Prevention and Control of Seasonal Influenza with Vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009, July 31

The United States Centers for Disease Control and Prevention (CDC) has recently published the recommendations of its Advisory Committee on Immunization Practices (ACIP) regarding the use of vaccine for the prevention and control of seasonal influenza for the 2009-10 season.(1) The major change from the 2008-9 season recommendations is that ACIP now recommends that vaccine should be given to all children aged 6 months -18 years.(2) At the same time though the Committee recommends that the primary focus for childhood immunisation should continue to be ensuring immunisation in all children aged 6 – 59 months and older children with increased risk for severe disease through having chronic illnesses or handicapping conditions. In addition ACIP recommends that influenza vaccine should be provided to all persons > 6 months who want to reduce risk of becoming ill with influenza or of transmitting it to others. However, emphasis should be on providing certain groups of all ages at higher risk for influenza infection or complications is advised.

The ACIP target groups for seasonal influenza vaccination for children and adults for the 2009/2010 season are summarized in the table below:

It is estimated by CDC that approximately 85% of the US population is included in one or more of the target groups. At the same time it is estimated that vaccination coverage for the 2008-2009 season was less than 40%.  Influenza vaccination coverage levels by population group in 2007- 2008 through National Health Interview Survey (NHIS) in the US are presented below:

One important difference from Europe is that intranasal live attenuated influenza virus vaccine (LAIV) is available in the United States as well as intramuscular trivalent inactivated influenza virus vaccine (TIV). LAIV may be used in the age groups 2- 49 years in the USA, however, the LAIV should not be administered to children <5 years with possible reactive airways.

The same MMWR report also summarizes safety data for influenza vaccines licensed in the United States including thiomersal-containing vaccines.

ECDC comment (9th September 2009):
This winter will be an unusual one for seasonal influenza in Europe and North America because of the A(H1N1) 2009 pandemic. It will be a season of transition and it is hard to predict which seasonal influenza viruses will be circulating in addition to the A(H1N1) 2009 pandemic strain. The pattern in the Southern Hemisphere temperate countries may be informative. To date where there has been robust sentinel surveillance the experience in the 2009 winter in the Southern Hemisphere few seasonal influenza B virus or seasonal influenza A(H1N1) virus isolates have been detected. As the season has gone on the pandemic strain has predominated more and more over other A viruses. However, some seasonal influenza A(H3N2) viruses have been observed  in Australia and Far East Asia (http://www.cdc.gov/h1n1flu/updates/international/map.htm) and  there are countries like South Africa where there have been significant amounts of influenza A(H3N2) preceding the pandemic strain. Data from countries in southern South America are lacking, since at least in Argentina intense diagnostic needs resulted in temporary cessation of virological sentinel surveillance and typing but in Chile the picture is similar to Australasia with little influenza A apart from A(H1N1) 2009. (www.minsal.cl)  The pressure from the pandemic is likely to introduce additional selection biases, for example through the majority of specimens coming from people affected by the pandemic and the number of the seasonal viruses are likely to be less in the surveillance system than are actually circulating in the community, eg among older people.

In addition, a few of the influenza A (H3N2) strains isolated in Australia and the Far East Asia have drifted in their antigenic characteristics to be outside the range that might be expected to be covered by current influenza vaccines.(I. Barr personal communication)  The implications of this and the pandemic virus will be considered later this month at the strain selection meeting for the Southern Hemisphere 2010. One epidemiological study has been published looking at the protective effect of seasonal vaccination against A(H1N1) 2009.(3) That confirmed the serological finding that the current seasonal vaccine is likely to be ineffective in giving protection against influenza A(H1N1) 2009.(4) Having had prior seasonal influenza neither increased nor decreased the risk of illness from the pandemic strain.(3) Hence, it can be argued that seasonal vaccination this autumn may be less useful than usual. Alternatively, influenza is not a predictable virus and not vaccinating with seasonal vaccines would leave older patients and people with chronic diseases vulnerable against any remaining B and A(H3N2) viruses. Also with hospitals possibly under strain from influenza A(H1N1)2009 even preventing a small number of hospitalizations with seasonal strains is of value.

The overview in this report of the safety of seasonal influenza vaccines will be helpful in the coming assessment of safety of pandemic vaccines.

1. CDC Prevention and Control of Seasonal Influenza with Vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009; 58 July 31
2. CDC Prevention and Control of Seasonal Influenza with Vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008; 57 August 8th
3. Kelly H, Grant K. Interim analysis of pandemic influenza (H1N1) 2009 in Australia: surveillance trends, age of infection and effectiveness of seasonal vaccination Eurosurveillance 2009 14; August 6^th http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19288
4. Centers for Disease Control and Prevention. CDC Serum Cross-Reactive Antibody Response to a Novel Influenza A (H1N1) Virus After Vaccination with Seasonal Influenza Vaccine. MMWR May 22, 2009/58(19);521–524.