Are We Going to Have a Severe Influenza Season?

Early testing of influenza samples by the Centers for Disease Control and Prevention (CDC) has shown that one of the most common strains circulating this season is influenza A strain H3N2. Additionally, this H3N2 strain has changed enough that the version included in the vaccine is no longer a good match. So, what does this mean for you and your family?

H3N2-dominated influenza seasons tend to be more severe

• When H3N2 strains of influenza are the most common strains infecting people, the influenza season tends to be more severe, meaning that more people are hospitalized and die from influenza infections and related complications.
• By the first week of December 2014, seven children have died from influenza already this season.
• Influenza season is just beginning; the number of cases continues to rise weekly.

Poor match of circulating strain and vaccine strain

• This year’s influenza vaccines contain three or four strains of influenza virus depending on which one you get. One of the strains included in all available vaccines, H3N2, is not well-matched to the version that is infecting people. The remaining two or three strains are well-matched.
• The H3N2 strain that is commonly infecting people so far this winter is an example of what is called “genetic drift.” Genetic drift means that as the virus reproduced itself in people, it accumulated changes that made it different enough from the strain in the vaccine that the antibodies produced by vaccination are less likely to recognize it and stop infection.
• Because influenza vaccine strains are determined early in the calendar year (around February), the World Health Organization (WHO) and the CDC must make the decision about which strains to include in the vaccine given in the fall based on the information they have at the time about which strains of influenza are infecting people. This year, as sometimes happens, a different H3N2 strain became common after the decision was made leading to the mismatch that we’re dealing with now.

What should I do to protect my family?

• Even with the mismatched H3N2 strain, the influenza vaccine is still the best way to protect yourself and your family from influenza.
  • Two or three other strains are well matched
  • The mismatched strain may still offer some protection leading to decreases in severity or length of illness
• Encourage your family to practice good hygiene:
  • Cover coughs and sneezes
  • Stay home when ill
  • Stay away from others who are ill
  • Wash hands frequently
• Be sure that high-risk members of your family have been vaccinated and stay away from others who are ill with influenza-like symptoms. High-risk groups include:
  • Children younger than 2 years of age
  • Adults 65 years and older
  • People with chronic conditions of the lungs, including asthma
  • People with chronic conditions of the heart, kidneys, liver, or blood, such as sickle cell disease
  • People with diabetes or other metabolic disorders
  • People with neurologic conditions, seizure disorders, or moderate to severe developmental or intellectual delay
  • People with immune-compromising conditions, including those on immunosuppressive medications and those with HIV
  • Pregnant women and those who recently delivered infants (within 2 weeks of delivery)
  • Persons younger than 19 years of age receiving long-term aspirin therapy
  • American Indians/Alaskan Natives
  • Persons considered morbidly obese (BMI greater than or equal to 40)
  • Residents of nursing homes and other chronic care facilities
• If you or a family member, particularly those in high-risk groups, experience influenza-like symptoms, such as fever, chills, muscle aches, headache, runny nose or cough, visit your healthcare provider early because antiviral medications are most effective when started within 48 hours of illness onset.

Additional resources

For additional information about influenza and influenza vaccines, check these resources:

• Influenza: What You Should Know – offered by the Vaccine Education Center at The Children's Hospital of Philadelphia
• CDC’s influenza website
• Check the levels of influenza in your state

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