What are the risk factors for severe illness with common respiratory viruses such as influenza and RSV?

27 Jul 2023

What the research says

Influenza (Kalil et al 2019; Vitoratou et al 2023; Zhao et al 2020))

  • Risk factors for severe illness associated with influenza infection include pregnancy and extreme obesity
  • Risk factors for the development of pneumonia associated with influenza infection include:
    • Younger age (<5 years)
    • Older age (>65 years)
    • Nursing home residency
    • Chronic lung or heart disease
    • History of smoking
    • Immunocompromised state

Respiratory syncytial virus (RSV)

  • Adults at highest risk for severe RSV include (CDC 2023a):
    • Older adults (>65 years)
    • Those with chronic heart of lung disease
    • Those with weakened immune systems
  • Infants and young children at highest risk for severe RSV include: (CDC 2023b)
    • Premature infants and those <6 months of age
    • Children <2 years with chronic lung disease or congenital heart disease
    • Children with weakened immune systems
    • Children with neuromuscular disorders are at increased risk for PCS (Tsampasian et al 2023).

What this means for your clinical practice

  • Continue to encourage patients, especially those at increased risk for more severe illness from influenza infection, to consider annual influenza vaccinations as recommended in your National Guidelines.
  • There are newly approved vaccines against RSV for use in adults (but not children) and more approved vaccines are expected in the coming months.
  • Maintain particular vigilance for individuals with multiple risk factors and promote preventive measures to reduce the risk for infection during endemic seasons.

Resource information

Respiratory conditions
  • COVID-19
Type of resource
IPCRG COVID-19 and respiratory Q&A


With grateful thanks to Tiago Maricoto, MD, PhD (Family Doctor, Beira Ria Family Health Unit, Aviero and University of Beira Interior, UBIAir – Clinical & Experimental Lung Centre, UBIMedical; CICS-UBI – Health Sciences Research Centre, Covilhã, Portugal) for and on behalf of the IPCRG practice driven answers review group.

Useful links and supporting references:

Centers for Disease Control and Prevention. Respiratory Syncytial Virus (RSV). Older Adults. 2023a. Available at: https://www.cdc.gov/rsv/high-risk/older-adults.html. Accessed May 2023.

Centers for Disease Control and Prevention. Respiratory Syncytial Virus (RSV). Infants & Young Children. 2023b. Available at: https://www.cdc.gov/rsv/high-risk/infants-young-children.html. Accessed May 2023.

Kalil AC, Thomas PG. Influenza virus-related critical illness: pathophysiology and epidemiology. Crit Care 2019;23:258. Available at: https://pubmed.ncbi.nlm.nih.gov/31324202/.  Accessed May 2023.

Vitoratou DI, et al. Obesity as a risk factor for severe influenza infection in children and adolescents: a systematic review and meta-analysis. Eur J Pediatrics 2023;182:363–374. Available at: https://pubmed.ncbi.nlm.nih.gov/36369400/. Accessed May 2023.

Zhao X, et al. Obesity increases the severity and mortality of influenza and COVID-19: A systematic review and meta-analysis. Front Endocrinol 2020;11:595109. Available at: https://pubmed.ncbi.nlm.nih.gov/33408692/. Accessed May 2023.