Which groups of people are more vulnerable to flu?  

#TogetherAgainstFlu - 17/11/21

Typically starting in October, seasonal flu lasts throughout March for the Northern hemisphere and April to September for the Southern hemisphere1. Every year, the flu threatens our health. 

Enjoying high-quality life and being able to function independently is for many the highest wish. Unfortunately, the seasonal flu can put a barrier to that. The flu usually hits suddenly, with symptoms such as headache, fever, running nose, sore muscles, and sometimes vomiting and diarrhea2. When we get sick with the flu we can’t be as active as before and are often forced to stay in bed to recover. Most people recover from the flu rather promptly however and can go back to life as before.  

But for some people, it can get serious and even life-threatening. In fact, each year, globally, there are an estimated 1 billion cases of flu, of which 3–5 million are severe cases and 290–650 thousand result in death3.  

Fighting flu is more difficult for some people 

Flu is a viral respiratory disease caused by the influenza virus that easily spreads through sneezing, coughing, and droplets transmitted on hands and tissues4. When we get sick with the flu the immune system’s antibodies are fighting the virus. Usually, the immune system can clear the virus, leaving no trace of it in the body, in about two weeks. However, when the immune system is weakened, it can’t control the virus; it can result in flu-related complications. So, a weak immune response is a reason for complications, not the flu virus itself5

Underlying chronic conditions can reduce the resistance to infection making the immune response less effective6Some people are more at risk for developing complications from flu because of underlying health conditions, making them more vulnerable to flu. The term vulnerability is defined as increased exposure to infection; increased susceptibility to severe disease, including complications, hospitalizations, and death; and lack of access to health care7.  

According to the World Health Organization (WHO), the following people are at risk: 

  • Pregnant women at any stage of pregnancy 
  • Children younger than 5 years 
  • Older people 
  • People with chronic medical conditions such as HIV/AIDS, asthma, heart and lung diseases and diabetes 
  • Health and care workers8.  

Life-threatening complications 

Flu can make chronic medical problems worse. For example, people with asthma may experience asthma attacks while they have flu, and people with chronic heart disease may experience a worsening of their heart condition9.  

The most common complication is pneumonia, an infection that inflames the lungs’ air sacs. The air sacs may fill up with fluid or pus, causing symptoms such as a cough, fever, chills, and trouble breathing10.  

Flu can also lead to:  

  • Inflammation of the airways to the lungs, causing cough (bronchitis); 
  • Inflammation of the heart muscle that reduces the heart’s capability to pump and cause rapid or abnormal heart rhythms (e.g. myocarditis); 
  • The brain’s tissues can get inflamed causing the brain to swell, which can lead to headache, stiff neck, sensitivity to light, mental confusion and seizures* (e.g. encephalitis); 
  • Multi-organ failure, for example respiratory and kidney failure; 
  • Inflammation of muscles (e.g. myositis or rhabdomyolysis)11

Below are the most common complications for each of the risk groups that the #TogetherAgainstFlu campaign currently focuses on: 

Risk group Complications Facts 
Older adults   serious sequelae, e.g. a long-term effect of a temporary disease; mortality12Adults aged 65 years or older account for 50%-70% of all flu-related hospitalizations and 85% flu-related deaths in the US13
People with diabetes (type 1, type 2, or gestational) worsening of diabetes (the body is producing additional glucose to fight the virus and that leads to higher blood sugar levels, which can be dangerous); chest infections which may develop into pneumonia; inflammation of the tonsils; inflammation of the protective membranes covering the brain and spinal cord; inflammation of the active tissues of the brain14In recent seasons, about 30% of adults hospitalized with flu reported to CDC had diabetes15.     
People with heart disease**  increased risk of stroke and heart attacks (the flu stresses the heart and vascular system and that can be overwhelming for an already weakened heart muscle). During recent flu seasons, heart disease was one of the most common chronic (long-term) conditions—about 50% of adults hospitalized with flu had heart disease16.  
People with lung disease***  pneumonia; other acute respiratory diseases17Among adults hospitalized with flu, 46% had respiratory diseases, such as asthma18

Flu can be seriously dangerous for a large part of the population.

In addition, the flu may interrupt necessary medical routine visits, vital medication refills19, tests, or monitoring, for example for older people, pregnant women, or people with diabetes, which can have devastating consequences. 

“While we start to understand more and more about the role of insulin, glucose variability and other characteristics of persons living with diabetes in the role of influenza and covid-19 Pathogenesis, vaccine coverage rates in this risk group is still below the target. We are in urgent need for a multidisciplinary translational approach to better understand why and how to protect these people.” 

Dr Marco Goeijenbier
Chair – Influenza Diabetes Community (IDC)

The flu vaccine is important  

The flu can be prevented, and health organizations such as the European Centre for Disease Prevention and Control (ECDC) highlight the importance of flu vaccination to reduce the risk of complications20. During recent flu seasons, 9 out of 10 people hospitalized with flu had at least one underlying health condition and therefore getting the flu vaccine each year is especially important for people with chronic health conditions21

“Incorporating a practice as simple as influenza vaccination can considerably reduce the risk of cardiovascular events in patients with pre-existing CV disease and in high-risk groups.” 

Dr Álvaro Sosa Liprandi
President – InterAmerican Society of Cardiology
School of Medicine, University of Buenos Aires

During the 2019-2020 season, flu vaccination prevented an estimated 7.52 million illnesses, 3.69 million medical visits, 105 thousand hospitalizations, and 6,300 deaths due to flu in the US alone22

With COVID-19 also circulating this season, it is especially important to stay protected against the flu to avoid getting infected by both the flu and COVID-19.

“The issue is that today, after the COVID-19 pandemic, nothing has changed. Health care workers are still not included in influenza programs that would contribute to flu vaccine uptake.”

Ber Oomen
Interim Executive Director – European Specialist Nurses Organisation (ESNO)

Even if co-infection is rare, data from the UK suggest that people with both diseases are more than twice as likely to die as those who only have COVID-1923. New studies show that it is safe to take the flu vaccine and COVID-19 vaccine at the same time24.  

For risk groups, getting the flu vaccine can help people stay independent, reduce illnesses, and avoid severe complications. It can minimize absenteeism from work for health care workers and help avoid spreading the flu to vulnerable patients25. What is more, vaccinated health and care workers can influence others to get vaccinated by setting an example and proving the importance of flu vaccination for the community.  

1 World Health Organization: How can I avoid getting the flu?
2 Centers for Disease Control and Prevention: Flu symptoms and complications
3 World Health Organization: Global Influenza Strategy 2019-2030. Prevent. Control, Prepare.
4 European Centre for Disease Prevention and Control: Factshttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC4504371/heet about seasonal influenza.
5 Science Daily. Immune response to influenza. 2019.
6 European Centre for Disease Prevention and Control: Factsheet about seasonal influenza.
7 US National Library of Medicine National Institutes of Health. Protecting Vulnerable Populations From
Pandemic Influenza in the United States: A Strategic Imperative. 2009.

8 World Health Organization: Influenza (Seasonal)
9 Centers for Disease Control and Prevention: Flu Symptoms & Complications
10 American Lung Association. Pneumonia Symptoms and Diagnosis.
11 Centers for Disease Control and Prevention: Flu Symptoms & Complications
12 Medscape. Preventing flu in older adults. 2021.
13 Medscape. Preventing flu in older adults. 2021.
14 Centers for Disease Control and Prevention. Flu and people with diabetes. Flu and people with diabetes.
15 Centers for Disease Control and Prevention.
16 Centers for Disease Control and Prevention. Flu & People with Heart Disease or History of Stroke
17 Centers for Disease Control and Prevention. Flu & People with asthma.
18 US National Library of Medicine National Institutes of Health. The disease burden of influenza beyond
respiratory illness. 2021.

19 US National Library of Medicine National Institutes of Health. Protecting Vulnerable Populations From
Pandemic Influenza in the United States: A Strategic Imperative. 2009
20 European Centre for Disease Prevention and Control. Guidance. Priority risk groups for influenza
vaccination. 2008.

21 Centers for Disease Control and Prevention. A Chronic Health Condition Can Increase Your Risk.
22 Centers for Disease Control and Prevention. Estimated Influenza Illnesses, Medical visits, and
Hospitalizations Averted by Vaccination in the United States — 2019–2020 Influenza Season.

23 International Federation of Pharmaceutical Manufacturers and Associations, and The Health Policy
Partnership. Influenza vaccination during the COVID-19 pandemic. 2021.

24 World Health Organization. Science in 5, Episode #59 – Flu & COVID-19
25 World Health Organization. WHO SAGE Seasonal Influenza Vaccination
Recommendations during the COVID-19 Pandemic Interim guidance. 2020.

26 Johns Hopkins Medicine. Types of Seizures.
27 Centers for Disease Control and Prevention. Heart Disease.
28 WebMd. Lung Diseases Overview.

*A seizure is a burst of uncontrolled electrical activity between brain cells that causes temporary abnormalities in muscle tone or movements (stiffness, twitching, or limpness), behaviors, sensations, or states of awareness. Seizures are not all alike26.

** The term ‘heart disease’ refers to several types of heart conditions. In the US, the most common type of heart disease is coronary artery disease (CAD)27. 

*** ‘Lung disease’ refers to several types of lung conditions, for example, asthma and Chronic obstructive pulmonary disease (COPD)28