The flu shot is a crapshoot… it may or may not work, even when the vaccine matches with the predominant virus.
“A northern Indiana hospital has fired eight employees who refused to get flu shots the hospital says are needed to protect patients from the potentially deadly illness.” [Associated Press]
How well do inactivated influenza vaccines work in randomized control trials?
As noted above, effectiveness varies with vaccine match and the age and immune function of the recipient. In general, the greatest benefits of influenza vaccines have been reported in randomized controlled trials (RCTs) conducted among healthy adults. For example, recent RCTs of inactivated influenza vaccine among adults under 65 years of age have estimated 50-70% vaccine efficacy during seasons in which the vaccines’ influenza A components were well matched to circulating influenza A viruses (Beran et al., 2009, 2006-2007 season; Jackson et al., 2010, 2005-2006 season; Monto et al., 2009, 2007-2008 season). As vaccine efficacy from a randomized clinical trial is the gold standard for how well a vaccine actually works, vaccine effectiveness estimates obtained from observational studies can equal, but not exceed, estimates of efficacy. Many factors that can result in substantial bias in effectiveness studies tend to bias the vaccine effect downwards.
How well do influenza vaccines work during seasons in which the vaccine strains are not well matched to circulating influenza viruses?
When vaccine strains are not well matched with circulating influenza viruses, the benefits of vaccination may be reduced. For example, inactivated influenza vaccine effectiveness against laboratory-confirmed influenza was 60% among healthy persons and 48% among those with high-risk medical conditions in a case-control study among people 50–64 years old during the 2003-2004 influenza season, when the vaccine strains were not optimally matched to viruses in circulation (Herrera et al., 2007). However, in a year when the influenza vaccine and predominant circulating influenza viruses were poorly matched, researchers were not able to measure an effect of influenza vaccination against the respective vaccine component (Bridges et al., 2000). It is not possible to predict how well the vaccine and circulating strains will be matched in advance of the influenza season, and how this match may affect vaccine effectiveness.