2009 H1N1 Influenza -- Just the Facts: Vaccine Essentials

John G. Bartlett, MD


November 23, 2009

In This Article

Seasonal Flu Vaccine

This vaccine is now available and should be promoted by physicians "early and intensely" according to Dr. William Shaffner of Vanderbilt University, Nashville, Tennessee. (Americans urged to get seasonal flu vaccine early. Medical News Today. September 11, 2009.) Efficacy in preventing seasonal flu is reported to be 50%-70% for children 6 months to 8 years of age and 70%-90% in healthy adults under 65 years. (Fiore AE, Shay DK, Broder K, et al; CDC. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR Recomm Rep. 2009;58[RR-8]:1-52.) Some states view the seasonal flu vaccine distribution as a dry run for the 2009 H1N1 vaccine. CVS Caremark announced that it will make seasonal influenza available at 500 walk-in Minute Clinics and some drugstores. The plan includes providing free vaccinations to 100,000 unemployed people.

Seasonal Flu Vaccine Supply Update

  • 82 million doses (71% of anticipated supply) have been distributed as of October 9, 2009.

  • The total supply expected is 114 million doses, including 50 million that are thimerosal-free and adjuvant-free. Most of the supply should be available by the end of October.

  • Some areas are experiencing seasonal flu vaccine shortages. This is due to: (1) delayed delivery by Sanofi, the largest supplier; (2) more providers purchasing vaccine early due to increased demand; and (3) the desire to complete seasonal flu vaccination before 2009 H1N1 vaccination.

On October 15, 2009, a news report indicated that the availability of the seasonal flu vaccine has become problematic for some due to a supply shortage attributed to heavy demand and manufacturer delays. The limited supply has prompted CVS Caremark Corporation to report that it will end seasonal flu shot clinics on October 22, 2009 instead of the end of November as originally planned. Rite Aid Corporation also reported delays, but the Walgreen Company claimed no problems. (Klayman B. CVS cuts short flu clinics due to shortage. Reuters. October 15, 2009.)

Severe shortages of seasonal flu vaccine are attributed to inadequate supply and early demand resulting from pandemic .The CDC estimates that 85 million Americans have received the seasonal flu vaccine compared with 61 million at this calendar point last year. In the 2008-2009 flu season, the total number of doses available was 113 million and the total taken was 103 million. The current shortage is attributed in part to the commitment of vaccine suppliers to produce about 200 million doses of 2009 H1N1 vaccine for the United States. (McNeil DG. Nation is facing vaccine shortage for seasonal flu. New York Times. November 4, 2009. Available at: http://www.nytimes.com/2009/11/05/health/05flu.html Accessed November 10, 2009.)

Does the Seasonal Flu Vaccine Protect Against H1N1?

Analysis of the pandemic influenza strain with strains in the H1N1 vaccine show only 72%-73% amino acid congruence with hemagglutinin; by comparison, the test for seasonal flu shows 97%-98% sequence identity. The CDC concluded that the seasonal flu vaccine will not provide any protection for the pandemic H1N1 influenza strain. This means that 2 entirely different influenza vaccines are required for the 2009-2010 influenza season, the standard seasonal influenza vaccine and a second for pandemic H1N1. (CDC. Serum cross-reactive antibody response to a novel influenza A (H1N1) virus after vaccination with seasonal influenza vaccine. MMWR Morb Mortal Wkly Rep. 2009;58:521-524.)

Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. This CDC study reported cross-reactive antibodies to 2009 H1N1 in persons who were blood donors or vaccinated with recent seasonal or 1976 swine influenza vaccines. (Hancock K, Veguilla V, Lu X, et al. Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med. 2009 Sep 10. [Epub ahead of print].)

Pre-existing cross-reaction:

  • Persons born after 1980, 4 of 107 (4%);

  • Persons born before 1980, 39 of 115 (34%); and

  • Vaccinated for A/NJ/76 45 of 83 (54%).

Four-fold increase after trivalent flu vaccine:

  • Age 6 months to 9 years, 0;

  • Age 18-60 years (n = 231), 12%-22%; and

  • Age > 60 years (n = 113), ≤ 5%.

The conclusion is that recent seasonal flu vaccines produce little or no cross-reactive antibody response, and persons under 30 years of age have little evidence of serologic protection to 2009 H1N1. Vaccination in 1976 substantially boosted cross-reactive antibodies to the 2009 H1N1 virus.

Testing of 795 clinical isolates of 2009 H1N1 virus showed that 1 (0.1%) had reduced titers with antisera directed at the vaccine strain A/California/07/2009 (H1N1). The remaining 99.9% are related to the A/California/07/2009 (H1N1) virus. Antigenic characterization of the 2009 H1N1 vaccine strain shows that this virus is antigenically and genetically unrelated to the seasonal influenza (H1N1) viruses. This suggests little or no cross protection. (CDC. FluView. Antigenic characterization. 2008-2009 Influenza Season Week 38 ending September 26, 2009. Available at: http://www.cdc.gov/flu/weekly/ Accessed October 5, 2009.)


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