Flu Shots & Your INR

By: Alere Staff

The flu virus or influenza, impacts the very young and patients 65 or older, those with pre-existing heart or lung condition, long term health condition or those coming in frequent contact with other high risk patients such as healthcare workers. The viruses that typically cause the flu are primarily categorized as influenza type A or type B. Since type A can mutate rapidly, a new form of the flu vaccine must be developed each year to protect people against the exact strains that are expected.

Since flu outbreaks typically occur during winter months, the "flu shots" or flu vaccine is usually offered during the fall and winter of any given year. According to the Centers for Disease Control and Prevention, the best way to prevent the flu is by getting a flu vaccine every year.1

The flu shot contains killed or inactive viruses, so it is not possible to get the flu from this type of vaccine; however, you may experience flu-like symptoms.1

Interactions with warfarin?

The impact of the flu vaccine on a patient's International Normalized Ratio (INR) level is becoming better understood. Each year the strain of the vaccine changes, so accurate measure of the impact of the flu vaccine is difficult. According to one study published in the Archives of Internal Medicine, it is non-conclusive on whether flu shots have an impact on patient INRs.2

Although, a case-controlled Italian study, presented in the peer-reviewed Haematologica Journel, proved somewhat quite different results. Following the vaccine, 49 out of 90 patients experienced an increase in their INR, before averaging 2.64, going to an average of 3.85 following the vaccine in this study. The remaining 41 patients showed no change in their INR following the vaccine.3

More frequent INR testing may help determine if the flu vaccine impacts your INR values. Before getting a flu vaccine, check with your doctor.

  1. Centers for Disease Control and Prevention. Key Facts about Influenza (Flu) & Flu Vaccine. October 2011.   
  2. Guna Raj, MD; Raminder Kumar, MD; W. Paul McKinney, MD. Safety of Intramuscular Influenza Immunization Among Patients Receiving Long-term Warfarin Anticoagulation Therapy. Arch Intern Med. 1995;155(14):1529-1531.
  3. Gresele, P, et. al. Significant potentiation of anticoagulation by flu-vaccine during the season 2001-2002. Haematologica. 2003. 88;599-600.