How COVID-19 Is Transmitted and Who Is Most at Risk

Risk factors indicate who could face a more severe course of infection

As the virus (SARS-CoV-2) that causes COVID-19 continues to become better understood, several transmission routes and risk factors have been identified. These insights have been summarized by Ryan Miller, DO, and Kristin Englund, MD, in a short review included in Cleveland Clinic Journal of Medicine’s collection of COVID-19 Curbside Consults.

Advertising Policy

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services Policy

Transmission of the virus occurs via respiratory secretions and saliva. Droplets are expelled through the mouth or nose and can directly land on the mucous membranes (mouth, nose, eyes) of another person. Indirect transmission is also possible either from droplets landing on the skin of another person who then touches his or her mucous membranes or from touching a surface that has been infected. The virus can live on surfaces for one to several hours, though the viability of the virus depends on the surface material. For that reason, surfaces that are touched frequently should be cleaned with antiviral solutions at least every 24 hours.

Based on the Diamond Princess cruise ship COVID-19 outbreak, the number of cases an infected person will cause, also known as reproductive number (R0), appears to be 2.28, which is much smaller than the R0 of measles (12 to 18) but greater than influenza (0.9 to 2.1).

Asymptomatic shedding of the virus does occur and may account for up to 60% of the cases. Viral shedding can occur up to 5 days before symptoms appear.

Advertising Policy

Several risk factors have been identified as being associated with a more severe course of COVID-19. These include:

  • Age (65 or older, living in a nursing home or extended care facility)
  • Respiratory issues (chronic lung disease, moderate-to-severe asthma, history of tobacco use)
  • Heart conditions
  • Drug treatments (chemotherapy, long-term use of steroids or an immunosuppressant)
  • Liver disease
  • Diabetes
  • BMI ≥ 40
  • Poorly controlled HIV/AIDS or other immune deficiencies
  • History of bone marrow or organ transplant

In addition to outlining the above evidence on transmission and risk factors for COVID-19, Drs. Miller and Englund review procedures during which aerosolization of the virus can occur and associated recommendations for the use of personal protective equipment. They also discuss best practices for social distancing and the use of cloth masks to contain community viral spread.

The full article, with references, from Cleveland Clinic Journal of Medicine’s COVID-19 Curbside Consults is available here.