H3N2 2015

To H3N2 or not to H3N2? That Is the Question

Teresa Wise, PharmD Intern and Jennifer Seltzer, PharmD

February 2015

The 2014-15 flu season has been severe so far, according to early data collected by the Centers for Disease Control and Prevention.1  Since the start of the flu season on September 28, 2014, hospitalization rates for people over 65 years are already the highest they have been since monitoring of the data began.   As of the third week in January, there have been 61 pediatric deaths, including 8 in Texas.2, 3 Due to an early onset, the season may also last longer than the average 13 weeks.

The influenza vaccine protects against three, sometimes four, strains of the virus.  Both the trivalent and the quadrivalent formulations will always cover a strain from H1N1, H3N2, and influenza B.  The quadrivalent includes a second influenza B strain.  The World Health Organization, in collaboration with other worldwide health organizations, identifies which strains to include in the annual vaccine in February of each year based on data obtained from participating labs worldwide.  A new vaccine is required each year because the strains will mutate, sometimes faster than others.4 The most widely used vaccine is a product made with eggs.

The process of manufacturing the vaccine is fairly simple, but time consuming.  A previously identified strain of the virus is injected into a fertilized chicken egg where it will replicate.  After only a few days the egg will contain millions of the virus.  They are then extracted and purified to be ready for injection. The process can take four to five months to complete.  Due to this lengthy processing time, the vaccine cannot be reformulated quickly for same year distribution when new strains become predominant.4

The most common strain currently identified in the United States has been influenza A H3N2, a strain that has been prevalent in previous years as well, and included in the vaccine.2  Unfortunately, roughly half of the H3N2 viruses analyzed to date have been variants, or viruses that have mutated, making this year’s vaccine less efficacious.5  Early estimates have demonstrated  effectiveness rates for a the current vaccine of only 23% (95% confidence interval, 8%-36%) based on laboratory confirmed cases.  More than two thirds of the H3N2 strains tested were genetically different then that covered in the vaccine.5

The CDC stresses that even with reduced efficacy, there is still benefit for vaccination.  The current vaccine offers protection from other circulating strains, and reduces the severity and complications associated with the illness.5  Individuals in high-risk categories should always obtain a vaccine   (e.g., children < 5 years of age, adults > 65 years, pregnant women, residents of long-term care faculties, immunocompromised, those with increased risk of complications due to concomitant disease states such as asthma).

Additional measures should be taken to help prevent viral spread including prompt treatment for those that do become ill as well as preventative measures to avoid becoming ill.1  The anti-viral medications oseltamivir and zanamivir, both neuraminidase inhibitors, can be effective in reducing flu duration.  A new medication was recently approved by the FDA, paramivir, for treatment in adults as well.2   Few cases of resistance to these antiviral medications have been reported.    All three work best if initiated within 24 hours of onset.  To prevent the spread of the flu, individuals should stay home for at least 24 hours after the fever has resolved, cover a cough or sneeze with a tissue and exercise good hand hygiene.6


  1. Centers for Disease Control and Prevention.  Early data suggests potentially severe flu season.  Available at:  http://www.cdc.gov/media/releases/2014/p1204-flu-season.html.  Accessed on January 30, 2015.
  2. Centers for Disease Control and Prevention.  Situation update: summary of weekly flu view. Available at:   http://www.cdc.gov/flu/weekly/summary.htm.  Accessed on January 30, 2015.
  3. Texas Department of State Health Services.  Infectious disease Control: Influenza (Flu) 2014-2015. Texas influenza surveillance activity report.  Available at: http://www.dshs.state.tx.us/idcu/disease/influenza/surveillance/2015/.   Accessed on January 30, 2015.
  4. Zhang S. Why the flu vaccine doesn’t always work. January 02, 2015. Available at: http://gizmodo.com/why-the-flu-vaccine-doesnt-always-work-1677122912.  Accessed on January 30, 2015.
  5. Centers for Disease Control and Prevention.  Morbidity and mortality weekly report (MMWR): Early estimates of seasonal influenza vaccine effectiveness – United States, January 2015. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6401a4.htm?s_cid=mm6401a4_w.   Accessed on January 30, 2015.
  6. PL DetailDocument, Influenza vaccines for 20142015. Pharmacists Letter/Prescribers Letter. December 2014.