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COVID-19 and Seasonal Flu: How Do They Stack Up?

As SARS-CoV-2 continues to circulate, surges in COVID-19 cases frequently overlap with flu season (seasonal influenza). As a result of increasing COVID-19 and seasonal flu-related admissions, hospitals are starting to feel the strain. So what can we learn from this situation? According to VA St. Louis Health Care System researchers, quite a lot.

While the world may no longer be on high alert, it is worth remembering that COVID-19 is still with us. In fact, there have been notable surges in the rates of infection and mortality this winter. In the 28 days between November 20th 2023 and December 17th 2023, COVID-19 cases increased 52% and deaths jumped 8% globally.1 The combined pressure of COVID-19 and seasonal flu on healthcare systems is a serious concern.2

Impact of COVID-19 and Seasonal Flu

In the early stages of the pandemic, global efforts to slow the spread of COVID-19 also reduced the spread of flu. While reducing influenza cases was helpful, it made assessing how similar the long-term effects of the two viruses were, tricky.

In a recent study published in the Lancet Infectious Diseases, researchers affiliated with the Veterans Affairs St. Louis healthcare System in Missouri, USA, evaluated both short- and long-term effects of COVID-19 and seasonal flu.3

The epidemiologists, led by Dr. Ziyad Al-Aly, report that while both SARS-CoV-2 and influenza can cause very serious illness and death, COVID-19 appears to trigger more extensive, long-lasting damage to the body. People admitted to hospital with COVID-19 between March 1st 2020 and June 30th 2022 were more likely to die than those admitted with flu between October 1st 2015 and February 28th 2019. They also used more healthcare resources than flu patients and suffered more harm to their overall health. Follow ups over 18 months demonstrated that COVID-19 caused worse long-term health outcomes than seasonal Flu.

Deep into Data

The researchers used US Department of Veterans Affairs healthcare databases to examine patient records from across the VA St. Louis Healthcare System. They divided the patient profiles into two groups. People who tested positive for SARS-COV-2 and people who tested positive for influenza. The researchers excluded patients who tested positive for both SARS-CoV-2 and influenza.They then picked patients who were admitted to hospital for COVID-19 or flu within 30 days of a positive test result. The researchers followed their treatment records and medical appointments over 18 months from the initial hospital admission date. The team collected 81,280 patient records for COVID-19 admissions and 10,985 seasonal flu admissions.

The large size of the cohort allowed the researchers to account for factors that could make the participants difficult to compare, such as age, race, socioeconomic background and obesity.

COVID-19 Carries More Severe Long-Term Effects than Flu

The scientists specifically compared the effects of COVID-19 and seasonal flu on various organ systems. Using the International Classification of Diseases 10th Revision (ICD-10) manual, they selected 94 health outcomes associated with COVID-19 or seasonal flu. They only counted a symptom as an “outcome” if the patient had not experienced it before they tested positive for the virus.

The categories of health outcomes they examined were the heart and circulation system, blood and clotting, gastrointestinal, kidney, mental health, metabolic (diabetes), musculoskeletal, neurologic and lungs. They then determined which of the two illnesses more often led to each health outcome.

The researchers established that COVID-19 had a higher risk than flu of death, following hospital admission. COVID-19 had more wide-ranging impacts compared to seasonal flu, which tended to be limited to the lungs and respiratory system. It also carried a higher risk for harmful outcomes in individual organ systems such as the cardiovascular, hematological, mental health and musculoskeletal systems.

COVID-19 patients were more likely to be readmitted to hospital, and had higher chances of admission to the intensive care unit. Notably, COVID-19 had higher risks of harm to all the organ systems except for the pulmonary system (lungs).

The long-term impact of COVID-19 could be more severe than seasonal flu due to the cumulative effect of all the different organ systems it harms. So while somebody recovering from flu might be burdened by lingering lung symptoms, fatigue and brain fog, COVID-19 patients might be bothered by these symptoms and more.

Both COVID-19 and Seasonal Flu Can Cause Severe Harm

This is not to say that the effects of seasonal flu were not serious or extended. In fact, the increase in risk is on top of an already high probability of harm. A good example is the effect on musculoskeletal outcomes. If we look at 100 COVID-19 patients, after 360 days, roughly 35 would have a musculoskeletal problem. Of 100 seasonal flu patients, around 23 might have the same outcome. We should also consider that the huge number of patients with COVID-19 compared to seasonal flu might distort the story. This study only followed patients who had been hospitalized. The cases examined were also not from the same time period. This would that prior to the pandemic, the resources available to treat each patient with flu may have been different to those available to COVID-19 patients.

The study showed that both COVID-19 and seasonal flu have long-term effects both on individuals and the healthcare systems at large, especially for cases that result in hospital admissions. This means it is prudent to take preventive measures that fend off the viruses or reduce their harm.

Protecting Yourself and Others

The impact of COVID-19 might not be as obvious as in 2020, because people have been vaccinated and we now have evidence-based treatment protocols. However, with tens of thousands of COVID-19 hospitalizations in the United States, every week COVID-19 is still a serious problem. What’s more, aggressive variants continue to arise, meaning this virus might be with us for a long time to come. The latest variant of the Omicron family, JN. 1 is currently responsible for over 80% of the COVID-19 cases in the United States of America.4 Experts are encouraging the uptake of updated vaccines because the viruses keep getting smarter in how they evade the immune system of the body.

Andy Pekosz, a professor in the Department of Molecular Microbiology and Immunology at the Johns Hopkins Bloomberg School of Public Health is quoted on the institution’s website saying that taking the COVID-19 vaccine protects against developing severe disease, even for the new variants.5

“When you get infected with COVID, you have symptoms from the virus that wreak havoc on your body. Whether or not you’ve been infected or vaccinated previously, the updated COVID vaccine is going to strengthen your immune responses to high levels and do so in a safe way. We’re also at a much different point in the pandemic than we were when the first vaccines rolled out in 2020. Exposure levels to COVID vary significantly: Some people may have gotten their last vaccine two months ago or 10 months ago, while some are still not vaccinated; some people still haven’t gotten COVID, while others have had it three times or more. That’s all to say that we’re now vaccinating a population that has some existing immunity, but ultimately updated vaccines will provide the safest and most effective protection against the currently circulating variants.” Andy Pekosz, PhD

What can I do?

You can contribute to the curbing of COVID-19 and flu spread by:

  • Getting your flu and COVID-19 shots
  • Wearing a mask when necessary, especially at the doctor’s surgery or hospital
  • Practising cough hygiene
  • Maintaining hand hygiene
  • Isolating when you test positive or develop COVID-19 or seasonal flu symptoms

As we adapt to living with the COVID-19 virus, it’s crucial that we all do our bit to reduce its transmission.

  1. COVID-19 epidemiological update—22 December 2023. World Health Organization.
    Accessed January 20, 2024.
  2. COVID burden to fall on primary care, Ontario needs new approach: science table | Accessed January 24, 2024.
  3. Xie Y, Choi T, Al-Aly Z. Long-term outcomes following hospital admission for COVID-19
    versus seasonal influenza: a cohort study. Lancet Infect Dis. Published online December
    14, 2023. doi:10.1016/S1473-3099(23)00684-91.
  4. CDC COVID Data tracker. Centers for Disease Control and Prevention. Accessed January
    20, 2024.
  5. What to Know About the Updated COVID-19 Vaccine for Fall/Winter 2023 | Johns Hopkins | Bloomberg School of Public Health. Published September 14, 2023. Accessed January 26, 2024.
Dr Anthony Onunga
Dr Anthony Onunga
Anthony Onunga is a practicing medical doctor with the compulsion to share medical knowledge and concepts to the public in a simple-to-understand manner. He enjoys researching and writing about medical topics and has experience working in the clinical setting. As a science correspondent, Anthony creates content backed with facts and scientific studies. In his spare time, he enjoys watching football, playing chess and exploring new travel destinations.


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