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Media Statement

For Immediate Release: October 7, 2009
Contact: Division of News & Electronic Media, Office of Communication
(404) 639-3286

Seasonal Influenza Vaccine and Risk of 2009 H1N1 Influenza

Recently there have been several media reports describing unpublished findings from seasonal influenza vaccine studies conducted in Canada. The findings from these studies suggest that receiving the 2008-09 seasonal influenza vaccine (given last influenza season) was a risk factor for developing influenza caused by the 2009 H1N1 virus. In the studies done in Canada, the increase in risk among persons vaccinated with a seasonal influenza vaccine was approximately double the risk for those who were not vaccinated with seasonal influenza vaccine. However, the risk for severe influenza was not increased. The results of these studies have been presented by the Canadian investigators to CDC scientists. However, the research findings from Canada have not been published in the medical literature or presented at any public scientific meetings. There has not yet been an opportunity to fully review the studies in detail.

There is no indication that the type of seasonal influenza vaccine received is an important factor. The Canadian study investigators are not able to determine which vaccines study participants received. The influenza virus strains used to make the Canadian seasonal influenza vaccine were the same as those used in the United States and many other countries. However, some seasonal influenza vaccines used in Canada are made by manufacturers not licensed in the United States and vice versa.

CDC Response
Preliminary results of studies conducted in the United States using methods similar to the Canadian studies did not indicate that receiving a seasonal influenza vaccine increased the risk of developing influenza caused by the 2009 H1N1 influenza virus. In addition, no other country has reported that seasonal influenza vaccine increases the risk of developing influenza caused by the 2009 H1N1 influenza virus. Only one study has been published on this issue, which reported data collected in Australia. The Australian study did not find any association between receipt of seasonal influenza vaccine and risk of developing influenza caused by the 2009 H1N1 influenza virus. Australian researchers recently published these findings: Kelly H, Grant K. Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination. Eurosurveillance Volume 14, Issue 31, 06 August 2009. ( In addition, seasonal influenza vaccination leading to increased susceptibility to other influenza viruses has never been reported before.

CDC is working with scientists in Canada, the World Health Organization (WHO) and other countries to further investigate the findings from Canada and other countries. Studies in the United States also are continuing. Thus far, no explanation has been found for the differences between results from studies in Canada compared to results of studies conducted in other countries.

CDC continues to recommend seasonal influenza vaccination for the 2009-10 influenza season. Currently the vast majority of influenza being reported to CDC is 2009 influenza A (H1N1). However, influenza is very unpredictable and seasonal influenza viruses might circulate at any point in the season. CDC does not recommend halting or deferring seasonal influenza immunization efforts.

The recommendations for who should receive seasonal influenza vaccine have not changed. CDC recommends seasonal influenza vaccination for anyone who wants to reduce their chances of getting seasonal flu. Yearly vaccination is particularly important for certain groups of people, including those who are at high risk of having serious seasonal influenza-related complications or people who live with or care for those at high risk for serious seasonal influenza-related complications, including:

  • Children aged 6 months up to their 19th birthday
  • Pregnant women
  • People 50 years of age and older
  • People of any age with certain chronic medical conditions
  • People who live in nursing homes and other long-term care facilities
  • People who live with or care for those at high risk for complications from flu,including:
    • Health care workers
    • Household contacts of persons at high risk for complications from the flu
    • Household contacts and out of home caregivers of children less than 6 months of age(these children are too young to be vaccinated)


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