The Flu Shot - Does it Work?
October is Flu Shot Awareness Month
Influenza or 'the flu' is a major cause of illness and death for the the very old and the very young. As we age, our immune systems have to work harder to fight the influenza virus. For this reason, new vaccines specific to older adults are being developed and tested. The major reason to administer influenza vaccination is to prevent complications of influenza (such as pneumonia). Although the scientific evidence establishing the benefits of vaccination in older adults is limited, it is still the most important strategy for prevention.
The spread of influenza is seasonal, typically from November to April in the Northern Hemisphere. Influenza is transmitted from person to person, through coughing, sneezing, hand to hand contact with subsequent touching (auto-inoculation) of the nose. Although strategies such as hand washing and covering your mouth (known as 'respiratory etiquette') may help reduce spreading of the virus, we don't yet have scientific evidence that these methods work. It is widely accepted that vaccination is the most important strategy to prevent influenza.
Why are influenza vaccinations important for your health?
It is important to recognize that the major reason to administer influenza vaccination (the flu shot) is to prevent complications of influenza. Influenza, particularly in the elderly, is most frequently complicated by pneumonia, often caused by a second infection with bacteria following the viral infection (called super-infection). This is what generally leads to hospitalization and death. People at the extremes of age, that is the very young and the very old, have the highest hospitalization rates due to influenza. The elderly have the highest rates of death, likely because of high rates of secondary infection with bacteria (such as Streptococcus pneumonia or Staphylococcus aureus). There is uncertainty about the exact mechanism for why these bacteria are more likely to cause a second infection in those with influenza. Temporary damage to the lining of the respiratory tract caused by influenza may be predisposing people to the secondary bacteria infection.
Vaccines work by stimulating the immune system to produce antibodies that can protect those vaccinated from infection with influenza. There are small changes that take place in the genetic structure of the influenza virus and these changes occur every year. These changes lead to a reduced ability of the antibodies produced by a vaccine to effectively bind to the surface of the influenza virus. For this reason, the virus strains in the vaccine must be updated on an annual basis. This is why it is important for people to be vaccinated yearly.
How well do vaccines for influenza work as we age?
Scientists and doctors believe that in the elderly immune mechanisms other than antibodies play a major role in protection. The study of these other immune mechanisms is still in the early stages. For this reason scientists focus on new vaccines that continue to measure antibodies related to protection from the influenza virus. The ability to produce an effective response to vaccines decreases as people age. The majority of studies examining changes due to age have been done in animal models. As well, some studies have not been linked to protection against actual infection. Nevertheless, it is safe to say that changes to the immune system that occur with aging, reduce the ability of the elderly to protect themselves against influenza. For this reason manufacturers have recently developed different types of influenza vaccines to better protect the elderly. These new vaccines are currently being tested to establish their effectiveness.
How well do influenza vaccinations work in older adults?
Estimates of how well vaccines work differ by age. In younger adults for example, recent studies that have pooled the most rigorous data, suggest that the influenza vaccine is about 60% effective in preventing influenza. In the elderly, an answer to the question about the effectiveness of influenza vaccines is less clear. This is because there have only been a few randomized controlled trials done in older adults. It has become increasingly evident in recent years that estimates of protection of the influenza vaccine against serious outcomes (such as death or hospitalization) may have been exaggerated. This is because of design flaws in the studies that have been conducted. The major flaw is simple to understand, healthier people have been more likely to be vaccinated than those who are frail and ill. Therefore, the effect of the vaccine to protect against hospitalization and death has not entirely been on the strengths of the vaccine itself. Rather, healthier people who are less likely to die and less likely be hospitalized were those who received the vaccinations. We need better research to know how effective influenza vaccinations are in preventing these outcomes in older adults.
Despite the limited scientific research public health officials still promote the use of influenza vaccinations. The goal of public health is to maximize the uptake of influenza vaccination. It is still important to decrease the amount of flu circulating in the community at large. Increasing the uptake of influenza vaccination in the community, particularly among children, can lead to indirect protection for the elderly. That is, the so-called herd effect can occur where groups that are typically key in spreading influenza, such as young children, are vaccinated. If children are vaccinated effectively, the influenza virus is less likely to spread and therefore less likely to infect the elderly.
Bottom line about influenza vaccinations in older adults
Senior citizens and older adults are at the highest risk for complications from influenza. Any reduction in such complications is important, even if it is difficult to arrive at a more precise estimate of effectiveness at this time. The development of new vaccines specific to the elderly, have recently been shown to reduce influenza compared to standard dosing in a large randomized trial. As well, studies evaluating the addition of immune stimulating agents to the vaccine have been shown to produce a stronger immune response (higher concentration of antibodies) compared to usual vaccines. These new vaccines specific to older adults show promising improvements to prevent influenza.
Visit the McMaster Optimal Aging Portal online at www.mcmasteroptimalaging.org for more information. The Portal is a free website for seniors that provides evidence-based information about healthy aging.