2009 H1N1 Influenza -- Just the Facts: Vaccine Essentials

John G. Bartlett, MD


November 23, 2009

In This Article

Allocation Priorities: HCWs and Influenza Vaccines

In the event of inadequate supply, SHEA provides guidance for allocation of vaccine to HCWs. (This is unlikely to be an issue with the seasonal flu vaccine, but it is anticipated for 2009 influenza A H1N1):

  1. Nature, degree, and duration of contact: close contact defined as within 3 ft, prolonged is "several minutes" and repeated contact;

  2. Degree of contact with patients at high risk for complications;

  3. Need to protect essential HCWs and services;

  4. Exposure in areas of high patient traffic, such as emergency departments and high-volume clinics; and

  5. Areas where HCWs have exposure to patients before they are placed on precautions, including first responders, acute care clinic employees, and emergency department staff. (Talbot TR, Bradley SE, Cosgrove SE, Ruef C, Siegel JD, Weber DJ. Influenza vaccination of healthcare workers and vaccine allocation for healthcare workers during vaccine shortages. Infect Control Hosp Epidemiol. 2005;26:882-890.)

Vaccine Response

Anthony S. Fauci, Director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, reported very promising, early 2009 H1N1 vaccine trial data. The vaccine was well tolerated and induced a "strong immunologic response" when a single, unadjuvanted 15-µg dose was given to healthy adults. The trial data with vaccine from sanofi-aventis showed a response rate of 96% in adults ages 18-64 years and 56% in persons older than 65 years. CSL Biotherapeutics' vaccine showed response rates of 80% in adults ages 18-64 years and 60% in persons older than 65 years. It was acknowledged that vaccine-response rates are expected to be lower in persons older than 65 years. (CNN News. Study: single dose of H1N1 flu vaccine may suffice for adults. September 11, 2009.)

Antigenic response to 2009 H1N1 vaccine -- preliminary report, October 9, 2009. The report is based on a trial from CSL Biotherapeutics that used a split virus vaccine in doses of 15 µg or 30 µg in 240 participants aged 18-64 years. The results at 21 days showed antibody titers of > 1:40 in 116/120 (97%) of those given the 15 µg dose and 112/120 (93%) of those given the 30 µg dose. Solicited adverse effects included systemic side effects in 30% (headaches, malaise and myalgia) which subjects attributed to the vaccine. (Greenberg ME, Lai MH, Hartel GF, et al. Response after one dose of a monovalent Influenza A (H1N1) 2009 vaccine -- preliminary report. N Engl J Med. 2009 Sep 10. [Epub ahead of print])

Vaccine interaction. A widely quoted unpublished study from Canada asserts that people who receive the 2008-2009 seasonal flu vaccine are more prone to acquire pandemic H1N1 influenza. (CBC News. Seasonal flu shot may increase H1N1 risk. September 23, 2009. Available at: http://www.cbc.ca/health/story/2009/09/23/flu-shots-h1n1-seasonal.html Accessed October 13, 2009) The implication is that this year's seasonal flu vaccine should be avoided to prevent this vaccine interaction. However, this interaction has not been found by the CDC or WHO, both of which continue to encourage people to get both vaccines.

Vaccine response in children. Dr. Anthony Fauci announced results of the 2009 H1N1 influenza vaccine trials in children. Using doses of 15 µg of Sanofi's killed 2009 H1N1 vaccine, the serologic response rate at 10 days (Table 3) was:

Table 3. Antigenic Response to 2009 H1N1 Vaccine (Sanofi) by Age

Age Group Response
10-17 years 76%
3-9 years 36%
6-35 months 25%

These results are similar to the response rate with seasonal flu vaccine. (NIAID, NIH News. Early results: In children, 2009 H1N1 influenza vaccine works like seasonal flu vaccine. September 21, 2009. Available at: http://www3.niaid.nih.gov/news/newsreleases/2009/H1N1PedTrial.htm Accessed September 30, 2009.)

Response to second dose of 2009 A (H1N1) vaccine. (National Institute of Allergy and Infectious Diseases. November 2, 2009. Bulletin: Updated Results: In Youngest Children A Second Dose of 2009 H1N1 Influenza Vaccine Elicits Robust Immune Response. Available at: http://www3.niaid.nih.gov/news/newsreleases/2009/intermimpedsdata.htm Accessed November 18, 2009.)

This clinical trial evaluated immune response in children 6 months to 17 years of age who received two 15- or 30-µg doses of 2009 A (H1N1) vaccine. Data announced September 21 indicated a good antigenic response in children ages 10 years of age or older following a single 15-µg dose of the vaccine, but younger children did not show a good response to a single dose of either 15 or 30 µg. Results with the second dose given 3 weeks later in children ages 6 months to 9 years were good (Table 4).

Table 4. Response Rates With Single Dose at 3 Weeks and With Second Dose at 3 Weeks

Age Single Dose Second Dose
6-35 months 25% 100%
3-9 years 55% 94%

Immune responses were similar with 2 doses of 15 or 30 µg, and good antibody levels were noted at 8-10 days after the second dose.

These trial results support the recommendation for a single vaccine dose of 15 µg for children 10 years of age or older. Children 6 months to 9 years of age should receive two 15-µg doses separated by at least 21 days.

Adjuvanted H1N1 vaccine. GSK reported initial results of 130 healthy volunteers vaccinated with the GSK split-virus 2009 H1N1 adjuvanted vaccine. This showed that a single dose of 5.25 µg of adjuvanted vaccine produced a 1:40 seroconversion in 98% of volunteers at 3 weeks. (GlaxoSmithKline. Pandemic (H1N1) 2009 Influenza Update: initial results from first clinical trial of GSK's H1N1 adjuvanted vaccine [press release]. London, United Kingdom: GlaxoSmithKline; September 14, 2009.)

Effectiveness and cost-effectiveness of vaccine for pandemic Influenza (H1N1) 2009. Researchers analyzed the effectiveness and cost-effectiveness of the 2009 H1N1 vaccine using epidemiologic models designed to predict the progression of pandemic influenza in a city with a population of 8.3 million. Vaccinating 40% of such a population in October would avert 2051 deaths, gain 69,000 quality-adjusted life-years, and save $469 million. Projected benefits are smaller if the epidemic peaks before mid-October. The results are summarized in Table 5.

Table 5. Results of Vaccination in October vs November 2009

Results October November
Deaths prevented 2051 1468
Gain in quality-adjusted life-years 69,679 49,422
Cost savings $469 million $302 million

The study authors concluded that earlier vaccination against pandemic H1N1 influenza prevents more deaths and is more cost-saving.

(Khazeni N, Hutton DW, Garber AM, Hupert N, Owens DK. Effectiveness and cost-effectiveness of vaccination against pandemic influenza (H1N1) 2009. Ann Intern Med. 2009 Oct 5 [Epub ahead of print])

2009 H1N1 influenza vaccine in China. A novel influenza A (H1N1) vaccine was developed in China and tested in 2200 volunteers stratified by age. The results showed a 97% serologic response rate with at least a 4-fold increase in hemagglutinin titer in volunteers 12-60 years of age at 21 days after a single 15 µg out adjuvant. Children younger than 12 years of age and adults older than 60 years of age had a lower response rate to a single dose but had good responses after a second dose at 21 days. (Zhu FC, Wang H, Fang HH, et al. A novel influenza A (H1N1) in various age groups. N Engl J Med. 2009 Oct 21. [Epub ahead of print].)


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