The Centers for Disease Control and Prevention (CDC) recently announced that so far this year, the predominant strain of influenza causing disease is a drifted strain of the influenza A virus H3N2. As of early December, more than 91 percent of CDC-tested samples were influenza A strains and the balance were influenza B. Here is what you should know:

  • Historically, H3N2-dominated influenza seasons have been more severe, causing higher rates of hospitalizations and deaths.
  • H3N2-dominated seasons tend to be particularly severe for those groups at highest risk from influenza, including children, elderly adults, pregnant women and those with chronic conditions, such as heart- and lung-associated diseases.
  • Although influenza vaccines contain an H3N2 strain of the virus, the circulating strain has drifted, such that to date, about 68 percent of H3N2 samples have been significantly different from the A/Texas/50/2012 strain included in the vaccine.
  • The drifted strain is classified as A/Switzerland/9715293/2013. When the strain suggestions were made for the Northern Hemisphere, this strain was not often detected. However, increasing numbers of specimens caused the World Health Organization (WHO) to recommend its inclusion in the 2015 Southern Hemisphere vaccines.
  • Although current influenza vaccines do not afford significant protection against the drifted strain, they contain two or three other strains and are the best opportunity to prevent influenza infections, so should still be encouraged.
  • Additionally, patients, especially those in high-risk groups, should be encouraged to seek treatment early for influenza-like illness since anti-virals can decrease the severity of disease if started early in infection.
  • By the beginning of December, seven pediatric deaths have been attributed to influenza.

Resources related to this situation

Resources related to influenza in general

Check these resources for general information related to influenza:

  • Influenza: What You Should Know from the Vaccine Education Center at CHOP; available in English [PDF, 188KB] and Spanish [PDF, 193KB].
  • New influenza-related PSA targeting college students from Families Fighting Flu
  • – influenza information for the public

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.