Is the 2012 Flu Vaccine Useful?

Motivation: The flu is rampant.  Emergency rooms are full.  And, what about the vaccine?  Did the vaccine miss the important strains or did most patients avoid the vaccine?  Speculation on this topic is common, and the answer is important for public health preparation for the year ahead.

As way of background, this year's vaccine consists of three strains:

• H1N1 virus: Type A influenza (A/California/7/2009)
• H3N2 virus: Type A influenza (A/Victoria/361/2011)
• Type B influenza strain: B/Wisconsin/1/2010 from B/Yamagata strain of viruses

Methods: Weekly Influenza Surveillance Report by CDC available at http://www.cdc.gov/flu/weekly.

Results:

Flu Positive Tests: In the week from December 30 to January 5, 12,876 samples were tested by collaborating laboratories, and 4,222 (32.8%) tested positive for influenza. 

Influenza Strains:  Between December 10 to January 5, the distribution of viral strains as tested in laboratories is as follows:

- 7340 samples (49.77%) of H3 strains of Influenza A

- 4328 samples (29.35%) of Influenza A (subtyping not performed)

- 2959 samples (20.07%) of Influenza B

- 120 samples (0.81%) of H1N1 strain of Influenza A

Detailed Antigenic Subtyping: Since October 2012, 521 influenza viruses have been characterized:

- 17 strains of 2009 H1N1 (included in vaccine)

- Of 327 strains of H3N2, 325 strains (99.4%) were A/Victoria subtype included in vaccine.  2 strains were of different subtype.

- Of 177 Influenza B subtypes tested, 118 strains (66.7%) were of the B/Wisconsin type included in vaccine.  59 additional strains (33.3%) were not included in the vaccine.

Neuraminidase Inhibitor Resistance: All tested strains in the US were susceptible to oseltamivir and zanamivir.  

Discussion: So far this season, the majority of virus strains are of the Influenza type A variety.  At least with the limited strain subtyping, most of the strains that are circulating were included in the year's vaccine.  From the surveillance data, the cause for this year's widespread flu infections is either lack of adequate vaccination or lack of efficacy of the vaccine.  Alternatively, the explanation could be a mix of the two.  In randomized trials, the efficacy of vaccines is about 50-70% with lower efficacy in people with high-risk medical comorbidities.  With a large unvaccinated pool and many elderly people, the disease could be propagated by both the unvaccinated and the elderly with failed efficacy.  In the next year, focus should be on higher levels of vaccination within the population.