Re: USA - Media Advisory - CDC Update: Flu Season and Vaccine Effectiveness Today 11:30am
Flu Vaccine Effectiveness: Questions and Answers for Health Professionals CDC 0ct. 2011 (Some of the questions and answers)
Which outcomes provide the best estimates of vaccine effectiveness?
Studies that use more specific outcomes, such as laboratory-confirmed influenza outcomes (e.g., culture positive or reverse-transcriptase polymerase chain reaction (RT-PCR) positive results), provide the best and most specific estimates of the impact of influenza vaccines in preventing influenza. In general, when non-laboratory-confirmed outcomes are used (e.g., all pneumonia hospitalizations or influenza-like illness, which include many non-influenza illnesses), vaccine effectiveness estimates are lower. For example, a study by Bridges et al (2000) among healthy adults found that the inactivated influenza vaccine was 86% effective against laboratory-confirmed influenza, but only 10% effective against all respiratory illnesses in the same population and season.
Why do estimates of influenza vaccine effectiveness vary widely?
Estimates of influenza vaccine effectiveness are affected by several factors, including the specific study biases discussed above, the match between the vaccine influenza strains and the circulating strains, host factors and the sample size of a specific study...However, vaccine effectiveness will always vary from season to season, based upon the degree of similarity between the viruses in the vaccine and those in circulation, as well as other factors. In years when the vaccine strains are not well-matched to circulating strains, vaccine effectiveness is generally lower. In addition, host factors also affect vaccine effectiveness.
In general, influenza vaccines are less effective among people with chronic medical conditions and among people age 65 and older, as compared to healthy young adults and older children.
How well do influenza vaccines work in people with chronic high-risk medical conditions?
The presence of chronic medical conditions may also affect the effectiveness of influenza vaccines. For example, in an observational study of people 50?64 years of age, the vaccine was 60% effective in preventing laboratory-confirmed influenza among otherwise healthy adults 50?64 years of age, but
only 48% effective among those who had high-risk medical conditions (Herrera et al., 2006). In general, vaccine efficacy and effectiveness estimates among people with high-risk conditions may be somewhat lower than among people of similar age without high-risk conditions. However, because the risk of influenza-related complications among this group is much higher, vaccination still provides important benefits.
Adults 65 years or older
Only one large randomized, controlled trial of influenza vaccine has been conducted among an elderly population. During the 1991-1992 influenza season, a group of Dutch people 60 years of age and older not living in long-term care facilities (e.g., nursing homes) was studied (Govaert et al., 1994). In this study, vaccine efficacy was 58% in preventing clinically-defined influenza with serologic confirmation of infection. There are no published studies of the efficacy or effectiveness of influenza vaccines in preventing laboratory-confirmed, serious outcomes of influenza such as hospitalization, primarily because the size of the study would be large, and therefore, such a study is very expensive to conduct. Published observational studies conducted among people 65 and older not living in long-term care facilities have used non-specific outcomes, such as pneumonia hospitalizations or all-cause mortality. These studies may be subject to substantial confounding and selection bias, and they use outcomes in which the proportion of illness associated with influenza virus infections vary by season (as other respiratory viruses can cocirculate). As a result, it is difficult to interpret the results of these studies.
Adults 65 years or older in long-term care facilities
All residents of long-term care facilities s (e.g., nursing homes) should receive annual influenza vaccination, as outbreaks of influenza can be explosive and result in substantial morbidity and mortality among residents of such facilities. There is evidence that vaccination prevents respiratory illnesses during periods of influenza circulation for elderly nursing home residents. For example,
one study conducted during the 1991-1992 influenza season found that vaccination was associated with a 34% reduction in total respiratory illnesses and a 55% reduction in pneumonia during the two-week peak of influenza activity (Monto, 2001). In addition, one study conducted in UK nursing homes found that vaccinating health care workers decreased deaths during periods of influenza activity during one season with substantial influenza circulation, but not during the next year, when influenza activity was low throughout the winter (Hayward, 2006).
Children
In a four-year randomized, placebo-controlled study of inactivated and live influenza vaccines among children aged 1?15 years, vaccine efficacy was estimated at 77% against influenza A (H3N2) and 91% against influenza A (H1N1) virus infection (Neuzil et al., 2001). A two-year study of children aged 6?24 months found that the vaccine was 66% effective in preventing laboratory-confirmed influenza in one year of the study (Hoberman et al., 2003). Only children who were fully vaccinated (i.e., had either two doses if not previously vaccinated, or one dose if previously vaccinated) versus unvaccinated children were included in the analysis. In the other year of this study, few cases of influenza occurred, making it difficult to assess the vaccine's efficacy (Hoberman et al., 2003). Children younger than 9 years of age who have not been vaccinated previously are recommended to receive two doses of vaccine the first year they get vaccinated. In subsequent years, they need only one dose. This recommendation was made because many children younger than 9 years of age have not been infected with influenza viruses previously, and a booster dose is needed for them to produce a protective immune response.
Questions and answers continue.
http://www.cdc.gov/flu/professionals/vaccination/effectivenessqa.htm
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