Seasonal influenza


Seasonal influenza summary1

Pathogenic agent

Influenza A virus
Influenza B virus




Breathing airborne (up to 1 meter or approximately 3 feet) virus-containing droplets dispersed by coughs and sneezes of infected people; virus-contaminated hand contact


  • Seasonal epidemics each winter in temperate climates; year-round in tropical climates
  • Annual epidemics cause an estimated 3 to 5 million cases of severe illness worldwide
  • Up to 500,000 influenza-related deaths per annum
  • Higher risk globally in pregnant women, children under 5 years, elderly, people with chronic conditions, healthcare workers

Influenza, commonly known as “the flu,” is an infectious respiratory disease caused by either the influenza A virus or the influenza B virus.2,3 Seasonal influenza outbreaks occur every year in the winter months in both the northern and southern hemispheres.2

In an average year, influenza will infect about 1 billion people worldwide, causing 3-5 million cases of severe illness and up to 500,000 deaths.2,3 In the US alone, almost 1 in 10 people experiences symptoms of influenza infection each year.4

18,000–96,000 infected people are admitted annually in the US to an intensive care unit (ICU) due to influenza,5 with costs to the healthcare system and society estimated at $11.2 billion per year in 2015.6

What characterizes seasonal influenza?

Influenza viruses travel up to 1 meter (approximately 3 feet) through the air in droplets produced by infected people when they cough, sneeze, or talk.1,7 Symptoms can appear suddenly and can range in severity from person to person.3,4,8 Common symptoms include fever, cough, sore throat, runny nose, body aches, and headache,3 but some people experience potentially lethal complications, including pneumonia, bronchitis, or worsening of chronic conditions, such as asthma, diabetes, and congestive heart failure.3,9

Anyone can get influenza, but very young and elderly people, as well as pregnant women and people with chronic conditions, are most vulnerable to severe influenza infection and death.3,8

Though it is clear who is in greatest need, protection can be challenging to provide because variants of the viruses appear frequently, and can often evade the body’s natural immune surveillance.3 It is these variants of influenza that are responsible for annual seasonal outbreaks (epidemics) and pandemics.3

Today, antiviral drugs (eg, oseltamivir and zanamivir) are available for the treatment of influenza in high-risk cases.9,10 For the majority of people, however, vaccination is the most effective way to help reduce the risk of serious illness and complications.10,11

Due to the frequent appearance of variants, scientists closely monitor the circulating influenza strains and modify the vaccine each year according to predicted variants.3,11–13 International influenza surveillance systems, such as the World Health Organization’s (WHO) Global Influenza Surveillance and Response System (GISRS)12 and the European Centre for Disease Control’s (ECDC) European Influenza Surveillance Network (EISN),14 keep track of the evolution of new strains, share data, and help to make sure that the world is prepared for the next influenza season.

Vaccination is proven to help reduce illness, especially severe illness in the elderly, as well as ICU admissions and deaths.4 Despite the benefits of vaccination, however, coverage remains low; it ranges from 1% to over 75% in older people and less than 40% among people with chronic diseases.11

Seasonal influenza and COVID-19

During the ongoing COVID-19 pandemic, the influenza season in the northern hemisphere did not last as long as it usually would, and infection rates were much lower than normal in the southern hemisphere.14 This could be potentially explained by enforced social distancing and enhanced hygiene measures that were introduced in response to COVID-19, or by higher numbers of people seeking influenza vaccinations.14 There are clearly some similarities between these two pandemic-causing respiratory viruses, but it is not known if or how the two viruses could converge to put pressure on healthcare systems during typical seasonal peaks.

Future of combination vaccination

Because of the evident long-term, global need for vaccines against influenza and SARS-CoV-2, one way to encourage regular and efficient vaccination for both diseases could be to provide immunizations together, either as two shots in a single visit or as a single combination vaccine. This approach may reduce healthcare staffing costs and increase compliance with vaccination schedules due to improved convenience. Studies need to be done to evaluate the safety and efficacy of combination vaccination.

  1. World Health Organization. Influenza (seasonal). Available at: [Accessed 27 Aug 2021].
  2. Krammer F, et al. Influenza. Nat Rev Dis Primers. 2018;4(1):3.
  3. Ghebrehewet S, et al. Influenza. BMJ. 2016;355:i6258.
  4. Chow EJ, et al. Influenza virus-related critical illness: prevention, diagnosis, treatment. Crit Care. 2019;23:214.
  5. Reed C, et al. Estimating influenza disease burden from population-based surveillance data in the United States. PLoS One. 2015;10(3):e0118369.
  6. Putri WCWS, et al. Economic burden of seasonal influenza in the United States. Vaccine. 2018;36(27):3960–3966.
  7. Stelzer-Braid S, et al. Exhalation of respiratory viruses by breathing, coughing, and talking. J Med Virol. 2009;81(9):1674–1679.
  8. Centers for Disease Control and Prevention (CDC). Key Facts About Influenza (Flu). Available at: [Accessed 27 Aug 2021].
  9. Nicholson KG, et al. Influenza. Lancet. 2003;362:1733–1745.
  10. Webster RG, Govorkova EA. Continuing challenges in influenza. Ann N Y Acad Sci. 2014;1323:115–139.
  11. World Health Organization. Influenza Vaccination Coverage and Effectiveness. Available at: [Accessed 27 Aug 2021].
  12. World Health Organization (WHO). Global Influenza Surveillance and Response System (GISRS). Available at: [Accessed 27 Aug 2021].
  13. European Centre for Disease Prevention and Control (ECDC). European Influenza Surveillance Network (EISN). Available at: [Accessed 27 Aug 2021].
  14. Jones N. How COVID-19 is changing the cold and flu season. Nature. 2020;588:388–390.