions we studied generated a complex magnetic field and a compressed plasma sheath, which is a layer of charged particles in the upper atmosphere that interacts with Earth’s magnetic field.

When this complex structure encountered Earth’s magnetosphere, it compressed the magnetosphere and triggered an intense geomagnetic storm.

This same process generated the geomagnetic storm from May 2024.

Between May 8-9, multiple Earth-directed CMEs erupted from the Sun. When these CMEs merged, they formed a massive, combined structure that arrived at Earth late on May 10, 2024. This structure triggered the extraordinary geomagnetic storm many people observed. People even in parts of the southern U.S. were able to see the northern lights in the sky that night.

More technology and higher stakes

Scientists have an expansive network of space- and ground-based observatories, such as the Parker Solar Probe, Solar Orbiter, the Solar Dynamics Observatory and others, available to monitor the heliosphere – the region surrounding the Sun – from a variety of vantage points.

These resources, coupled with advanced modeling capabilities, provide timely and effective ways to investigate how CMEs cause geomagnetic storms. The Sun will reach its solar maximum in the years 2024 and 2025. So, with more complex CMEs coming from the Sun in the next few years and an increasing reliance on space-based infrastructure for communication, navigation and scientific exploration, monitoring these events is more important than ever.

Integrating the observational data from space-based missions such as Wind and ACE and data from ground-based facilities such as the e-Callisto network and radio observatories with state-of-the-art simulation tools allows researchers to analyze the data in real time. That way, they can quickly make predictions about what the CMEs are doing.

These advancements are important for keeping infrastructure safe and preparing for the next solar maximum. Addressing these challenges today ensures resilience against future space weather.

Shirsh Lata Soni, Postdoctoral Research Fellow, University of Michigan

This article is republished from The Conversation under a Creative Commons license. Read the original article.